OpenGL weird rendering

Question

I'm trying to get a working OpenGL renderer, but I'm stuck at a weird spot. I'm generating a Model, View and Projection matrices that seem correct. GLIntercept gives me the following log:

glClear(GL_COLOR_BUFFER_BIT)
glEnableVertexAttribArray(0)
glUniformMatrix4fv(0,1,false,[... see below ...])
glBindBuffer(GL_ARRAY_BUFFER,1)
glVertexAttribPointer(0,3,GL_FLOAT,false,0,0000000000000000)
glDrawArrays(GL_POLYGON,0,3) GLSL=3 
glDisableVertexAttribArray(0)
wglSwapBuffers(FFFFFFFF9D011AB3)=true

My MVP matrix is as follows:

 [
   0,050000 0,000000 0,000000 0,500000
   0,000000 0,050000 0,000000 0,500000
   0,000000 0,000000 5,000000 0,000000
   0,000000 0,000000 -50,000000 1,000000
 ]

Here are the vertex shader I'm using:

#version 330

layout (location = 0) in vec3 vertex;
uniform mat4 mvp;

out vec4 color;

void main() {
    gl_Position = mvp * vec4(vertex, 0.0);
    color = vec4(1, 1, 1, 1);
}

and the fragment shader:

#version 330

in vec4 color;
out vec4 fragColor;

void main() {
    fragColor = color;
}

This is how I calculate the projection matrix, and the coordinates are the simplest possible, a flattened cube:

[
    new Vector3f(-1, -1, 0),
    new Vector3f(1, -1, 0),
    new Vector3f(1, 1, 0),
    new Vector3f(-1, 1, 0)
]

Yet that renders to a weird thing: rendering screenshot

If I multiply the matrix with the same coordinates in-app, however, I get expected results:

(0,45 0,45 0,00 1,00)
(0,55 0,45 0,00 1,00)
(0,55 0,55 0,00 1,00)
(0,45 0,55 0,00 1,00)

Could someone point me in a direction to solve this?

Your matrix doesn't look correct. You have a row with all zeros in it. Seems like you tranform `w` to 0.0, which will screw up everything. — derhass, Dec 19 '14 at 00:03
@derhass See the edit, I fixed (did i?) the MVP matrix calculation, but the result is still *exactly* the same... Also, I'm particularly interested in why the GLSL and in-app calculations seem to differ. — Délisson Junio, Dec 19 '14 at 00:41

score 1 · Accepted Answer · answered Dec 19 '14 at 04:50

The main problem is in your vertex shader:

gl_Position = mvp * vec4(vertex, 0.0);

When expanding regular 3D coordinates to homogeneous coordinates, you need to set the w component to 1.0. So the statement should look like this:

gl_Position = mvp * vec4(vertex, 1.0);

It's also slightly odd that you have 4 vertices, but pass 3 as the number of vertices (last argument) to the draw call:

glDrawArrays(GL_POLYGON, 0, 3);

OpenGL weird rendering

1 Answers1