Here is my Base64 encoder shader:

https://github.com/Rezmason/excel_97_egg/blob/main/glsl/base...

I got it down to about thirteen lines of GLSL:

https://github.com/Rezmason/excel_97_egg/blob/main/glsl/base...

I use it for Cursed Mode of my side project, which renders the WebGL framebuffer to a Base64-encoded, 640x480 pixel, indexed color BMP, about 15 times per second:

https://rezmason.github.io/excel_97_egg/?cursed=1