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Author Topic: A free high quality resampling tool for ImageMagick users  (Read 189732 times)

BartvanderWolf

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A free high quality resampling tool for ImageMagick users
« on: July 17, 2014, 11:47:12 AM »

Hi folks,

As a result of a discussion in another thread in this forum, and with permission of Nicolas Robidoux who suggested the prodedure, you are free to download and use the attached ImageMagick resampling script in a ZIP archive. NOTE, an updated version is available here.

It is formatted as a Windows batch file that uses ImageMagick for the actual number crunching, and it allows to drag and drop an image on the batch-file icon which can be situated on the desktop or in a folder. It will ask the user for a percentage value by which to resize the image pixel dimensions. Do *not* enter a percentage sign, or any other non-numerical input, just a number. A number larger than 100 will enlarge the image, a number smaller than 100 will reduce the image size. So e.g. just entering 25 will reduce the linear pixel dimensions to one quarter (25%), and 300 will triple the linear pixel dimensions (300%). Decimal separators, e.g. 33.33333 for a reduction to a third of the dimensions are allowed (regional OS settings may allow other separators, I'm not sure).

For ease of use in a Windows computer system, the file can also be added to the User's 'SendTo' file location (e.g. 'C:\Users\YOURNAME\AppData\Roaming\Microsoft\Windows\SendTo' for Windows 7), which will then allow to right mouse-click on an image file in the File Explorer and send it to the script to be processed by ImageMagick (which is assumed to be already installed on the user's computer).

The Resampled image file with a PNG file type and extension will be placed in the same folder or directory as the original file, with a postfix to the filename that indicates the resampling percentage that was used. The script currently will *not* ask if overwriting with that (assumed new) filename is allowed, should it already exist (which is unlikely anyway, but you are now assumed to have been warned!).

Currently the script assumes input files with an sRGB gamma and colorspace. This is something that can be changed in the file itself, or may be added to a future edition as a selection option.

Which brings us to the following. This file is the product of a free exchange of ideas on various fora. People are encouraged to add and share improvements, and versions adapted to other operating systems for which ImageMagick is available, such as UNIX and Mac OS.

Feature requests may be considered, but implementing them is merely a request to volunteers who may or may not wish to spend they spare free time to actually implement and test such modifications. Consider this to be an open collaborative project, so your input is also valued.

Use at your own peril or for enjoyment.

Cheers,
Bart

P.S. This is a works in progress, and the initial file can be improved I'm sure. Feel free to tell me how ...
« Last Edit: September 19, 2014, 10:08:59 AM by BartvanderWolf »
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NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #1 on: July 17, 2014, 04:33:53 PM »

A more radical approach is to use a pair of gammas, that is, replacing, for example, the linear RGB result (used to compute the luminance alpha channel) by the result of ... -gamma .3333333333333333 -filter LanczosRadius -distort Resize 800% -gamma 3 when blending.
Somehow, I think that sticking to linear light is likely to be better for the "not too much light halo" component, but who knows?
P.S. I just gave a quick and very dirty try to a version in which instead of using luminance I used the average of the through gamma 1/3 and the through gamma 3 results, and the nice thing about it is that it restores the "black/white symmetry". I'm not saying this is the best way to do things, just that it did not turn out badly. HDRI (floating point computation toolchain) may be necessary here. Not sure.
« Last Edit: July 17, 2014, 04:43:51 PM by NicolasRobidoux »
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BartvanderWolf

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Re: A free high quality resampling tool for ImageMagick users
« Reply #2 on: July 17, 2014, 04:49:26 PM »

A more radical approach is to use a pair of gammas, that is, replacing, for example, the linear RGB result (used to compute the luminance alpha channel) by the result of ... -gamma .3333333333333333 -filter LanczosRadius -distort Resize 800% -gamma 3 when blending.
Somehow, I think that sticking to linear light is likely to be better for the "not too much light halo" component, but who knows?

Indeed Nicolas, that idea had crossed my mind. If it works for one layer, it should also work for the other layer, with opposite gamma boosts, QED.

I will explore that alternative after looking a bit more at some potential optimizations of the current code. And since the results are that well behaved as far as haloing is concerned, sharpening the result is also a possible option to add in a multi-line script. Having a working script will make implementing such alternatives almost trivial.

I've also tried a 'null' conversion, 100 percent same size resampling, which produced only minimal differences in high spatial frequency areas. We'd have to be careful to keep such neutral behavior intact. Need to test that some more.

Cheers,
Bart
« Last Edit: July 17, 2014, 05:09:16 PM by BartvanderWolf »
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FranciscoDisilvestro

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Re: A free high quality resampling tool for ImageMagick users
« Reply #3 on: July 17, 2014, 06:58:55 PM »

Hi Bart,

Thanks for sharing the script. I have followed with interest the other thread and will be trying this over the weekend.
I have a couple of questions related to the script (I'm on the road so I can't test for myself now):

I see that you convert to 16  bits, but does the input file has to be in a specific format or will it work with tiff, jpg or png?

When you mention sRGB gamma, are you referring to sRGB TRC or an approximate gamma (e.g. 2.2)?

From what I can follow in the script, you separate the RGB channels and then recombine adding the calculated alpha channel.
If the script is modified, will it work for monochrome images (not RGB, only gray channel)? Pardon my ingenuity, but I'm thinking of skipping the channel separation part, computing the alpha channel and then recombine the gray with the alpha.

Regards

NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #4 on: July 18, 2014, 03:56:45 AM »

Bart:

I have high hopes that something good can be squeezed of this research direction.

1) The reason for using "a pair of opposite gamma spaces" is to restore some black/white symmetry. I think that, however, the alpha should then be computed from either some sort of average of the two results or from linear light, in which case there would be three resampled images used to recombine: through RGB gamma 3 (for example), through RGB gamma 1/3, and linear RGB (gamma 1). These thoughts are a bit rough. Maybe one should work in a pair of direct/reverse video after normalizing the input image, then de-normalizing? Assuming the normalization anchors things at the black and white points (roughly), this would definitely restore black/white symmetry. (Quickly brainstorming, here.)

Poking really quickly into the first idea above, however, one would need a very accurate floating point toolchain to pull off an initial -gamma .3333333333333. Not sure, so this is more a "watch for this" than a true "jump up and down warning". But I'm a bit concerned that Q16 is not enough.

Mixing linear with gamma 3 sounds like a reasonable practical approach. But it definitely does not respect black/white symmetry.

2) EWA LanczosRadius is not interpolatory. Unless you explicitly used a hard wired "null" operation, there will be a difference when do "no-op" resampling. In practice, this difference could be argued to be a good thing (with EWA LanczosSharpest 4, for example). The real question is whether the final answer differs significantly from an infinite precision version of the result. I'm just making sure here, that you know (the reasonable assumption, namely that you do know, can be dangerous) that there are two sources for deviation from the original when resizing to the exact same dimensions: round off error and the fact that the filter actually does some filtering in all cases.

3) Fred Weinhaus has started optimizing the code further and poking around. We'll post things in the IM forums as soon as I've had time to check things over and organize results (and my thoughts) into something halfway intelligible.
« Last Edit: July 18, 2014, 04:03:56 AM by NicolasRobidoux »
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BartvanderWolf

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Re: A free high quality resampling tool for ImageMagick users
« Reply #5 on: July 18, 2014, 03:57:17 AM »

Hi Bart,

Thanks for sharing the script. I have followed with interest the other thread and will be trying this over the weekend.
I have a couple of questions related to the script (I'm on the road so I can't test for myself now):

I see that you convert to 16  bits, but does the input file has to be in a specific format or will it work with tiff, jpg or png?

Hi Frank,

Good question.

In principle ImageMagick supports a whole lot of file formats. Here are the ones it reports on my Microsoft Windows [Version 6.1.7601] installation:
>identify -list format
   Format  Module    Mode  Description
-------------------------------------------------------------------------------
      3FR  DNG       r--   Hasselblad CFV/H3D39II
        A* RAW       rw+   Raw alpha samples
      AAI* AAI       rw+   AAI Dune image
       AI  PDF       rw-   Adobe Illustrator CS2
      ART* ART       rw-   PFS: 1st Publisher Clip Art
      ARW  DNG       r--   Sony Alpha Raw Image Format
      AVI  MPEG      r--   Microsoft Audio/Visual Interleaved
      AVS* AVS       rw+   AVS X image
        B* RAW       rw+   Raw blue samples
      BGR* BGR       rw+   Raw blue, green, and red samples
     BGRA* BGR       rw+   Raw blue, green, red, and alpha samples
      BIE* JBIG      rw-   Joint Bi-level Image experts Group interchange format
 (1.6)
      BMP* BMP       rw-   Microsoft Windows bitmap image
     BMP2* BMP       -w-   Microsoft Windows bitmap image (V2)
     BMP3* BMP       -w-   Microsoft Windows bitmap image (V3)
      BRF* BRAILLE   -w-   BRF ASCII Braille format
        C* RAW       rw+   Raw cyan samples
      CAL* CALS      rw-   Continuous Acquisition and Life-cycle Support Type 1
           Specified in MIL-R-28002 and MIL-PRF-28002
     CALS* CALS      rw-   Continuous Acquisition and Life-cycle Support Type 1
           Specified in MIL-R-28002 and MIL-PRF-28002
   CANVAS* XC        r--   Constant image uniform color
  CAPTION* CAPTION   r--   Caption
      CIN* CIN       rw-   Cineon Image File
      CIP* CIP       -w-   Cisco IP phone image format
     CLIP* CLIP      rw+   Image Clip Mask
CLIPBOARD* CLIPBOARD rw-   The system clipboard
     CMYK* CMYK      rw+   Raw cyan, magenta, yellow, and black samples
    CMYKA* CMYK      rw+   Raw cyan, magenta, yellow, black, and alpha samples
      CR2  DNG       r--   Canon Digital Camera Raw Image Format
      CRW  DNG       r--   Canon Digital Camera Raw Image Format
      CUR* CUR       rw-   Microsoft icon
      CUT* CUT       r--   DR Halo
      DCM* DCM       r--   Digital Imaging and Communications in Medicine image
           DICOM is used by the medical community for images like X-rays.  The
           specification, "Digital Imaging and Communications in Medicine
           (DICOM)", is available at http://medical.nema.org/.  In particular,
           see part 5 which describes the image encoding (RLE, JPEG, JPEG-LS),
           and supplement 61 which adds JPEG-2000 encoding.
      DCR  DNG       r--   Kodak Digital Camera Raw Image File
      DCX* PCX       rw+   ZSoft IBM PC multi-page Paintbrush
      DDS* DDS       rw+   Microsoft DirectDraw Surface
    DFONT* TTF       r--   Multi-face font package (Freetype 2.5.3)
     DJVU* DJVU      ---   Déjà vu
           See http://www.djvuzone.org/ for details about the DJVU format.  The
           DJVU 1.2 specification is available there and at
           ftp://swrinde.nde.swri.edu/pub/djvu/documents/.
      DNG  DNG       r--   Digital Negative
      DOT  DOT       ---   Graphviz
      DPS  DPS       ---   Display Postscript Interpreter
      DPX* DPX       rw-   SMPTE 268M-2003 (DPX 2.0)
           Digital Moving Picture Exchange Bitmap, Version 2.0.
           See SMPTE 268M-2003 specification at http://www.smtpe.org

     DXT1* DDS       rw+   Microsoft DirectDraw Surface
     DXT5* DDS       rw+   Microsoft DirectDraw Surface
      EMF  WMF       r--   Windows Enhanced Meta File
     EPDF  PDF       rw-   Encapsulated Portable Document Format
      EPI  PS        rw-   Encapsulated PostScript Interchange format
      EPS  PS        rw-   Encapsulated PostScript
     EPS2* PS2       -w-   Level II Encapsulated PostScript
     EPS3* PS3       -w+   Level III Encapsulated PostScript
     EPSF  PS        rw-   Encapsulated PostScript
     EPSI  PS        rw-   Encapsulated PostScript Interchange format
      EPT  EPT       rw-   Encapsulated PostScript with TIFF preview
     EPT2  EPT       rw-   Encapsulated PostScript Level II with TIFF preview
     EPT3  EPT       rw+   Encapsulated PostScript Level III with TIFF preview
      ERF  DNG       r--   Epson RAW Format
      EXR  EXR       ---   High Dynamic-range (HDR)
      FAX* FAX       rw+   Group 3 FAX
           FAX machines use non-square pixels which are 1.5 times wider than
           they are tall but computer displays use square pixels, therefore
           FAX images may appear to be narrow unless they are explicitly
           resized using a geometry of "150x100%".

     FITS* FITS      rw-   Flexible Image Transport System
      FPX  FPX       ---   FlashPix Format
  FRACTAL* PLASMA    r--   Plasma fractal image
      FTS* FTS       rw-   Flexible Image Transport System
        G* RAW       rw+   Raw green samples
       G3* FAX       rw-   Group 3 FAX
      GIF* GIF       rw+   CompuServe graphics interchange format
    GIF87* GIF       rw-   CompuServe graphics interchange format (version 87a)
 GRADIENT* GRADIENT  r--   Gradual linear passing from one shade to another
     GRAY* GRAY      rw+   Raw gray samples
   GROUP4* TIFF      rw-   Raw CCITT Group4
       GV  DOT       ---   Graphviz
     HALD*           r--   Identity Hald color lookup table image
      HDR* HDR       rw+   Radiance RGBE image format
HISTOGRAM* HISTOGRAM -w-   Histogram of the image
      HRZ* HRZ       rw-   Slow Scan TeleVision
      HTM* HTML      -w-   Hypertext Markup Language and a client-side image map

     HTML* HTML      -w-   Hypertext Markup Language and a client-side image map

      ICB* TGA       rw-   Truevision Targa image
      ICO* ICON      rw+   Microsoft icon
     ICON* ICON      rw-   Microsoft icon
     INFO  INFO      -w+   The image format and characteristics
   INLINE* INLINE    r--   Base64-encoded inline images
      IPL* IPL       rw+   IPL Image Sequence
   ISOBRL* BRAILLE   -w-   ISO/TR 11548-1 format
      J2C* JP2       rw-   JPEG-2000 Code Stream Syntax (2.1.0)
      J2K* JP2       rw-   JPEG-2000 Code Stream Syntax (2.1.0)
      JBG* JBIG      rw+   Joint Bi-level Image experts Group interchange format
 (1.6)
     JBIG* JBIG      rw+   Joint Bi-level Image experts Group interchange format
 (1.6)
      JNG* PNG       rw-   JPEG Network Graphics
           See http://www.libpng.org/pub/mng/ for details about the JNG
           format.
      JNX* JNX       r--   Garmin tile format
      JP2* JP2       rw-   JPEG-2000 File Format Syntax (2.1.0)
      JPC* JP2       rw-   JPEG-2000 Code Stream Syntax (2.1.0)
     JPEG* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
      JPG* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
      JPT* JP2       rw-   JPEG-2000 File Format Syntax (2.1.0)
     JSON  JSON      -w+   The image format and characteristics
        K* RAW       rw+   Raw black samples
      K25  DNG       r--   Kodak Digital Camera Raw Image Format
      KDC  DNG       r--   Kodak Digital Camera Raw Image Format
    LABEL* LABEL     r--   Image label
        M* RAW       rw+   Raw magenta samples
      M2V  MPEG      rw+   MPEG Video Stream
      M4V  MPEG      rw+   Raw MPEG-4 Video
      MAC* MAC       r--   MAC Paint
      MAP* MAP       rw-   Colormap intensities and indices
     MASK* MASK      rw+   Image Clip Mask
      MAT  MAT       rw+   MATLAB level 5 image format
    MATTE* MATTE     -w+   MATTE format
      MEF  DNG       r--   Mamiya Raw Image File
     MIFF* MIFF      rw+   Magick Image File Format
      MNG* PNG       rw+   Multiple-image Network Graphics (libpng 1.6.10)
           See http://www.libpng.org/pub/mng/ for details about the MNG
           format.
     MONO* MONO      rw-   Raw bi-level bitmap
      MOV  MPEG      rw+   MPEG Video Stream
      MP4  MPEG      rw+   MPEG-4 Video Stream
      MPC* MPC       rw+   Magick Persistent Cache image format
     MPEG  MPEG      rw+   MPEG Video Stream
      MPG  MPEG      rw+   MPEG Video Stream
      MRW  DNG       r--   Sony (Minolta) Raw Image File
      MSL* MSL       rw+   Magick Scripting Language
     MSVG  SVG       rw+   ImageMagick's own SVG internal renderer
      MTV* MTV       rw+   MTV Raytracing image format
      MVG* MVG       rw-   Magick Vector Graphics
      NEF  DNG       r--   Nikon Digital SLR Camera Raw Image File
      NRW  DNG       r--   Nikon Digital SLR Camera Raw Image File
     NULL* NULL      rw-   Constant image of uniform color
        O* RAW       rw+   Raw opacity samples
      ORF  DNG       r--   Olympus Digital Camera Raw Image File
      OTB* OTB       rw-   On-the-air bitmap
      OTF* TTF       r--   Open Type font (Freetype 2.5.3)
      PAL* UYVY      rw-   16bit/pixel interleaved YUV
     PALM* PALM      rw+   Palm pixmap
      PAM* PNM       rw+   Common 2-dimensional bitmap format
    PANGO* PANGO     r--   Pango Markup Language (Pangocairo 1.36.0)
  PATTERN* PATTERN   r--   Predefined pattern
      PBM* PNM       rw+   Portable bitmap format (black and white)
      PCD* PCD       rw-   Photo CD
     PCDS* PCD       rw-   Photo CD
      PCL  PCL       rw+   Printer Control Language
      PCT* PICT      rw-   Apple Macintosh QuickDraw/PICT
      PCX* PCX       rw-   ZSoft IBM PC Paintbrush
      PDB* PDB       rw+   Palm Database ImageViewer Format
      PDF  PDF       rw+   Portable Document Format
     PDFA  PDF       rw+   Portable Document Archive Format
      PEF  DNG       r--   Pentax Electronic File
      PES* PES       r--   Embrid Embroidery Format
      PFA* TTF       r--   Postscript Type 1 font (ASCII) (Freetype 2.5.3)
      PFB* TTF       r--   Postscript Type 1 font (binary) (Freetype 2.5.3)
      PFM* PFM       rw+   Portable float format
      PGM* PNM       rw+   Portable graymap format (gray scale)
    PICON* XPM       rw-   Personal Icon
     PICT* PICT      rw-   Apple Macintosh QuickDraw/PICT
      PIX* PIX       r--   Alias/Wavefront RLE image format
    PJPEG* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
   PLASMA* PLASMA    r--   Plasma fractal image
      PNG* PNG       rw-   Portable Network Graphics (libpng 1.6.10)
           See http://www.libpng.org/ for details about the PNG format.
    PNG00* PNG       rw-   PNG inheriting bit-depth and color-type from original

    PNG24* PNG       rw-   opaque or binary transparent 24-bit RGB (zlib 1.2.7)
    PNG32* PNG       rw-   opaque or transparent 32-bit RGBA
    PNG48* PNG       rw-   opaque or binary transparent 48-bit RGB
    PNG64* PNG       rw-   opaque or transparent 64-bit RGBA
     PNG8* PNG       rw-   8-bit indexed with optional binary transparency
      PNM* PNM       rw+   Portable anymap
      PPM* PNM       rw+   Portable pixmap format (color)
  PREVIEW* PREVIEW   -w-   Show a preview an image enhancement, effect, or f/x
       PS  PS        rw+   PostScript
      PS2* PS2       -w+   Level II PostScript
      PS3* PS3       -w+   Level III PostScript
      PSB* PSD       rw+   Adobe Large Document Format
      PSD* PSD       rw+   Adobe Photoshop bitmap
     PTIF* TIFF      rw+   Pyramid encoded TIFF
      PWP* PWP       r--   Seattle Film Works
        R* RAW       rw+   Raw red samples
RADIAL-GRADIENT* GRADIENT  r--   Gradual radial passing from one shade to anothe
r
      RAF  DNG       r--   Fuji CCD-RAW Graphic File
      RAS* SUN       rw+   SUN Rasterfile
      RGB* RGB       rw+   Raw red, green, and blue samples
     RGBA* RGB       rw+   Raw red, green, blue, and alpha samples
     RGBO* RGB       rw+   Raw red, green, blue, and opacity samples
      RGF* RGF       rw-   LEGO Mindstorms EV3 Robot Graphic Format (black and w
hite)
      RLA* RLA       r--   Alias/Wavefront image
      RLE* RLE       r--   Utah Run length encoded image
      RMF  DNG       r--   Raw Media Format
      RW2  DNG       r--   Panasonic Lumix Raw Image
      SCR* SCR       r--   ZX-Spectrum SCREEN$
SCREENSHOT* SCREENSHO r--   Screen shot
      SCT* SCT       r--   Scitex HandShake
      SFW* SFW       r--   Seattle Film Works
      SGI* SGI       rw+   Irix RGB image
    SHTML* HTML      -w-   Hypertext Markup Language and a client-side image map

SPARSE-COLOR* TXT       -w+   Sparse Color
      SR2  DNG       r--   Sony Raw Format 2
      SRF  DNG       r--   Sony Raw Format
  STEGANO* STEGANO   r--   Steganographic image
      SUN* SUN       rw+   SUN Rasterfile
      SVG  SVG       rw+   Scalable Vector Graphics (RSVG 2.40.1)
     SVGZ  SVG       rw+   Compressed Scalable Vector Graphics (RSVG 2.40.1)
     TEXT* TXT       rw+   Text
      TGA* TGA       rw-   Truevision Targa image
THUMBNAIL* THUMBNAIL -w+   EXIF Profile Thumbnail
     TIFF* TIFF      rw+   Tagged Image File Format (LIBTIFF, Version 4.0.3)
   TIFF64* TIFF      rw-   Tagged Image File Format (64-bit) (LIBTIFF, Version 4
.0.3)
     TILE* TILE      r--   Tile image with a texture
      TIM* TIM       r--   PSX TIM
      TTC* TTF       r--   TrueType font collection (Freetype 2.5.3)
      TTF* TTF       r--   TrueType font (Freetype 2.5.3)
      TXT* TXT       rw+   Text
     UBRL* BRAILLE   -w-   Unicode Text format
      UIL* UIL       -w-   X-Motif UIL table
     UYVY* UYVY      rw-   16bit/pixel interleaved YUV
      VDA* TGA       rw-   Truevision Targa image
    VICAR* VICAR     rw-   VICAR rasterfile format
      VID* VID       rw+   Visual Image Directory
     VIFF* VIFF      rw+   Khoros Visualization image
     VIPS* VIPS      rw+   VIPS image
      VST* TGA       rw-   Truevision Targa image
     WBMP* WBMP      rw-   Wireless Bitmap (level 0) image
     WEBP* WEBP      rw-   WebP Image Format (libwebp 0.4.0 (0202))
      WMF  WMF       r--   Windows Meta File
      WMV  MPEG      rw+   Windows Media Video
      WPG* WPG       r--   Word Perfect Graphics
      X3F  DNG       r--   Sigma Camera RAW Picture File
      XBM* XBM       rw-   X Windows system bitmap (black and white)
       XC* XC        r--   Constant image uniform color
      XCF* XCF       r--   GIMP image
      XPM* XPM       rw-   X Windows system pixmap (color)
      XPS  XPS       r--   Microsoft XML Paper Specification
       XV* VIFF      rw+   Khoros Visualization image
        Y* RAW       rw+   Raw yellow samples
    YCbCr* YCbCr     rw+   Raw Y, Cb, and Cr samples
   YCbCrA* YCbCr     rw+   Raw Y, Cb, Cr, and alpha samples
      YUV* YUV       rw-   CCIR 601 4:1:1 or 4:2:2

* native blob support
r read support
w write support
+ support for multiple images

However, as you know, e.g. TIFF is a flexible format that can contain complex image data, including e.g. Photoshop Layers. IM uses the public LIBTIFF, Version 4 library to decode TIFFs. It therefore depends on others how well that works.

In my experience, that can cause confusion in the interpretation, and ImageMagick can report TAG errors as a result. It may even abort operation. I want to explore that further, to either have the IM code adjusted or find another way around it, to take away the hassle for users. PNG, and JPEG, and GIF seem to convert withot issues, and probably SVG's as well although that may require additional open-source resources. The IM documentation is constantly being updated.

To prevent such issues, one can try using a flattened version of the TIFF, or export it as a PNG (16-b/ch PNGs are fully supported, and produce relatively compact files). While inconvenient, we are after all talking about a specific targeted operation such as preparation for web-publishing. What's more, scripts can be adjusted to do batch processing of all files in subdirectories (even recursive), so one can prepare large volumes of flattened files in advance in their own subdirectories for unattended processing of multiple output sizes can be created, e.g. thumbnails and web display size and mobile device display size, all in their own subdirectories. All it takes is a bit of coding in an easy batch script language.

It may also be a good thing to investigate/test the use of ImageMagick's own MIFF file format, instead of the current PNGs since it seems to also retain color profile conversions.
 
Quote
When you mention sRGB gamma, are you referring to sRGB TRC or an approximate gamma (e.g. 2.2)?

I have to go by the documented description of that IM option. The sRGB colorspace conversion option (-colorspace) is described as:
Quote
sRGB
if R ≤ .0.0031308 then Rs=R/12.92 else Rs=1.055 R ^ (1.0 / 2.4) ? 0.055
if G ≤ .0.0031308 then Gs=B/12.92 else Gs=1.055 R ^ (1.0 / 2.4) ? 0.055
if B ≤ .0.0031308 then Bs=B/12.92 else Bs=1.055 R ^ (1.0 / 2.4) ? 0.055
So that would be a correct sRGB TRC adjustment with linear slope shadow detail based on linear RGB.

Quote
From what I can follow in the script, you separate the RGB channels and then recombine adding the calculated alpha channel.

Correct. The current alpha channel calculation uses the Gray colorspace (Gray = 0.298839*R+0.586811*G+0.114350*B) conversion. But that is just one of many possible alpha channel creations. One could even add a Sigmoidal S-curve to the alpha channel, should that provide better blending between the other layers (one with 'shadow halos' and the other with 'highlight halos)'. It's flexible enough to optimize any way one desires.

Quote
If the script is modified, will it work for monochrome images (not RGB, only gray channel)? Pardon my ingenuity, but I'm thinking of skipping the channel separation part, computing the alpha channel and then recombine the gray with the alpha.

Exactly what I was looking for, a way of avoiding the channel separation and recombination, and 'simply' adding the Alpha channel. That might require promoting a single channel file to a 4 channel RGBA file, with identical RGB channels. This has yet to be tested.

Thanks for thinking along.

Cheers,
Bart
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NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #6 on: July 18, 2014, 04:16:43 AM »

Courtesy of Fred Weinhaus, first pass (shell script)
# setup arguments

infile="input_small.png"
scale=800
colorspace1="RGB"
filt="LanczosRadius"
filtering="-filter $filt"
gammaval=3
igammaval=`convert xc: -format "%[fx:1/$gammaval]" info:`
inname=`convert $infile -format "%t" info:`

# resize in RGB with and without gamma and blend by (intensity of non-gamma resized image)
# line1: read input, convert to 16bits and desired colorspace (RGB)
# line2: clone the first image and use LanczosRadius to scale by 800 (in RGB)
# line3: clone the first image, apply gamma, use LanczosRadius to scale by 800 and apply inverse gamma
# line4: clone the resized RGB (no gamma applied) image and convert to gray (intensity) and use -auto-level to stretch to full dynamic range
# line5: composite (-compose over) the images from lines 2,3,4 (in RGB) using the last image as a mask to do the blending
# line6: delete the temps
# line7: convert back to sRGB and save output
convert \( $infile -depth 16 -set colorspace sRGB -colorspace $colorspace1 \) \
\( -clone 0 $filtering -distort Resize $scale% -write tmp1.png \) \
\( -clone 0 -gamma $gammaval $filtering -distort Resize $scale% -gamma $igammaval -write tmp2.png \) \
\( -clone 1 -colorspace gray -auto-level \) \
\( -clone 2 -clone 1 -clone 3 -compose over -composite -write tmp3.png \) \
-delete 0-3 \
-set colorspace $colorspace1 -colorspace sRGB ${inname}_${filt}_blended_s${scale}_g${gammaval}_$colorspace1.png
The -write commands are unnecessary. Quoting Fred: "I also save 3 tmp.png files to show the non-gamma applied resize in RGB, the gamma applied resize in RGB and blended resize in RGB just to be sure the blending was working."
P.S. As pointed out by Bart van der Wolf below, there was a typo, which is now fixed in the now correct code above.
« Last Edit: July 19, 2014, 04:20:14 AM by NicolasRobidoux »
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NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #7 on: July 18, 2014, 04:22:10 AM »

Another brainstorm: Fix the pair of gammas so that near the relevant midtones (which are generally quite low in linear RGB) the blended combination of them resembles linear RGB.
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BartvanderWolf

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Re: A free high quality resampling tool for ImageMagick users
« Reply #8 on: July 18, 2014, 04:46:48 AM »

Bart:

I have high hopes that something good can be squeezed of this research direction.

Nicolas: Same hopes that I have. Thanks for sparking that idea, the results already look promising. A down sample of a star target or a zoneplate/rings target look very nice, which is important for photographers who produce web publishing content. Low aliasing and ringing/haloing is paramount for such operations.

Linear Light mixing of colors with downsampling avoids color issues that can arise with gamma pre-compensated downsampling. So that is also something to keep in mind.

Quote
1) The reason for using "a pair of opposite gamma spaces" is to restore some black/white symmetry. I think that, however, the alpha should then be computed from either some sort of average of the two results or from linear light, in which case there would be three resampled images used to recombine: through RGB gamma 3 (for example), through RGB gamma 1/3, and linear RGB (gamma 1). These thoughts are a bit rough. Maybe one should work in a pair of direct/reverse video after normalizing the input image, then de-normalizing? Assuming the normalization anchors things at the black and white points (roughly), this would definitely restore black/white symmetry. (Quickly brainstorming, here.)

Chaos rules the universe, so a bit of brain storm cannot hurt ;)

Quote
Poking really quickly into the first idea above, however, one would need a very accurate floating point toolchain to pull off an initial -gamma .3333333333333. Not sure, so this is more a "watch for this" than a true "jump up and down warning". But I'm a bit concerned that Q16 is not enough.

Definitely something to keep in mind, especially since only 16-b/ch pre-compiled binaries are available for download and installation by mere mortals. Having said that, we can 'blend away' the erroneous ranges by a good choice of thresholds or shape of the Alpha channel response curve(s). Lot's of testing ahead.

Quote
Mixing linear with gamma 3 sounds like a reasonable practical approach. But it definitely does not respect black/white symmetry.

Agreed.

Quote
2) EWA LanczosRadius is not interpolatory. Unless you explicitly used a hard wired "null" operation, there will be a difference when do "no-op" resampling. In practice, this difference could be argued to be a good thing (with EWA LanczosSharpest 4, for example). The real question is whether the final answer differs significantly from an infinite precision version of the result. I'm just making sure here, that you know (the reasonable assumption, namely that you do know, can be dangerous) that there are two sources for deviation from the original when resizing to the exact same dimensions: round off error and the fact that the filter actually does some filtering in all cases.

Yes, that's why it was only a quick test on a single image to see if there are huge issues with the current implementation. Things are not totally screwed up, they on the contrary produce only small changes in suspect regions with the current implementation.

Quote
3) Fred Weinhaus has started optimizing the code further and poking around. We'll post things in the IM forums as soon as I've had time to check things over and organize results (and my thoughts) into something halfway intelligible.

Great, Fred knows IM scripting like few others do, although he uses a specific scripting dialect. Anyway, the image results should be identical if the scripting can be ported to other dialects.

Cheers,
Bart
« Last Edit: July 28, 2014, 02:42:56 PM by BartvanderWolf »
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NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #9 on: July 18, 2014, 04:51:51 AM »

For now, http://www.imagemagick.org/discourse-server/viewtopic.php?f=22&t=25935 will be the centralized point for discussion of this new approach within the ImageMagick Forums.
I'll jump back and forth between there and here.
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Re: A free high quality resampling tool for ImageMagick users
« Reply #10 on: July 18, 2014, 05:19:02 AM »

With very preliminary results courtesy of Fred Weinhaus (on the "wizard" image, which is a drawing, not a photograph), enlarging through gamma 3 looks better than enlarging through L*a*b*; I suspect that this has to do with the "non-smooth" correspondence between linear light and L*a*b*).
« Last Edit: July 18, 2014, 05:08:03 PM by NicolasRobidoux »
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Re: A free high quality resampling tool for ImageMagick users
« Reply #11 on: July 18, 2014, 05:56:42 AM »

...
Correct. The current alpha channel calculation uses the Gray colorspace (Gray = 0.298839*R+0.586811*G+0.114350*B) conversion. But that is just one of many possible alpha channel creations. One could even add a Sigmoidal S-curve to the alpha channel, should that provide better blending between the other layers (one with 'shadow halos' and the other with 'highlight halos)'. It's flexible enough to optimize any way one desires.
...
IMHO it's really important that the alpha channel be "stable" (this roughly means "boring", with as few kinks and slope discontinuities and whatever) w.r.t. image data so that the blending not add aliasing. A straight line in linear light between the min and max image values (possibly filtered to reject outliers, although noise reduction should have been performed before resampling) fits the bill. It may be possible to do better, but this is a case of "proceed with caution".
« Last Edit: July 18, 2014, 06:39:50 AM by NicolasRobidoux »
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Re: A free high quality resampling tool for ImageMagick users
« Reply #12 on: July 18, 2014, 12:57:23 PM »

Again courtesy of Fred Weinhaus, the Windows version of the basic command:
convert ( image.png -depth 16 -set colorspace sRGB -colorspace RGB ) ^
( -clone 0 -filter LanczosRadius -distort Resize 800% ) ^
( -clone 0 -gamma 3 -filter LanczosRadius -distort Resize 800% -gamma 0.33333333 ) ^
( -clone 1 -colorspace gray -auto-level ) ^
( -clone 2 -clone 1 -clone 3 -compose over -composite ) ^
-delete 0-3 ^
-set colorspace RGB -colorspace sRGB ^
result.png
P.S. As pointed out below by Bart van der Wolf, there was a typo in the code above, which is now fixed.
« Last Edit: July 19, 2014, 04:21:56 AM by NicolasRobidoux »
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Re: A free high quality resampling tool for ImageMagick users
« Reply #13 on: July 18, 2014, 03:42:08 PM »

Again courtesy of Fred Weinhaus, the Windows version of the basic command:
convert ( image.png -depth 16 -set colorspace sRGB -colorspace RGB ) ^
( -clone 0 -filter LanczosRadius -distort Resize 800% ) ^
( -clone 0 -gamma 3 -filter LanczosRadius -distort Resize 800% -gamma 0.33333333 ) ^
( -clone 1 -colorspace gray -auto-level ) ^
( -clone 1 -clone 2 -clone 3 -compose over -composite ) ^
-delete 0-3 ^
-set colorspace RGB -colorspace sRGB ^
result.png

Hi Nicolas,

Sweet, nice and compact, and totally in memory, no temporary files on disk. My compliments to Fred.

Just one remark, an error IMHO:
( -clone 1 -clone 2 -clone 3 -compose over -composite ) ^
should be
( -clone 2 -clone 1 -clone 3 -compose over -composite ) ^

Clone 1 and Clone 2 should be reversed to remove the shadow-edge undershoot.

Also, '%' signs must be preceded by another percentage sign like this %%, if used in a batch script file instead of on the commandline. So 800% becomes 800%% as Resize percentage in a batch file.

With a small alteration, a batch file also becomes more flexible in drag-and-drop behavior (it accepts any filename dragged on the batch file, and drops a resampled PNG version in the same directory as the original):
convert ( "%~dpnx1" -depth 16 -set colorspace sRGB -colorspace RGB ) ^
( -clone 0 -filter LanczosRadius -distort Resize 800%% ) ^
( -clone 0 -gamma 3 -filter LanczosRadius -distort Resize 800%% -gamma 0.33333333 ) ^
( -clone 1 -colorspace gray -auto-level ) ^
( -clone 2 -clone 1 -clone 3 -compose over -composite ) ^
 -delete 0-3 ^
 -set colorspace RGB -colorspace sRGB ^
 "%~dpn1_800%%.png"

I'd also have to take another look at the '-auto-level' option to see if it is the best choice for non-synthetic images, rather than a '-contrast-stretch 0x0' or similar.

I'm also still looking for a better solution to retain the original file's color profile (important in a color managed environment), e.g. Adobe RGB or ProPhoto RGB, and assign it again to the output file. If Fred has any suggestions, I'd be a happy camper.

Cheers,
Bart
« Last Edit: July 18, 2014, 06:11:28 PM by BartvanderWolf »
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NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #14 on: July 18, 2014, 05:05:49 PM »

...
Clone 1 and Clone 2 should be reversed to remove the shadow-edge undershoot.
...
Indeed.
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Re: A free high quality resampling tool for ImageMagick users
« Reply #15 on: July 18, 2014, 07:13:52 PM »

As a result of the code optimization suggested by Fred Weinhaus, and a few tweaks of my own doing, I've updated the Windows batch script file that I posted earlier. I've attached it as a Zip archive.

An important change to the size input was made!

Now one does need to enter a percentage sign (%) after the value if one wants to do a resizing based on percentage. So a 50% entry will reduce the size of the image to half of the original size. The reason I changed it was to add more flexibility in getting an exact output size, instead of a percentage with lots of decimals to avoid 1 pixel round-off errors.

Now, one can alternatively enter the required size of the output in pixels, e.g. 800x800. This will fit the largest original image dimension to 800 pixels in output, regardless whether it's in a landscape or portrait orientation, and scale the shorter dimension proportionally.

I've also made some preparations that will save the resampled image in the highest possible JPEG or PNG quality, by preventing chroma sub-sampling, and setting the compression quality to the maximum, i.e. 100. This is in preparation to a fix (if possible) that will maintain the original file's embedded ICC color profile and embed it in the resampled version as well. For the time being, the output will be a PNG filetype without embedded profile.

Enjoy.

Cheers,
Bart
« Last Edit: July 19, 2014, 05:56:56 AM by BartvanderWolf »
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Re: A free high quality resampling tool for ImageMagick users
« Reply #16 on: July 19, 2014, 07:31:07 AM »

I've also made some preparations that will save the resampled image in the highest possible JPEG or PNG quality, by preventing chroma sub-sampling, and setting the compression quality to the maximum, i.e. 100. This is in preparation to a fix (if possible) that will maintain the original file's embedded ICC color profile and embed it in the resampled version as well. For the time being, the output will be a PNG filetype without embedded profile.

*** UPDATE ***

I've found a solution for the embedding of the color profile, it seems ...

-convert "input.tif" -write "IMsource.icc" apparently saves a copy of the embedded color profile, and
-profile ÏMsource.icc" will assign that profile to the file that will be written. I'm still searching for a way to do it without having to write a temporary file ...

The script (or rather ImageMagick) will produce warning messages if no profiles are embedded, but the conversion is still performed as intended, just without profile.

Attached is an updated version (version 1.0.3). The tool now copies the original profile that is embedded in the original image, and embeds that in the resized version. It also retains the original file type, so a JPEG remains a JPEG, and a TIF remains a TIF, it just has a different size, as indicated by the new file name, but now with the same profile as the original source.

Cheers,
Bart
« Last Edit: July 19, 2014, 08:44:14 AM by BartvanderWolf »
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NicolasRobidoux

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Re: A free high quality resampling tool for ImageMagick users
« Reply #17 on: July 19, 2014, 08:36:27 AM »

Fred Weinhaus http://www.fmwconcepts.com/fmw/fmw.html has asked me what's wrong with sigmoidization, which does suppress both dark and light halos and has been found to work fairly well. Wrong to the extent that I declare it "dead" as a general purpose resampling method.
(Sigmoidization is an ancestor of the methods discussed here and is described here: http://www.imagemagick.org/Usage/resize/#resize_sigmoidal.)
Basically, what is wrong with it is that it leads to "color separation", and this failing means that if sigmoidization is strong, one has to visually inspect the images to make sure taming the halos did not lead to other artifacts.
This is (apparently) not the case with the new methods discussed here: Resampling through a pair of gamma spaces, and blending with an alpha which is the same over all three channels (a normalized luminance or luma), means that the resulting method satisfies http://www.luminous-landscape.com/forum/index.php?topic=77949.msg745796#msg745796. I believe that, provided one works in a colorspace in which channels are monotone w.r.t. a reasonable luminance (RGB or XYZ, for example), this is a very very important property.
-----
There is something else that I believe >>may<< be wrong with sigmoidization: As much as possible, the intermediate color space(s) should >>locally<< be linear, and if nonlinear it should be nonlinear in a very smooth way. The sigmoidal color spaces have "kinks". I believe that in some situations this adds aliasing and color artifacts. "Gamma spaces" are very smooth, and consequently they "locally" (in the sense of near a given color value) are "almost linear". The same could be said of sigmoidal color spaces, but for the same amount of (one of the types of) halo suppression, the extent over which the tangent to a sigmoidal curve approximates it is smaller, overall, than a similarly strong gamma space.
-----
(My apologies if this is not particularly well explained. I edited this poor explanation a bit after Fred Weinhaus made it clear that some of it was more confusing than informative.)
« Last Edit: July 19, 2014, 02:08:42 PM by NicolasRobidoux »
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Re: A free high quality resampling tool for ImageMagick users
« Reply #18 on: July 19, 2014, 11:32:24 AM »

Suggested variant:
Instead of blending the gamma resample with linear light, negate the linear light version of the input image, use the same gamma, resample the same way, undo the gamma, negate again (to be back in linear light).
Then, use the luminance of the average of the two results instead of the linear light resample's luminance (which is not computed anymore), normalized of course, as alpha (blending parameter).
Why? The core of the method is then black/white symmetric.
-----
I am far from certain that this, or something like it, would give better results than the "blend linear light with gamma" version. The HVS (like "the real world) is >>not<< black/white symmetric.
The other difficulty is that for this to work well we most likely need to set the "white" used by the negation operators to fit the input and resampled images well.
« Last Edit: July 19, 2014, 11:41:07 AM by NicolasRobidoux »
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Jack Hogan

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Re: A free high quality resampling tool for ImageMagick users
« Reply #19 on: July 19, 2014, 02:01:03 PM »

Great work guys, this is very useful.  A couple of questions:

1) Is the script optimized for enlarging as opposed to shrinking?  I am asking because I see gammas greater than one flying around :-)
2) If one works in AdobeRGB, is it enough to simply change all references of 'sRGB' to 'AdobeRGB' in the script?
3) I sometimes generate a B&W image directly from a Bayer raw file. The resulting full resolution 'grayscale' TIF is still linear (gamma=1).   Assuming one wanted to downsize only, could all the colorspace changing, gamma and cloning be replaced by a single line such as
convert "%~dpnx1" -filter LanczosRadius -distort Resize %Msize% ?
4) If one wanted a TIFF as an output would they simply leave out the -quality line?

Thank you very much!
Jack
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