CODEWITHSUNDEEP
copenglopengl-esvertex-shader
jtheoof
jtheoof

Reputation: 585

Vertex shader matrix multiplication not doing anything

I've been struggling with an issue with my vertex shader. I have a sprite with the following position attributes:

sprite->vertices[0][0] = -1.0f;
sprite->vertices[0][1] = +1.0f;
sprite->vertices[1][0] = -1.0f;
sprite->vertices[1][1] = -1.0f;
sprite->vertices[2][0] = +1.0f;
sprite->vertices[2][1] = +1.0f;
sprite->vertices[3][0] = +1.0f;
sprite->vertices[3][1] = -1.0f;

And I use the attribute this way:

glUseProgram(sprite->program);

glEnableVertexAttribArray(sprite->position_attrib);
glVertexAttribPointer(sprite->position_attrib, 2, GL_FLOAT, GL_FALSE, 0,
                      sprite->vertices);

Here is my vertex shader:

static const char character_vshader_g[] =
    "#ifdef GL_ES\n"
    "precision mediump float;\n"
    "#endif\n"
    "uniform   mat4  mvp_u;\n"
    "attribute vec4  position_a;\n"
    "attribute vec2  texture_a;\n"
    "varying   vec2  texture_v;\n"
    "mat4  tmp;\n"
    "void main() {\n"
    "    tmp = mat4(\n"
    "              vec4(0.5, 0  , 0  , 0  ),\n"
    "              vec4(0  , 0.5, 0  , 0  ),\n"
    "              vec4(0  , 0  , 0.5, 0  ),\n"
    "              vec4(0  , 0  , 0  , 0.5) \n"
    "          );\n"
    "    gl_Position = tmp * position_a;\n"
    "    texture_v = texture_a;\n"
    "}\n";

Basically I'm trying to manage my own mvp uniform matrix and in order to debug it, I hard coded this basic scaling matrix tmp.

My issue is that this multiplication:

gl_Position = tmp * position_a;

Does not change anything at all unless I put all 0 in tmp. Then my quad disappears. Otherwise I see my quad occupying the full viewport (I do not have any kind of projection as you can see so the default viewport is -1,-1 to 1,1).

So it's almost like tmp is not doing anything.

Can anyone help?

Upvotes: 1

Views: 660

Answers (1)

datenwolf
datenwolf

Reputation: 162164

Oh, your matrix gets applied just fine. The problem is, that the ww element is 0.5, which when multiplied with a vec2 will yield some vec4(0.5x, 0.5y, 0, 0.5). However one of the last, hardcoded steps in the transform pipeline is the homogenous perspective divide v' = v/v.w. But a vec4(0.5x, 0.5y, 0, 0.5) / 0.5 == vec4(x, y, 0, 1) which looks just like as if nothing happened at all.

Solution to your problem: Make the w-row of your matrix 0 0 0 1, i.e. in your code

"    tmp = mat4(\n"
"              vec4(0.5, 0  , 0  , 0 ),\n"
"              vec4(0  , 0.5, 0  , 0 ),\n"
"              vec4(0  , 0  , 0.5, 0 ),\n"
"              vec4(0  , 0  , 0  , 1 ) \n"

Upvotes: 4

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