How to implement multiple render target in opengles 3.0 - opengl-es

Question

I want to implement MRT in opengl es 3.0. Thus has created a framebuffer with texture as a GL_COLOR_ATTACHMENT0 attachment of type GL_RGBA32UI. Rendering a textured image of GL_RGBA32UI on that framebuffer. Then reading the framebuffer data as a texture and applying it as a texture in default buffer. (Basically render to texture using INTEGER texture)
I am trying to use same fragment shader for both my customize FBO and default one.
precision highp float;
uniform highp usampler3D stexture;
in vec4 out_TexCoord;
uniform highp uint range;
layout(location = 0) out uvec4 uex_colour;
layout(location = 1) out vec4 ex_colour;
void main(void)
{
uex_colour = uvec4(texture(stexture, out_TexCoord.xyz));
ex_colour = vec4(texture(stexture, out_TexCoord.xyz))/(range);
ex_colour = vec4(vec3(ex_colour.xyz), 1.0);
}
Want to use uex_color for rendering into the customize framebuffer and ex_color to render in default framebuffer.
Tried using glDrawBuffer(1, {GL_COLOR_ATTACHMENT0 }) for my FBO, but not able get how to use ex_colour for default framebuffer.

Answer 1

I think you would save yourself a lot of headache by using two different shaders. The allowed sets of values for glDrawBuffers are much more restrictive in ES 3.0 than in full OpenGL.
Specifically in your case, with the fragment shader you're trying to use, what you want in the back buffer is output 1. But for the default framebuffer, you can have only one draw buffer, which has to be GL_BACK. So you can only use output 0 for drawing the back buffer.
It actually looks like you might be trying to render to the texture and use the texture in the same rendering pass. If that's the case, it's a really bad idea. It sets up what the specs call a "rendering feedback loop". You can read up on the details, but it's generally not going to work. And you couldn't render to an FBO and the default framebuffer at the same time anyway.
You need to do one pass that renders to the FBO to generate your texture, then another pass to render to the default framebuffer, sampling the texture. You will use different shaders for these two passes.