How do you write a GLSL vertex program that transforms verticies? - glsl

Question

...and have it actually work. I get the principle, you write a vertex program, something like, this say:
attribute vec3 v_pos;
attribute vec4 v_color;
attribute vec2 v_uv;
attribute vec3 v_rotation; // [angle, x, y]
uniform mat4 modelview_mat;
uniform mat4 projection_mat;
varying vec4 frag_color;
varying vec2 uv_vec;
void main (void) {
mat4 trans_in = mat4(
1.0, 0.0, 0.0, 50.0, // <--- Transformation matrix
0.0, 1.0, 0.0, 50.0,
0.0, 0.0, 1.0, 50.0,
0.0, 0.0, 0.0, 1.0
);
vec4 pos = trans_in * vec4(v_pos,1.0); // <--- apply to input
// Mark a vertex using color to prove a transformation is actually happening...
if (v_rotation[0] > 10.0) {
frag_color = vec4(1.0, 0.0, 0.0, 1.0);
gl_Position = projection_mat * vec4(pos[0], pos[1], 1.0, 1.0);
}
// And leave all the other verticies untouched.
else {
frag_color = v_color;
gl_Position = projection_mat * vec4(v_pos, 1.0); // <--- Untransformed output
}
uv_vec = v_uv; // <--- Pass UV to fragment program
}
The problem is, this doesn't actually work.
After applying the matrix transformation trans_in * v_pos, I expect a point [1, 2, 3] to become [51, 52, 53, 1].
...but it doesn't. In fact, it renders this:
(ie. no transformation of the point location; pos = trans_in * v_pos == vec4(v_pos, 1.0)!!!!!! O_o)
Notice the red marked vertices that prove that I am actually setting the gl_Position for them; indeed, if I do this:
gl_Position = projection_mat * vec4(1.0, 1.0, 1.0, 1.0);
Each of those red points is jumped down to the bottom corner, as you would expect.
I've also tried various 3x3 matrix multiplications and it seems that while the scale operations work, and to some extent, the rotation operations work, I cannot for the life of me get any 2d translation operations to run; the matrix multiplication just seems to... do nothing.
What am I doing wrong?

Answer 1

You got the matrix order wrong. GLSL uses column-major oder, so each row in your intializer will become a column of the matrix. This refelcts the same convention which was used with the (now deprecated) GL matrix stack. It is also consistent to the setting of the transpose parameter of glUinformMatrix*() calls which has to be set to GL_FALSE for column major input (where translation part are elements m[12],m[13],m[14] in an 1D array).
Your matrix actually only alters the w component of your vector, which you then ignore, so it does not have any visible effect.