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A-pack/Shaders11/deContrast.hlsl

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+// $MinimumShaderProfile: ps_4_0
+#define S -0.07
+#define n 4 
+
+/* --- deContrast (dx11) --- */
+
+/* v1.50 (2023-11) released by butterw under GPLv3
+(with S<0, ex: -0.07) Reduces midtone contrast. Also boosts contrast in shadows/highlights.
+n:4 (1 texture, 6 arithmetic).
+
+# Sn brightness/contrast curve
+out = f(x, S) = x + S/2*(2*x-1) *(1-(2*x-1)^n) = x + S/2*x1 *(1-x1^n)
+    with n even >0 (ex: 4, 6, 8, 16, etc.)
+    and x1 = 2*x -1
+    and x, out pixel.rgb in [0, 1.0].
+
+# Brightness
+f(x:0)=0; f(0.5)=0.5; f(1)=1.
+S<0: brightness is increased below the midpoint (x: 0.5) and decreased above the midpoint.
+
+# Contrast
+- contrast(x:0.5): 1+S
+- contrast(x:0)=contrast(x:1): 1-S*n
+
+
+Parameter S [-0.15, 0.15], 0: no effect
+- S>0: This symmetrical S-curve is a smoothed version of the contrast/levels linear curve. It provide a constant contrast increase (S) for a wide midtone range (with n=4+), and while it doesn't clip, contrast is decreased (up to S*n) in the highlights/shadows region. Because of this the sigmoid (sCurve shader) is probably a better choice for increasing contrast.
+- S<0: inverse S-curve. Now contrast is decreased by a constant value (S) in a wide midtone range and contrast is increased in the Shadows/Highlights (by |S|*n at x=0 and x=1).
+The plot https://github.com/butterw/bShaders/blob/master/img/deContrast_vs_contrast.png compares the contrast for different adjustment methods. This shader can be used to correct over-contrasted videos to get a more natural looking result. It's also good for restoring contrast/improving brightness in dark or bright videos.
+
+Parameter n, even integer [4: default, 8, 16, 6, ..]
+The higher n is, the wider the midtone contrast decrease range (with S<0) but also the stronger the contrast increase in highlights/shadows.
+n:8 (1 texture, 7 arithmetic)
+n:4 (1 texture, 6 arithmetic), default
+n:2 (1 texture, 5 arithmetic), S3(S/2) Hermite smoothstep interpolation. no flat midtone contrast decrease range.
+*/
+Texture2D tex: register(t0);
+SamplerState samp: register(s0);
+
+float4 main(float4 pos: SV_POSITION, float2 coord: TEXCOORD): SV_Target {
+    float4 c0 = tex.Sample(samp, coord);
+
+	float3 c1 = 2*c0.rgb -1;
+	c0.rgb+= 0.5*S *c1 *( 1.0 -pow(c1, n) );	
+	return c0; 
+}