How to handle alpha in a manual “Overlay” blend operation?

Question

I\'m playing with some manual (walk-the-pixels) image processing, and I\'m recreating the standard \"overlay\" blend. I\'m looking at the \"Photoshop math\" macros here:

Answer 1

After blending the base color and the blend color, mix the original base color and the color resulting from the blending using the alpha of the blend color:

vec4 baseColor = ...;
vec4 blendColor = ...;
vec4 blendedColor = blend(baseColor, blendColor);
vec4 fragmentColor = (1.0 - blendColor.a) * baseColor + blendColor.a * blendedColor;

I use this for "overlay" blending an opaque base color and a blend texture which has a lot of (semi) transparent pixels.