Topic: Banding when applying Gaussian Blur (Read 22150 times)

Etienne Cassar · « **on:** May 20, 2009, 03:16:06 pm »

I am trying to blur to outer third of a picture that I have by applying 100 pixel Gaussian blur, and then applying a circular gradient mask to this layer to mask the central part . However I am getting banding in an particular area of the picture where I want to blur. This particular area forms part of the background and happens to have no detail at all and is completely bare (it is part of a wall behind the photographed subject). I am editing my picture in 16 bit mode, but when I change to 8 bit the banding disappears. Any ideas how to avoid getting this banding effect? I would have loved to post the picture here for you to see what I mean, but to save it as JPEG, I have to change it to 8 bit mode, and the banding will disappear. as TIFF it will be too large to post here.

Schewe · « **Reply #1 on:** May 20, 2009, 06:45:52 pm »

Quote from: ecassar

I am editing my picture in 16 bit mode, but when I change to 8 bit the banding disappears.

What you are hitting is the fact that while the image is in 16 bit, it's being displayed on an 8 bit/channel video system and essentially the banding is a symptom of the fact that an 8 bit display can't show 16 bits worth of data...

Try zooming into 100-200% and see if you are still seeing the banding...

sergio · « **Reply #2 on:** May 20, 2009, 07:25:30 pm »

Try adding noise to the mask. That usually fixes it.

Schewe · « **Reply #3 on:** May 20, 2009, 09:45:40 pm »

Quote from: sergio

Try adding noise to the mask. That usually fixes it.

No, you SERIOUSLY don't want to be adding noise if the banding you are seeing is a display only phenomenon...that would be a very bad idea in that you would be screwing up your image quality for no real and valid reason...

Etienne Cassar · « **Reply #4 on:** May 21, 2009, 05:13:21 am »

Thanks for you replies.
When zooming in to 100% I can still see the banding. Only way that I managed to remove it is to change over to 8 bit mode and it won't reappear when I change it to 16 bit again. Adding noise to the mask won't help because the banding appears as soon as I apply the Gaussian blur to the image before applying the mask. I tried to appy some monochromatic gaussian blur to the image before applying the blur, but this did not help either. I am not sure if I am correct in saying that the banding appears to be a little bit posterized too. I am using a NEC Spectraview Reference 2690 monitor to view my pictures, but I guess that the digital output from the video card will still be 8 bit in the Windows environment.
Maybe I should try to print the picture as is (with the banding) and see if the banding appears on the print as well, but I have to wait until my new printer arrives. I have ordered a Canon iPF5100 4wks ago and it still hasn't arrived. This is going to be my first printer and I can't wait to start printing my pictures myself.

jbrembat · « **Reply #5 on:** May 21, 2009, 07:41:12 am »

Quote from: ecassar

Thanks for you replies.
When zooming in to 100% I can still see the banding. Only way that I managed to remove it is to change over to 8 bit mode and it won't reappear when I change it to 16 bit again. Adding noise to the mask won't help because the banding appears as soon as I apply the Gaussian blur to the image before applying the mask. I tried to appy some monochromatic gaussian blur to the image before applying the blur, but this did not help either. I am not sure if I am correct in saying that the banding appears to be a little bit posterized too. I am using a NEC Spectraview Reference 2690 monitor to view my pictures, but I guess that the digital output from the video card will still be 8 bit in the Windows environment.
Maybe I should try to print the picture as is (with the banding) and see if the banding appears on the print as well, but I have to wait until my new printer arrives. I have ordered a Canon iPF5100 4wks ago and it still hasn't arrived. This is going to be my first printer and I can't wait to start printing my pictures myself.

It's surprising that a gaussian blur (a smoothing operation) works exactly on the opposite way!!!!

Are you sure what you see is banding?

Jacopo

madmanchan · « **Reply #6 on:** May 21, 2009, 09:38:07 am »

I am guessing the reason you don't see the banding when you do your 16->8->16 conversion is because going from 16 to 8 applies dither which masks the banding. My understanding is that normally the 16-bit display process via the OS causes an 8-bit readout without dither; so display of the original 16-bit data may show banding.

Jonathan Wienke · « **Reply #7 on:** May 21, 2009, 11:54:20 am »

Quote from: ecassar

I am trying to blur to outer third of a picture that I have by applying 100 pixel Gaussian blur, and then applying a circular gradient mask to this layer to mask the central part . However I am getting banding in an particular area of the picture where I want to blur. This particular area forms part of the background and happens to have no detail at all and is completely bare (it is part of a wall behind the photographed subject). I am editing my picture in 16 bit mode, but when I change to 8 bit the banding disappears. Any ideas how to avoid getting this banding effect?

Ignore it; it is a screen artifact only, and does not actually exist in the image, You're seeing a limitation in your monitor's ability to smoothly display color gradients. If you view the image on a different display, it may go away entirely, or appear in a different area of the gradient. Ignore the advice to add noise to solve the problem, it's a stupid solution to a non-problem. Converting to 8-bit is adding noise to the image (dithering), which masks the problem the same way as adding noise. Neither solution is desirable or necessary.

Print the image; it's very unlikely you'll see the same banding in the print. If you don't, then the best solution to the problem is to do absolutely nothing.

JeffKohn · « **Reply #8 on:** May 21, 2009, 12:38:35 pm »

One other thing I've noticed is that sometimes when you have an image with multiple layers, it will show banding in Photoshop but as soon as you flatten the image the banding goes away.

sergio · « **Reply #9 on:** May 21, 2009, 09:58:57 pm »

Quote from: Schewe

No, you SERIOUSLY don't want to be adding noise if the banding you are seeing is a display only phenomenon...that would be a very bad idea in that you would be screwing up your image quality for no real and valid reason...

Of course not. If it's only in the monitor why worry about the print.

Etienne Cassar · « **Reply #10 on:** May 22, 2009, 03:16:16 am »

Thanks to all for your input. I guess I wait for my printer to arrive (hope that it doesn't take long) and see whether this will be evident on a print as well. I did try viewing it on my laptop and the banding effect was still visible. Have not tried flattening the image to see if the banding disappears. Will try it out. What is a bit strange is the fact that it is the first time that I have tried doing the same thing with other photographs, and didn't get the same result.

Jonathan Wienke · « **Reply #11 on:** May 22, 2009, 08:56:04 am »

Quote from: ecassar

What is a bit strange is the fact that it is the first time that I have tried doing the same thing with other photographs, and didn't get the same result.

Not really. The tonal ranges where banding will show up depend on the characteristics of the monitor and the monitor profile. Images that show banding on one monitor may not show any banding on another, and vice versa.

Doug Peterson · « **Reply #12 on:** May 22, 2009, 10:05:56 am »

A high-end photo-oriented monitor like an Eizo CG produced far less banding on smooth gradients than your typical general-purpose monitor. It's one of the easiest ways to show the differences; we use it in demos all the time.

I know this topic on a personal level since I work on smooth long gradients for hours and hours when touching up my Bodyscape series. I can only work on these on an Eizo because it IS possible for real banding to crop up even in 16 bit because of the enormous contrast I am adding, so I need to quickly know what banding is real and what banding is just on the monitor which is impossible on, for instance the Apple Cinema Display next to the Eizo on our custom workstation.

Doug Peterson (e-mail Me)
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Etienne Cassar · « **Reply #13 on:** May 23, 2009, 11:20:24 am »

I managed to find a way to show the banding effect that I am talking about. I made a screen grab in photoshop using the PrintScrn button, then pasted it in new window in Photoshop. I assigned the monitor profile to the new image to get the colours correct (after pasting they were completely off) and then converted to sRGB. I then saved is a Jpeg to be able to upload it here. You can see exactly what I am talking about. Incidentally the screen grab is of part of the image at 50% size.

hsmeets · « **Reply #14 on:** May 23, 2009, 01:46:54 pm »

Quote from: ecassar

I managed to find a way to show the banding effect that I am talking about. I made a screen grab in photoshop using the PrintScrn button, then pasted it in new window in Photoshop. I assigned the monitor profile to the new image to get the colours correct (after pasting they were completely off) and then converted to sRGB. I then saved is a Jpeg to be able to upload it here. You can see exactly what I am talking about. Incidentally the screen grab is of part of the image at 50% size.

Etienne,

When I open a jpg file created with a consumer digicam and apply gaussian blur I get directly banding. (invariant if this is in 8 or 16b mode)

When I take a raw file from my camera: banding is less, hardly noticable. When instead of the gaussian blur I use Lens Blur the banding is away.

Lens blur on the jpeg has also less banding then gaussian but is still visible to the critical eye.

Seems to be that Gaussian blur has so it's limitations in PS

Done this on latest 24" iMac with PS CS4

Jonathan Wienke · « **Reply #15 on:** May 23, 2009, 06:32:27 pm »

ITS A MONITOR ARTIFACT, NOT A FILE OR BLUR PROBLEM!

Get a good quality monitor (Eizo, etc) and you will NOT see this. This banding will also not show up in a print.

Etienne Cassar · « **Reply #16 on:** May 24, 2009, 05:37:10 am »

I thought that I had a good quality monitor! I know that Eizo may be better monitors, but I have the impression that many Pros use NEC Spectraview units, like the one I have. Even Andrew Rodney has wrote positive reviews about this monitor. I am not saying that it is not an aritifact, but I wasn't expecting it to get on this monitor. I won't get an Eizo, because they are much more expensive and for me Photography is just a hobby. Spending what I spent for the NEC was already a hell lot of money for me.

hsmeets · « **Reply #17 on:** May 24, 2009, 07:05:11 am »

Quote from: Jonathan Wienke

ITS A MONITOR ARTIFACT, NOT A FILE OR BLUR PROBLEM!

Get a good quality monitor (Eizo, etc) and you will NOT see this. This banding will also not show up in a print.

Send the file to the printer --> banding is there but less obvious as on the screen :-( luckily I don't do much of gaussian blur (only to masks) :-)

(Nikon 12b Raw file --> 16b PS --> 16b print on iPF5100 (although it's internally 12b processing))

FYI: Most displays are currently hookup to a computer via single link DVI. On single link DVI connections the bit depth is limited to 24b for RGB, thus 8 per channel. Even editing in 16b will limit the tones visible on your monitor as soon as the data is send to the monitor becuase of the DVI. Even if a monitor/lcd panel would be able to display 16b per channel: the bottle neck is still the DVI connection.

Duallink dvi allows for more resolution at 24b or single-link resolution at 48b color depth. The latter application I've never seen before.

Etienne Cassar · « **Reply #18 on:** May 24, 2009, 11:05:45 am »

Thanks for explaining that to me Huib. My printer still has not arrived and I can't wait to start printing my pictures. I don't usually apply Gaussian blur to my prints, but I happened to have a picture of my niece which turned out really nice with the peripheral blur. That is when I noticed to banding effect. I was wondering whether I was doing something wrong myself and whether there is a way to minimise it. I didn't know that it was something to do with hardware limitations.

hsmeets · « **Reply #19 on:** May 24, 2009, 12:00:19 pm »

Quote from: ecassar

Thanks for explaining that to me Huib. My printer still has not arrived and I can't wait to start printing my pictures. I don't usually apply Gaussian blur to my prints, but I happened to have a picture of my niece which turned out really nice with the peripheral blur. That is when I noticed to banding effect. I was wondering whether I was doing something wrong myself and whether there is a way to minimise it. I didn't know that it was something to do with hardware limitations.

Hopefully you printer arrives soon,

Don't worry to much about the banding in the print: only with my 'nose poked into the print' it was visible.

Author Topic: Banding when applying Gaussian Blur (Read 22150 times)