GLSL Code:

void main() { gl_FragColor=vec4(1,0,0,1); // draw red //gl_FragColor=vec4(texcoords.x,texcoords.y,0,1); // draw texture coords //if (length(texcoords)<0.5) gl_FragColor=vec4(0,0,1,1); // circle //gl_FragColor = texture2D(tex0,4.0*texcoords); // read checkerboard }

A product of Dr. Lawlor, UAF CS.