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manipulating colors

Mar 7, 2014, 20:38
Post: #1
manipulating colors

I've noticed that the past couple images I've been learning Gimp on have turned out okay, except the colors are pixelated in certain areas of the image. For example, A dancer wearing purple tights- the top of the tights are nicely purple but lower on the legs are pixels of blue and red.

I've tried stretch-contrast, manipulating layers ; hues; gaussian blur techniques; rotating colors; simple cloning really changes the color too much- is there a best way to learn color manipulation so I have better outcomes with the images I work on?

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Mar 9, 2014, 18:32
Post: #2
RE: manipulating colors
(Mar 7, 2014 21:40)ofnuts Wrote:  This is caused mostly because you are processing an 8-bit image, which can have only 256 values in each of the R, G and B channels. All the color processing consists in mapping these 256 values into another set of values. But a fundamental constraint is that the mapping should keep the values in the same order, i.e., the darkest of two pixels before the mapping should still be the darkest of the two after. And this constraint has a dire consequence: you cannot map your 256 values into 256 other values; you will always map your 256 values into less than 256 values (in some bad case, into as few as 120 values). This means that subtly different colors in the original image can map into the same color (this creates flat areas without texture) or into visually more different colors (hard transition, aka "banding"). In the histogram this translates into a "haircomb".

To avoid this:
  • if your camera supports it, use the RAW format (this produces images with 10 to14 bit per channel depending on the camera) and do the color/level processing in a specialized tool such as a the RAW file processing software that came with your camera. Then export to a high quality JPG that your process with Gimp (localized editing...)
  • otherwise
    • do the color/levels processing in as few steps as possible; ideally, in one single pass, using the Curves tool (the only one to give enough control to achieve this in all cases)
    • do the color/levels processing on the full-size picture, before scaling it down. The scaling down operation will even the colors in most cases (you trade spatial resolution for color resolution)
    • you can also mitigate the "banding" by using the spread noise filter.


I get this! I was just trying to "fix" another image that has been colorized by someone and again, I get the pixelated image...I see why now that you've explained it.

Thank you for explaining this- for a non-photographer and newbie Gimp user your help is invaluable. I use tutorials online but they often appeal to more advanced users or are too basic.

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