Levels control shader
January 28, 2009A little piece of code to reproduce the Levels control of Photoshop…
Input levels:
I already talked about the gamma correction (mid-tone slider), and I won’t explain what the shadows and highlights (black/white points) sliders are doing (excellent article here) but basically these can be used to remap the tonal range of the image. Here is how it’s calculated:
#define GammaCorrection(color, gamma) pow(color, vec3(1.0 / gamma))
#define LevelsControlInputRange(color, minInput, maxInput) min(max(color – vec3(minInput), vec3(0.0)) / (vec3(maxInput) – vec3(minInput)), vec3(1.0))
#define LevelsControlInput(color, minInput, gamma, maxInput) GammaCorrection(LevelsControlInputRange(color, minInput, maxInput), gamma)
Example with values from the 1st screenshot (blackpoint = 90/255, gamma = 4, whitepoint = 150/255), red: original color, green: blackpoint & whitepoint modified, blue: same with gamma:
Output levels:
This is useful to shorten the tonal range meaning compressing it to reduce contrast and shift it, details here.
#define LevelsControlOutputRange(color, minOutput, maxOutput) mix(vec3(minOutput), vec3(maxOutput), color)
Example with values from the 1st screenshot (min output = 40/255, max output = 180/255), red: original color, green: output levels applied:
Putting it all together:
#define LevelsControl(color, minInput, gamma, maxInput, minOutput, maxOutput) LevelsControlOutputRange(LevelsControlInput(color, minInput, gamma, maxInput), minOutput, maxOutput)
Same example but both input and output levels taken into account, red: original color, green: final result:
So these macros make it quite easy to increase or reduce contrast, shift and clip tonal range, lighten or darken shadows and highlights. I added the (GLSL / HLSL) code to the Photoshop Math shaders.
Projects, Software, Programming, GLSL - Romz Blog 
Just passing by.Btw, you website have great content!
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Hey these shaders and others in your articles are great. Ive been working on some film shaders myself, technicolor processing and lomo looks using LUTs. Very similar work. Great content. What license do you release these as? I may want to port some of these to Quartz Composer.
Thanks!
hello, great explanation.
i need to implement a similar program logic into one of my matlab projects. could you please explain how changing the input level sliders would work mathematically?
thank you!
The macro I gave in the post:
#define LevelsControlInputRange(color, minInput, maxInput) min(max(color – vec3(minInput), vec3(0.0)) / (vec3(maxInput) – vec3(minInput)), vec3(1.0))
Easier to read like this:
color = max(color – minInput, 0.0) / (maxInput – minInput);
color = min(color, 1.0);
Remember that in high level shaders the color is normalized in the range [0, 1].
The min level (left slider), clamps the color in the lower values (that’s why we take the max(color – minInput, 0.0). Any color < minInput will be 0 (instead of going negative).
The color (or tonal range) needs to be remapped between minInput and maxInput. That’s why we divide by (maxInput – minInput).
At the end, the max level (right slider) clamps the color in the higher values (that’s why we take the min(color, 1.0). Any color > maxInput will be 1.0.
Make sure you read the levels article I linked in the post: http://www.cambridgeincolour.com/tutorials/levels.htm
This has really helped me understand fragment-shaders!
Could you explain how the curves tool in photoshop translates?