How can I simulate "Glow Dodge" blending by using OpenGL Shader?

Question

I wanted to simulate 'glow dodge' effect on clip studio using opengl and its shader. So I found out that following equation is how 'glow dodge' works.

final.rgb = dest.rgb / (1 - source.rgb) 

Then I came up with 2 ways to actually do it, but neither one doesn't seem to work.

First one was to calculate 1 / (1 - source.rgb) in the shader, and do multiply blending by using glBlendfunc(GL_ZERO, GL_SRC_COLOR), or glBlendfunc(GL_DST_COLOR, GL_ZERO).

but as khronos page says, all scale factors have range 0 to 1. which means I can't multiply the numbers over 1. so I can't use this method cuz most of the case goes more than 1.

Second one was to bring background texels by using glReadPixel(), then calculate everything in shader, then do additive blending by using glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA).

regardless the outcome I could get, glReadPixel() itself takes way too much time even with a 30 x 30 texel area. so I can't use this method.

I wonder if there's any other way to get an outcome as glowdodge blending mode should have.

Answer 1

With the extension EXT_shader_framebuffer_fetch a value from the framebuffer can be read:

This extension provides a mechanism whereby a fragment shader may read existing framebuffer data as input. This can be used to implement compositing operations that would have been inconvenient or impossible with fixed-function blending. It can also be used to apply a function to the framebuffer color, by writing a shader which uses the existing framebuffer color as its only input.

The extension is available for dektop OpenGL and OpenGL ES. An OpenGL extension has to be enabled (see Core Language (GLSL)- Extensions. The fragment color can be retrieved by the built in variable gl_LastFragData. e.g.:

#version 400
#extension GL_EXT_shader_framebuffer_fetch : require

out vec4 fragColor;

# [...]

void main 
{
    # [...]

    fragColor = vec4(gl_LastFragData[0].rgb / (1.0 - source.rgb), 1.0);  
}