Applying a smooth-step S-curve to output color increases contrast and creates a filmic look

The smoothstep cubic x²(3-2x) maps [0,1]→[0,1] but with flat slopes at 0 and 1, making darks stay darker longer and brights push to white faster. Applied to the final color (after gamma and tonemapping), this is a simple but effective contrast increase that mimics photographic S-curves from film. Unlike a linear contrast multiply it avoids pure-white clipping and maintains gradients. The result: the image gains a cinematic quality without elaborate tone-mapping math. Blend weight controls the intensity of the effect.

Examples

col = mix(col, col*col*(3.0-2.0*col), 0.5); — 50% S-curve blend applied post-gamma.

Assessment

Compare the effect of smoothstep contrast on a mid-grey (0.5,0.5,0.5) vs a near-white (0.9,0.9,0.9) color and describe what changes in each.