2-pass shader possible with Processing?

MartinVDL

Some of the shadertoy shaders use 2or more passes using the built in buffers, for example https://www.shadertoy.com/view/XsG3z1#. Is it possible to use these buffers from within processing or do they have to be created in the GLSL shader code? If so, any examples I could learn from?

codeanticode

You could apply multiple passes if you use PGraphics, by doing something like this:

pg1.beginDraw();
pg1.shader(pass1);
pg1.rect(0, 0, pg1.width, pg1.heigth);
pg1.endDraw(); 

pg2.beginDraw();
pg2.shader(pass2);
pass2.set("prevOutput", pg1);
pg2.rect(0, 0, pg2.width, pg2.heigth);
pg2.endDraw(); 

Here I'm assuming that the prevOutput uniform in pass2 shader is a texture sampler you use to read the output from the previous pass, but not sure if that's the technique used by ShaderToy.