using geometry shader to create new primitives types

Question

Is it possible to output new primitive type from geometry shader other than was input? I'd like to input a point and render a triangle. The point would be used just as center for this triangle. If not, is there other option to input just point and render some other piece of geometry defined by that point?

With the help from answer here is geometry shader doing just what I asked for (if anyone ever needed):

#version 120
#extension GL_EXT_geometry_shader4 : enable

layout(points) in;
layout(triangle_strip) out;

void main()
{   
    gl_Position = gl_in[0].gl_Position;
    EmitVertex();
    gl_Position = gl_in[0].gl_Position+vec4(1,0,0,0);
    EmitVertex();
    gl_Position = gl_in[0].gl_Position+vec4(0, 1, 0, 0);
    EmitVertex();
    EndPrimitive();
}

Christian Rau · Accepted Answer · 2011-09-22 17:15:41Z

4

Yes, this is perfectly possible, that's what the geometry shader is there for. Just specify the input primitive type as point and the output primitive type as triangle (or rather triangle strip, no matter if it's only a single triangle) using either glProgramParameteri in the application or using the more modern layout syntax directly in the shader.

answered Sep 22, 2011 at 17:15

Christian Rau

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2 Comments

Nicol Bolas Over a year ago

Note that you don't really get a choice of syntax. Or rather, your choice of syntax is based on whether you are using the extension version of geometry shaders or the core functionality. Don't try to mix the two.

Christian Rau Over a year ago

@Nicol I actually haven't worked with geometry shaders, yet. Thanks for the clarification.

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