OpenGL ES 2.0: matrix multiplication problem

I'm trying to draw a quadrilateral using OpenGL ES 2.0. My vertex shader looks like this:

uniform mat4 mvp;
attribute vec4 position;
attribute vec4 color;
varying vec4 colorVarying;

void main()
{
    gl_Position = mvp * position;
    colorVarying = color;
}

If I use these vertices:

-1.0f, -1.0f, 0.f, 1.f,
 1.0f, -1.0f, 0.f, 1.f,
-1.0f,  1.0f, 0.f, 1.f,
 1.0f,  1.0f, 0.f, 1.f

and this matrix (in column-major order, as are all following matrices):

0.5,0,0,0,
0,  1,0,0,
0,  0,1,0,
0.5,0,0,1

the quad covers the right half of the viewport. Indeed, if I multiply those two matrices I get the vertices

0,    -1,     0,     1,
1,    -1,     0,     1,
0,     1,     0,     1,
1 ,    1,     0,     1,

and if I use those vertices and an identity matrix, I get the same result. So it seems that the multiplication is done correctly.

Now, if I multiply these vertices (A):

-100.0f, -100.0f, 301.0f, 1.0f,
 100.0f, -100.0f, 301.0f, 1.0f,
-100.0f,  100.0f, 301.0f, 1.0f,
 100.0f,  100.0f, 301.0f, 1.0f,

by this matrix (M):

0.4170         0         0         0
     0    0.5560         0         0
     0         0   -1.2220   -1.0000
     0         0  444.4440         0

I get these vertices (B):

 0.开发者_开发问答2075,    0.2766,   -0.9892,    1.0000
-0.2075,    0.2766,   -0.9892,    1.0000,
 0.2075,   -0.2766,   -0.9892,    1.0000,
-0.2075,   -0.2766,   -0.9892,    1.0000,

and if I pass my shader the B vertices and an identity matrix, it renders a small rectangle in the middle of the viewport. But if I pass it the A vertices and the M matrix, it renders nothing.

What's going on?

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The problem is that gl_Position after the vertex shader completes goes through clipping where it is compared against the range [-W W] and clipped ( not drawn ) if it gl_Position falls outside of that range.

IF you multiply A * M and set the result to gl_Position, then given the matrices you are using the W coordinate will be -301, a negative number, which will invert the clipping comparison.

Try multiplying the matrix M by -1 before doing the multiplication, that should yield the same results as multiplying matrix B by the identity matrix.

From the "Red Book":

Avoid using negative w vertex coordinates and negative q texture coordinates. OpenGL might not clip such coordinates correctly and might make interpolation errors when shading primitives defined by such coordinates.

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The math library you are using might not use the same matrix order as GLSL. Try transposing your matrix M before passing it to the shader.

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The problem was that with that -1 in the wz cell, the triangles ended up with their normal in the opposite directions, and since they are one-sided they were not drawn. I solved the problem by changing M to this:

0.4170         0         0         0
     0    0.5560         0         0
     0         0    1.2220    1.0000
     0         0 -444.4440         0

I also changed the sign of the z column (actually row since the matrix is transposed) in order to keep -1 as near and +1 as far on the z axis.