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Removing red block pixels

Nov 29, 2011, 03:52
Post: #1
Removing red block pixels
I've tried adjusting levels, curves, color balance and the heal plug-in, but I can't remove the red blocky pixels in a series of pictures that look like the one below:

[Image: testpic.jpg]

Any idea on how I could fix up these pictures? Do I need to use a red-eye or noise removal method? I'm using GIMP 2.6.11 if that helps.
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Nov 29, 2011, 10:13
Post: #2
RE: Removing red block pixels
Yes, a denoiser is needed, but you since have a picture that is both noisy and underexposed the result won't be pretty. When you look at the pixel values (Histogram dialog) the reds are all confined in the 0-50 range, and it is even worse on the red and blue channels. So when you will lighten the picture, you'll try to get values in the 0-150 range instead. But since you only have 50 different start values, you get only 50 different final values, even if they are spread on a wider range. This translates into quite visible color transitions.

If you work on a large picture and reduce it at the end, some these problems get attenuated by the downscaling.

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Dec 4, 2011, 17:47
Post: #3
RE: Removing red block pixels
Okay, I played with the picture and managed to get the left side of the picture looking decent, but the right side still has problems.

[Image: Untitled.jpg]

I tried using color correction on Irfanview, and with GIMP, I tried heal, levels and curves. Any ideas on how to clear up the purple pixels? I managed to get the red pixels to blend in a bit, but the purple is just being stubborn.
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Dec 4, 2011, 18:05 (This post was last modified: Dec 4, 2011 18:06 by ofnuts.)
Post: #4
RE: Removing red block pixels
(Dec 4, 2011 17:47)leoak Wrote:  Okay, I played with the picture and managed to get the left side of the picture looking decent, but the right side still has problems.
I tried using color correction on Irfanview, and with GIMP, I tried heal, levels and curves. Any ideas on how to clear up the purple pixels? I managed to get the red pixels to blend in a bit, but the purple is just being stubborn.
Possible fix:
- Colors/Component/Decompose->RGB
- This creates a new image with three layers (red, green, blue).
- Notice that most of the variation is on the red layer
- Blur the red layer (gaussian, 20 pixels)
- Colors/Component/Recompose

[attachment=765]

This works because the green (which is where your eye gets most of the details) isn't blurred. You can also try a similar technique using other decomposition modes.

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