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  Creating a projection matrix  (Read 1633 times)
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Offline Sublixt

Senior Newbie





« Posted 2012-07-15 23:14:07 »

I'm trying to create a projection matrix to use in my shaders. I've tried to use the algorithm from here(http://openglbook.com/the-book/chapter-4-entering-the-third-dimension/) to create the matrix, but when I multiply it with the model and view matrices that I've created nothing displays on the screen.
This is my translation of the C code.
Sorry if there are syntax errors. I wrote this in Scala and translated it to Java.
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public static Matrix4f projection(fov: Float, aspectRatio: Float, near: Float, far: Float) {
Matrix4f out = new Matrix4f();
float yScale = 1.0f / scala.math.tan((fov / 2.0f).toRadians).toFloat;
float xScale = yScale / aspectRatio;
float frustumLength = far - near;
         
float[] tempArray = new float[16];
tempArray[0] = xScale;
tempArray[5] = yScale;
tempArray[10] = -((far + near) / frustumLength);
tempArray[11] = -1;
tempArray[14] = -((2 * near * far) / frustumLength);
         
FloatBuffer buf = BufferUtils.createFloatBuffer(16);
buf.put(tempArray);
buf.flip();
out.load(buf);
return out;
}

//This is the method call
Matrix.projection(60.0f, 800.0f / 600.0f, 1.0f, 100.0f);

//and when I print the matrix that is returned I get this
1.2990382 0.0 0.0 0.0
0.0 1.7320509 0.0 0.0
0.0 0.0 -1.020202 -2.020202
0.0 0.0 -1.0 0.0
Offline ra4king

JGO Kernel


Medals: 322
Projects: 2
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« Reply #1 - Posted 2012-07-18 05:41:58 »

That xScale doesn't look right. The article says f/aspect....you've interpreted 'f' to be the yScale, but how do you know?

Offline Sublixt

Senior Newbie





« Reply #2 - Posted 2012-07-18 19:10:58 »

I got yScale for f from the code sample at the top.
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Matrix CreateProjectionMatrix(
   float fovy,
   float aspect_ratio,
   float near_plane,
   float far_plane
)
{
   Matrix out = { { 0 } };

   const float
      y_scale = Cotangent(DegreesToRadians(fovy / 2)),
      x_scale = y_scale / aspect_ratio,
      frustum_length = far_plane - near_plane;

   out.m[0] = x_scale;
   out.m[5] = y_scale;
   out.m[10] = -((far_plane + near_plane) / frustum_length);
   out.m[11] = -1;
   out.m[14] = -((2 * near_plane * far_plane) / frustum_length);
   
   return out;
}
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Offline Sublixt

Senior Newbie





« Reply #3 - Posted 2012-07-18 20:34:49 »

I found the problem. I multiplied the matrices in the wrong order.
I used
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gl_Position = model * view * perspective * in_Position

when its should be
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gl_Position = perspective * model * view * in_Position
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