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iosvideometalvideo-editingimage-editing
Max Adamyan
Max Adamyan

Reputation: 177

Right sequence of applying Color Adjustments to Image

I am working on Video Editing application for iOS powered with Metal. I'm implementing Filters and Adjustments for Videos and Images. For now I have all appropriate shaders to change Video's or Image's brightness, exposure, contrast, saturation, hue and vibrance. The part I do not understand is the right order to apply adjustments, because the final result depends on the order (and the results are very different).

For example should I apply

Brightness -> Exposure -> Contrast -> Saturation

or

Exposure -> Contrast -> Brightness -> Saturation

or any other order?

I cannot find any resource online that can clarify the right sequence of applying adjustments. If there is any sequence that is considered right, what it is, and why?

Thanks.

Upvotes: 1

Views: 1040

Answers (1)

meaning-matters
meaning-matters

Reputation: 22986

When making image adjustment it's important to avoid R, G, and/or B values of pixels to clip at 0 (left side) or 255 (right side). In an RGB histogram this can be seen as a high spike at 0 and/or 255.

Let's see what four adjustments you mentioned do with the histogram roughly. I describe only a positive adjustment (the negative is simply the reverse).

  • Exposure: Stretches it out to the right. Unless the histogram has an empty right side, clipping will be guaranteed.
  • Contrast: Stretches it out to the left and right, but much faster to the left. Unless the histogram has an empty left and/or right side(s), clipping will be guaranteed.
  • Brightness: Left side is pushed down and right side is pulled up. If you go too far the right side will clip.
  • Saturation: G will be more or less untouched, R and B peaks will move left or right and can be smeared out. Clipping can occur but not so severely as with the other adjustments.

Note that especially exposure and contrast adjustments have overlapping effects and that brightness does similar things as well.

Based on the above (and my experience as a photographer plus having built the image processing pipeline of the Lapse app), I'd say that to get the best results you should:

  • First do exposure adjustment, but only if the histogram has an (almost) empty far-right area. Also this adjustment should depend on the size of that empty right side.
  • Then an empty far-left area of the histogram can be filled by increasing the contrast.
  • However, if there's an empty area left and right you could skip exposure correction and try to resolve this by increasing contrast.
  • Now that the histogram is 'filled' from left to right you can make the look a bit darker or lighter by adjusting brightness.
  • Finally, you can make color adjustments like saturation.

As you can see, which adjustment you apply depends on the image. And, there are many many more adjustments that, when combined, can result on almost the same end result.

Personally I hardly use brightness when I edit images manually.

For some more background on histograms and image adjustments have a look at these fabulous articles from Cambridge in Colour:

And, there's a whole list of related ones.

Upvotes: 1

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