fragment shader and glDrawArrays

Question

is it possible to use fragment shader to set one color channel of a rendering and

glColorPointer(...);
glDrawArray();

to set the 2 others colors channels. If yes how can I do that ?

ratchet freak · Accepted Answer · 2015-11-10 16:10:04Z

3

If you are stepping to the programmable pipeline then I also suggest stepping away from glColorPointer glVertexPointer and the matrix stack and instead use glVertexAttribPointer and pass the transformation matrices as uniforms.

In the fragment shader you would do

gl_FragColor = vec4(channel1, channels2_3.xy, 1);

Where channel1 and channel2_3 can come from either a uniform or a attribute passed from the vertex shader.

edited Nov 10, 2015 at 16:10

answered Nov 10, 2015 at 15:15

ratchet freak

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4 Comments

Thomas Kostas Over a year ago

I'm new to openGl your explanation is little bit tricky for me, does it mean i should write a vertexshader ? should I still use glDrawArrays? glDrawArrays() will still work fine if I use glVertexAttributePointer(...) instead of glVertexPointer(..)?

ratchet freak Over a year ago

You'd have to create the vertex shader anyway. glDrawArrays will still work fine.

Bartek Banachewicz Over a year ago

Don't you need to declare the output variable explicitely? I think gl_FragColor is a reserved name that can't be used.

ratchet freak Over a year ago

@BartekBanachewicz that depends on the version openGL 3.3 would use gl_FracColor while 4.x needs explicit declaration (IIRC)

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