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Michael Wojcik

OpenGL Orthogonal matrix issue

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Hello everyone.

I've ran into a strange issue in my engine. With openGL ES 2.0, when I render my scene with a projection matrix, my scene renders correctly. With a orthogonal matrix, nothing renders at all. I'm using my own projection matrix functions to generate the matrices, so naturally, my first suspicion would be that my orthogonal matrix is being constructed erroneously but even openGL's orthoM() function bears the same problem.

Here is my GLSL vertex shader.

[code]
uniform mat4 u_projection_matrix;
uniform mat4 u_view_matrix;
uniform mat4 u_model_matrix;
attribute vec3 a_position;
attribute vec3 a_normal;
attribute vec2 a_texCoord0;
varying vec2 v_texCoord0;

void main()
{
v_texCoord0 = a_texCoord0;
gl_Position = ((u_projection_matrix * u_view_matrix * u_model_matrix) * vec4(a_position.xyz, 1.0));
}
[/code]

Here is my orthogonal projection matrix generation function just in case..

[code]
public void CreateOrthoMatrix(float left, float right, float bottom, float top, float near, float far)
{
// These paramaters are lens properties.
// The "near" and "far" create the Depth of Field.
// The "left", "right", "bottom" and "top" represent the rectangle formed
// by the near area, this rectangle will also be the size of the visible area.

Matrix.setIdentityM(projectionMatrix, 0);

// First Column
projectionMatrix[0] = 2.0f / (right - left);
projectionMatrix[1] = 0.0f;
projectionMatrix[2] = 0.0f;
projectionMatrix[3] = 0.0f;

// Second Column
projectionMatrix[4] = 0.0f;
projectionMatrix[5] = 2.0f / (top - bottom);
projectionMatrix[6] = 0.0f;
projectionMatrix[7] = 0.0f;

// Third Column
projectionMatrix[8] = 0.0f;
projectionMatrix[9] = 0.0f;
projectionMatrix[10] = -2.0f / (far - near);
projectionMatrix[11] = 0.0f;

// Fourth Column
projectionMatrix[12] = (right + left) / (right - left);
projectionMatrix[13] = (top + bottom) / (top - bottom);
projectionMatrix[14] = (far + near) / (far - near);
projectionMatrix[15] = 1;
}
[/code]

Anyone have any ideas whats going on?

My appreciations.

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Try

[code] // Fourth Column
projectionMatrix[12] = -(right + left) / (right - left);
projectionMatrix[13] = -(top + bottom) / (top - bottom);
projectionMatrix[14] = -(far + near) / (far - near);
projectionMatrix[15] = 1; [/code]

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[quote name='NumberXaero' timestamp='1312667268' post='4845560']
Try

[code] // Fourth Column
projectionMatrix[12] = -(right + left) / (right - left);
projectionMatrix[13] = -(top + bottom) / (top - bottom);
projectionMatrix[14] = -(far + near) / (far - near);
projectionMatrix[15] = 1; [/code]
[/quote]

Thank you, but still no go after replacing my fourth column code the code you posted :(. If it helps this is how I call the method.

[code]
CreateOrthoMatrix(0.0f, width, 0.0f, height, 0.1f, 100f);
[/code]

Where the width and height is the width and height of my viewport. Which are matched to the pixel width and height of my device's screen.

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Switching from a perspective to orthogonal matrix sometimes isn't simply as easy as changing the matrix. You also have to look at the scale of things that were being drawn before, and how they are being drawn now. Some objects might be too large, some might be too small, etc... For instance, a very small object (say, less than 1 unit in length/width/height) would occupy a large portion of the screen if viewed through a projection matrix, if it were close enough to the camera. However, with an orthogonal matrix, regardless of how near the camera it is, that object would always be drawn as less than one pixel in size (given a 1:1 mapping of projection space to screen space) since it's real size is less than one.

I'd say, test out your projection code with a controllable test case first, before you switch your in-project matrix, then when you are assured the matrix code is working correctly, begin the process of switching your game over.

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[quote name='JTippetts' timestamp='1312673160' post='4845601']
Switching from a perspective to orthogonal matrix sometimes isn't simply as easy as changing the matrix. You also have to look at the scale of things that were being drawn before, and how they are being drawn now. Some objects might be too large, some might be too small, etc... For instance, a very small object (say, less than 1 unit in length/width/height) would occupy a large portion of the screen if viewed through a projection matrix, if it were close enough to the camera. However, with an orthogonal matrix, regardless of how near the camera it is, that object would always be drawn as less than one pixel in size (given a 1:1 mapping of projection space to screen space) since it's real size is less than one.

I'd say, test out your projection code with a controllable test case first, before you switch your in-project matrix, then when you are assured the matrix code is working correctly, begin the process of switching your game over.
[/quote]

jTippetts, Extrodenary Information!! I am making my engine unit relative to the metric meter. Yet I was contructing my orthogonal projection matrix's bounds to be relative to pixils, so everything in the scene was way too small. I am glad to now know that aspect in the nature of working with an othrogonal projection matrix.

NumberXaero, Thank you as well! That fixed up the implementation of my orthogonal projection matrix and now it works!

Thanks guys, you both saved me boat loads of time and made my day!

[b]
[/b]

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You might want to consider using glm.
http://glm.g-truc.net/
It is a really nice math library for Vectors and Matrices especially designed for OpenGL and written against the GLSL ShaderSpec.
It saved [b]me[/b] a lot of time implementing that on my own.

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[quote name='NicoG' timestamp='1312675908' post='4845611']
You might want to consider using glm.
[url="http://glm.g-truc.net/"]http://glm.g-truc.net/[/url]
It is a really nice math library for Vectors and Matrices especially designed for OpenGL and written against the GLSL ShaderSpec.
It saved [b]me[/b] a lot of time implementing that on my own.
[/quote]

I'll be sure to check it out, thanks!

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