fragment shader for stipple pattern - opengl

Question

I'm trying to draw a stipple pattern using a shader, like as in glStipple. However it does not seem to be working as expected.
The idea is to compare pixel coordinates against a stipple mask and discard fragments if the pixel coordinate mod mask size is 0.
However, two odd things are occurring which suggests the approach has some flaw I'm missing. The first is that the resulting stipple pattern does not match the mask. A single diagonal line in an 8x8 mask results in what appears to be diagonals spaced 1 pixel apart in the drawn shape. The second is that testing against 1, instead of 0, does not give a similar (but shifted) pattern.
The vertex shader looks like this:
static const char *vertexShaderSource =
"attribute highp vec4 posAttr;\n"
"attribute highp vec4 colAttr;\n"
"varying lowp vec4 col;\n"
"varying highp vec2 coord;\n"
"uniform highp mat4 matrix;\n"
"void main() {\n"
" col = colAttr;\n"
" gl_Position = matrix * posAttr;\n"
" coord = gl_Position.xy;\n"
"}\n";
and the fragment shader:
static const char *fragmentShaderSource =
"varying lowp vec4 col;\n"
"varying highp vec2 coord;\n"
"uniform int stipple[64];\n"
"uniform int width;\n"
"uniform int height;\n"
"void main() {\n"
" if (stipple[abs(mod(coord.x * width,8)) + abs(mod(coord.y * height,8)) * 8] == 1)\n"
" {\n"
" discard;\n"
" }\n"
" gl_FragColor = col;\n"
"}\n";
where width and height are the w/h of the viewport and the stipple is e.g.
GLint stipple[64] = {
0, 0, 0, 0, 0, 0, 0, 1,
0, 0, 0, 0, 0, 0, 1, 0,
0, 0, 0, 0, 0, 1, 0, 0,
0, 0, 0, 0, 1, 0, 0, 0,
0, 0, 0, 1, 0, 0, 0, 0,
0, 0, 1, 0, 0, 0, 0, 0,
0, 1, 0, 0, 0, 0, 0, 0,
1, 0, 0, 0, 0, 0, 0, 0
};
Any ideas appreciated, thanks.

Answer 1

You can use gl_FragCoord. It contains the pixel's center coordinate. So for a resolution of 800x600 then gl_FragCoord would be in the range of vec2(0.5, 0.5) and vec2(799.5, 599.5). Thereby there's no need to multiply by the resolution, since we already basically have window coordinates.
Since gl_FragCoord wouldn't result in a negative number you can remove abs().
All in all with the few modifications, the fragment shader ends up looking like this:
varying vec4 color;
uniform int stipple[64];
void main()
{
ivec2 coord = ivec2(gl_FragCoord.xy - 0.5);
if (stipple[int(mod(coord.x, 8) + mod(coord.y, 8) * 8)] == 0)
discard;
gl_FragColor = color;
}