Levels control shader

A 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.

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Photoshop gamma correction shader

After reproducing contrast, hue, saturation, brightness controls of Photoshop in pixel shaders, here is the gamma correction filter 🙂

Photoshop

There are 2 ways of changing gamma in Photoshop:

• Image | Adjustments | Exposure…
• Image | Adjustments | Levels… (or CTRL+L) and then move the midtone slider.

Gamma correction is not the same thing than Brightness at all, even if it can give the impression it is. For example here is the histogram of my original image:

Then after setting the gamma to 0.5 (it compresses the highlights and stretches the shadows):

And here it is after lowering the brightness (Image | Adjustments | Brightness/Contrast…):

Here you can see it clipped the values after some threshold in the shadows (and also in the highlights) and you’re loosing a lot of lighting information in this case. That’s actually why I wanted to have also a gamma control in my post-processing effects.

Shader

A little macro:

// Gamma from 9.99 to 0.1
#define GammaCorrection(color, gamma)   pow(color, 1.0 / (gamma))

color = GammaCorrection(color, 0.1);

Here are the curves it produces with extreme values (limits of Photoshop [9.99, 0.1]):

So you see it stays in the same range and give a non-linear luminance (if correction value is different than 1). By the way I used this awesome web function grapher here.

By zooming and taking a very close look I noticed a few tiny differences with Photoshop in the shadows (seriously you need to toggle screenshot from shader and photoshopped image quickly and scan the image to see where it’s not the same). Photoshop on the left, shader version on the right (gamma = 0.1).

I also saw that the gamma in Photoshop (on the left) produced some banding artefacts somewhere, and it didn’t in my shader (on the right),  well.. :):

I added the code in my Photoshop Math (GLSL/HLSL) shaders.

Photoshop math with HLSL shaders

See my original post about Photoshop math in GLSL (blending modes, contrast, desaturation, RGB to HSL). Now it’s also in HLSL!

Download PhotoshopMath.hlsl