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Author Topic: How to avoid aliased edges  (Read 2388 times)

Guigui

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How to avoid aliased edges
« on: June 28, 2009, 06:52:04 pm »

Hello,

Once again, I have a very basic question. Did a forum search, couldn't find a clear answer, so I'm opening a new topic. If I missed another one on the same subject, feel free to close this thread !

Here's my problem :

I want to make a 30cm x 20cm print of a 3456 x 2304 px file.
If I do nothing in photoshop, the resolution will be 292,608 PPI, and the print will come out with visible aliased edges (although the image shows no aliasing on screen).

If I resample the image to make it EXACTLY 360 PPI, and sharpen it accordingly, the image will print fine without any visible aliasing.

However, in the "From Camera to Print" tutorial, Jeff clearly says that one should not up-res an image besides a x2 or x4 enlargement. In my example, I'm increasing the size of the file by approximately 23%. But it's the only way I've found to have a satisfactory result on the paper.

So, two questions :

- Have I misunderstood something in what Jeff & Michael said about resolutions ?
- Does the PPI resolution have to be an absolutely round number, e.g. 180, 240, 300, 360...or is it ok to print a file which has an in between resolution, such as 343.342 PPI ?


Thanks !


Details :
Printer : Epson Stylus Pro 7900
Printing from : Photoshop CS 3 (Windows Xp)
« Last Edit: June 29, 2009, 09:53:02 am by Guigui »
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Ernst Dinkla

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How to avoid aliased edges
« Reply #1 on: June 29, 2009, 06:18:56 am »

Quote from: Guigui
Hello,

Once again, I have a very basic question. Did a forum search, couldn't find a clear answer, so I'm opening a new topic. If I missed another one on the same subject, feel free to close this thread !

Here's my problem :

I want to make a 30cm x 20cm print of a 3456 x 2304 px file.
If I do nothing in photoshop, the resolution will be 292,608 PPI, and the print will come out with visible aliased edges (although the image shows no aliasing on screen).

If I resample the image to make it EXACTLY 360 PPI, and sharpen it accordingly, the image will print fine without any visible aliasing.

However, in the "From Camera to Print" tutorial, Jeff clearly says that one should not up-res an image bedises a x2 or x4 enlargement. In my example, I'm increasing the size of the file by approximately 23%. But it's the only way I've found to have a satisfactory result on the paper.

So, two questions :

- Have I misunderstood something in what Jeff & Michael said about resolutions ?
- Does the PPI resolution have to be an absolutely round number, e.g. 180, 240, 300, 360...or is it ok to print a file which has an in between resolution, such as 343.342 PPI ?


Thanks !


Details :
Printer : Epson Stylus Pro 7900
Printing from : Photoshop CS 3 (Windows Xp)

Guigui,

I personally would trust my eyes first, make the best print and after that I might analyse why the learned rules didn't work this time. It's the way to get knowledge of the technology and leads to the creation of rules that work for you.

For what reason this image didn't  resample well in the Epson driver isn't clear to me, this isn't a big blow up.

I doubt the 7900 has an image quality driver setting that will ask for 180 PPI input but it would be a nice explanation for what you see. If you had used that low image quality by mistake (Qimage always gives good feedback to the user on the driver requested PPI input). Downsampling in printer drivers is still worse than in some applications and anti-aliasing then isn't done properly if done at all (would make the driver terribly slow too).  However the 360 PPI anti-aliased image you made in PS would survive that downsampling to a 180 PPI image quality level much better, (even) if the downsampling routine in the driver isn't better than nearest neighbour. I used a similar method with the Epson 9000 in the past, feeding it vector and font designs rasterised to anti-aliased 720 PPI while the 9000 printer driver only knew a 360 PPI highest image quality input. It was better than feeding it 360 PPI files, not to mention an odd PPI number file.

Pixar format bitmap. Not PCX I guess. First what comes to mind is that the kind of anti-aliasing available in the Epson driver is not coping with that image in the first place. If you save it as a flattened Tiff first in PS at the original resolution and then print it again with that resolution, will that improve the image? Is there a chance you can export the image as a Tiff from the original Pixar software at 360 PPI + anti-aliasing applied right away at export?

If you want to resample before printing (and there are enough reasons not to do it, for example the existence of Qimage) then forget about rounded off numbers, think 360 or 720 PPI for Epsons and 300 or 600 PPI for Canon and HP printers. In some special cases like vectors and font designs for small prints, make 1200 PPI images and use a 1200 PPI image quality path in the C and HP printer driver. Most of the time extrapolations of photographic images with the ratio you mentioned should be good with a modern printer driver so you can feed the odd PPI number. Resampling to 180, 240, PPI etc  means that the total extrapolation is divided over two stages as the printer driver in the end will need 360 or 720 PPI input. I do not see any advantage in that and it obscures  possible flaws in the workflow(what went wrong where).


met vriendelijke groeten, Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/
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Wayne Fox

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How to avoid aliased edges
« Reply #2 on: June 29, 2009, 03:25:11 pm »

Quote from: Guigui
- Does the PPI resolution have to be an absolutely round number, e.g. 180, 240, 300, 360...or is it ok to print a file which has an in between resolution, such as 343.342 PPI ?


Details :
Printer : Epson Stylus Pro 7900
Printing from : Photoshop CS 3 (Windows Xp)

No ... you do not need to force the resolution to a round number.  Why you see aliasing I'm not sure but is not normal.  I'm not familiar with windows and the printer driver for windows, but would suspect something in your setup, or perhaps some step in your workflow.

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Guigui

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How to avoid aliased edges
« Reply #3 on: June 29, 2009, 07:41:28 pm »

Quote from: Wayne Fox
No ... you do not need to force the resolution to a round number.  Why you see aliasing I'm not sure but is not normal.  I'm not familiar with windows and the printer driver for windows, but would suspect something in your setup, or perhaps some step in your workflow.
Driver setup :
Paper type : Ilford Gold Fiber Silk
Quality : Max Quality (Level 5 -  2880x1440 dpi)
Edge smoothing : off.
Finest detail : off.

Do you think I should turn Edge smoothing on ?


@Ernst Dinkla :

Thank you for your extensive reply. I must admit I still don't have enough experience to understand everything you said. I've downloaded Qimage earlier today though, and will try and print from this software and see if I get better results.
« Last Edit: June 29, 2009, 08:08:14 pm by Guigui »
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reburns

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How to avoid aliased edges
« Reply #4 on: June 29, 2009, 11:31:44 pm »

Guigui,

You could start Qimage at max resolution, hybrid interpolation and default 5 sharpening.  You are 1/3-way around the world, but I do have that Ilford paper to try printing your file out here.  If you want to upload it to my web FTP, drop me a message.  Ralph
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Guigui

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How to avoid aliased edges
« Reply #5 on: June 30, 2009, 10:53:54 am »

Quote from: reburns
Guigui,

You could start Qimage at max resolution, hybrid interpolation and default 5 sharpening.  You are 1/3-way around the world, but I do have that Ilford paper to try printing your file out here.  If you want to upload it to my web FTP, drop me a message.  Ralph

I did exaclty that, and the print came out without any aliasing. Thanks !

Looks like Qimage really is a great help.

However, I still can't explain why printing from Photoshop CS 3 causes so much aliasing problems. If anyone has any clue as to why this happened to me, I'm all ears.

Thanks everyone for their great help so far.

PS : Colors and blacks on Ilford Gold Fiber Silk truly are amazing. Almost makes me forget I just wasted so much ink on resolving clog issues on my 7900...
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