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sufimaster

OpenGL
Orthographic matrix not working

13 posts in this topic

Hi all,

 

I am having trouble creating an orthographic projection in opengl (without using glOrtho).  And before anyone tells me to use glOrtho, lets just pretend I'm using opengl 3.0 as an exercise.  Here is the way I have set up my matrix:

 

projectionMatrix.m00 = 2/WIDTH;
projectionMatrix.m11 = 2/HEIGHT;
projectionMatrix.m22 = -2/(far_plane - near_plane);
projectionMatrix.m32 = -((far_plane + near_plane)/(far_plane - near_plane));
projectionMatrix.m33 = 1;

This is your typical ortho projection matrix, taken straight from the opengl spec def. for glOrtho. 

 

When I set up a frustrum and use a perspective matrix, this works fine, but I can't use my regular screen coordinates properly.  But when I use the glOrtho matrix above, I see nothing but my background color. Anyone know what's going on? Is my matrix wrong?

 

My shader looks like this:

 

 

#version 150 core
uniform mat4 projectionMatrix;
uniform mat4 viewMatrix;
uniform mat4 modelMatrix;


in vec4 in_Position;
in vec4 in_Color;
in vec2 in_TextureCoord;


out vec4 pass_Color;
out vec2 pass_TextureCoord;


void main(void){
    gl_Position = in_Position;
    
    //override gl position with new calculated position
    gl_Position = projectionMatrix * viewMatrix * modelMatrix * in_Position;
    
    pass_Color = in_Color;
    pass_TextureCoord = in_TextureCoord;


}

 

Any help appreciated.

 

Thanks!

 

 

 
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Try this:

 

void glOrtho(float* out, float left, float right,float bottom, float top,float near, float far)
{
    
    float a = 2.0f / (right - left);
    float b = 2.0f / (top - bottom);
    float c = -2.0f / (far - near);
    
    float tx = - (right + left)/(right - left);
    float ty = - (top + bottom)/(top - bottom);
    float tz = - (far + near)/(far - near);
    
    float ortho[16] = {
        a, 0, 0, 0,
        0, b, 0, 0,
        0, 0, c, 0,
        tx, ty, tz, 1
    };
    
    memcpy(out, ortho, sizeof(float)*16);
}

 

Your vertex program should be fine if it works with your perspective matrix.  Let me know if this fixes your problem, this has been confirmed to work for OpenGL ES 2.0.

 

Shogun.

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Thanks Shogun - but isn't that matrix the same as the one I have, if I have 0 as left and 680 as right for example (and same for top/bottom/height)?

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What geometry are you feeding it?

 

A mistake I see sometimes is that people will be drawing, say, a quad: [b](-1,-1,0) -> (1,1,0)[/b] or some other geometry on a very small scale. Which will work fine with a perspective matrix, since you can always move your camera to near enough the object that it appears as large in the viewport as you need it to be. But in a screen-size orthographic projection, that 2x2 size quad will only ever be 2x2 pixels on screen, regardless of where you put your camera. This makes switching from perspective to ortho tricky, in that you have to think about the scale of your objects. If you make your base unit to be a meter for example (ie, 1 unit=1 meter) then you might have a character model that is 2 units tall. In an orthographic projection, this object will only be 2 pixels tall.

 

So if your geometry is small, you either need to scale the geometry or include a scaling factor when constructing your matrix.

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Well right now all I am trying to do is get a 40 pixel by 40 sprite displayed on the screen.  I have tried messing around with the size of the sprite to no avail.  Once I get home I will take a look at the code and give you some more details about it, but I have a feeling you ar right about the scaling/size/etc.

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Ok, so I have a sprite with x,y,z (10,10,0)  with w/h 10fx10f

 

I have left as 0 and right as 680

top as as 480, bottom as 0

 

Still I see nothing.

 

Also, I am using lwjgl, in which if you have a matrix4f, I think m23 means 2nd column, 3rd row. 

 

 

 

  

 

              Sprite sprite = new Sprite("assets/texture/redman.png", 50f, 50f, 10f, 10f );
 
projectionMatrix.m00 = 2/WIDTH;
        projectionMatrix.m11 = 2/HEIGHT;
        projectionMatrix.m22 = -2/(far_plane - near_plane);
        projectionMatrix.m30 = -1;
        projectionMatrix.m31 = -1;
        projectionMatrix.m32 = -((far_plane + near_plane)/(far_plane - near_plane));
        projectionMatrix.m33 = 1;

         cameraPos = new Vector3f(0,0,-1); ...

 

 

//Translate camera        
Matrix4f.translate(cameraPos, viewMatrix, viewMatrix);

        //Scale, translate and rotate model
        Matrix4f.scale(modelScale, modelMatrix, modelMatrix);
        Matrix4f.translate(modelPos, modelMatrix, modelMatrix);
        Matrix4f.rotate((float)Math.toRadians(modelAngle.z), new Vector3f(0,0,1), modelMatrix, modelMatrix);
        Matrix4f.rotate((float)Math.toRadians(modelAngle.y), new Vector3f(0,1,0), modelMatrix, modelMatrix);
        Matrix4f.rotate((float)Math.toRadians(modelAngle.x), new Vector3f(1,0,0), modelMatrix, modelMatrix);
 

 

 

..

Edited by sufimaster
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Have you tried disabling back face culling?

If your sprite doesn't have the correct winding, and back face culling is enabled , then it will not be drawn.

 

Also, in OpenGL the Y coordinate goes from bottom to top, so make sure that you are positioning the sprite inside the view (unlike the projection matrix where point 0.0, 0.0, 0.0, is the center of the screen, on an orthographic projection, it is the left bottom corner).

Although you can change this when you build the matrix.

You can also try a bigger sprite, like with a size of (100, 100).

 

One last note. You should try the function Shogun posted to calculate your matrix. Somehow, and i might be wrong, but it doesn't seem like your matrix is being calculated like it should (again, it may be just me).

 

Don't know if this is your problem, but try it and see.

Edited by __SKYe
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Ok- so i used Shogun's method, I messed around with the camera position and near and far plane.

 

Now:

near plane is 1f, far plane is 100f (though I dont see why this matters in an ortho projection)

Camera is located at 0,0,-1

 

Sprite is at 0,0,0 dim: 10x10 pixels.

 

I can actually see it on the screen now, but it seems like everything is vanishing at the origin point, as attached:

 

This is supposed to be a quad, but the perspective is all wonky. Any idea what's going on now?

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How are you sending this to your shader?  Are you multiplying it by your previous projection matrix before doing so?  These are two other areas where things can go wrong, and you haven't provided sufficient info to diagnose.

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This is how I send it to the shader: I know this part works since the perspective projection matrix works, but the ortho produces wierd results. 

 

As for my previous projection matrix - this is my first projection matrix - so I am not multiplying it by anything before sending it tothe shader. Should I be?

///upload matrices to uniform varis in shader
        GL20.glUseProgram(shaderObject.pId);
        
        projectionMatrix.store(matrix44Buffer);matrix44Buffer.flip();            
        GL20.glUniformMatrix4(projectionMatrixLocation, false, matrix44Buffer);
        
        viewMatrix.store(matrix44Buffer);matrix44Buffer.flip();
        GL20.glUniformMatrix4(viewMatrixLocation, false, matrix44Buffer);
        
        modelMatrix.store(matrix44Buffer);matrix44Buffer.flip();
        GL20.glUniformMatrix4(modelMatrixLocation, false, matrix44Buffer);
        
        
        GL20.glUseProgram(0);
 
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I managed to figure this out - apparently I was not accessing the matrix elements correctly - transposing their accessors = m03 instead of m30 for example for the translation matrix.  All is well now! Thanks everyone. 

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I noticed you're transforming your matrices as so:

 

projectionMatrix * viewMatrix * modelMatrix

 

This is also how I do it, but a lot of people do it opposite (and I think glOrtho does too). If this is the case, than the glOrtho() code provided above may be correct, but it just has to be transposed. Here's my camera's ortho code:

void Camera::Ortho(float width, float height, float zNear, float zFar)
	{
		Ortho(0.0f, width, 0.0f, height, zNear, zFar);
	}
	
	
	void Camera::Ortho(float left, float right, float top, float bottom, float zNear, float zFar)
	{
		// find the translation vector
		const float tx = - (right + left)/(right - left);
		const float ty = - (top + bottom)/(top - bottom);
		const float tz = - (zFar + zNear)/(zFar - zNear);
		
		// column 1
		projMat.m[ 0] = 2.0f / (right - left);
		projMat.m[ 1] = 0;
		projMat.m[ 2] = 0;
		projMat.m[ 3] = 0;
		
		// column 2
		projMat.m[ 4] = 0;
		projMat.m[ 5] = 2.0f / (top - bottom);
		projMat.m[ 6] = 0;
		projMat.m[ 7] = 0;
		
		// column 3
		projMat.m[ 8] = 0;
		projMat.m[ 9] = 0;
		projMat.m[10] = -2.0f / (zFar - zNear);
		projMat.m[11] = 0;
		
		// column 4
		projMat.m[12] = tx;
		projMat.m[13] = ty;
		projMat.m[14] = tz;
		projMat.m[15] = 1;
		
		mode = PROJECTION_MODE_ORTHOGONAL;
		Update(0.0f); // update the camera
	}

 

The first method is just a shortcut I created where the origin is in the upper-left corner of the screen is treated as the origin. The second method is what would emulate glOrtho() from the old OpenGL days, only it should be multiplied first like you have it.

 

Also, keep in mind that all matrix elements are an array of 16 floats stored on a column-by-column basis:

m[ 0] m[ 4] m[ 8] m[12]
m[ 1] m[ 5] m[ 9] m[13]
m[ 2] m[ 6] m[10] m[14]
m[ 3] m[ 7] m[11] m[15]

 

When you feed the matrix as a 4x4 to OpenGL, make sure that the transpose argument is GL_FALSE. That's very important.

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When you feed the matrix as a 4x4 to OpenGL, make sure that the transpose argument is GL_FALSE. That's very important.

 

Not really; please see http://www.opengl.org/archives/resources/faq/technical/transformations.htm

 

Column-major versus row-major is purely a notational convention. Note that post-multiplying with column-major matrices produces the same result as pre-multiplying with row-major matrices. The OpenGL Specification and the OpenGL Reference Manual both use column-major notation. You can use any notation, as long as it's clearly stated.

 

Also

 

Sadly, the use of column-major format in the spec and blue book has resulted in endless confusion in the OpenGL programming community. Column-major notation suggests that matrices are not laid out in memory as a programmer would expect.

 

OpenGL is perfectly capable of using either row-major or column-major matrices (and all the more so with the programmable pipeline); the only important thing is that you be consistent in your code.  That just means getting the multiplication orders correct in both your C/C++ code and your shader code.  You don't have to use the same major-ness in your shaders as you use in your C/C++: either transpose before sending (either manually or by using GL_TRUE in your glUniformMatrix call) or flip the multiplication order in your shader code.

 

So what's actually important is that you know which major-ness each element of your code uses and that you set things up appropriately for that; otherwise the importance of row-major versus column-major is hugely overstated.

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      I do not want to deal with static tiles, since I plan to slowly animate the starfield. So, I am trying to figure out how to generate a random starfield for the entire map.
      I feel that using a uniform distribution for the stars will not do the trick. Instead I would like something similar to the screenshot below, taken from the game Star Wars: Empire At War (all credits to Lucasfilm, Disney, and so on...).

      Is there someone who could have an idea of a distribution which could result in such a starfield?
      Any insight would be appreciated
    • By afraidofdark
      I have just noticed that, in quake 3 and half - life, dynamic models are effected from light map. For example in dark areas, gun that player holds seems darker. How did they achieve this effect ? I can use image based lighting techniques however (Like placing an environment probe and using it for reflections and ambient lighting), this tech wasn't used in games back then, so there must be a simpler method to do this.
      Here is a link that shows how modern engines does it. Indirect Lighting Cache It would be nice if you know a paper that explains this technique. Can I apply this to quake 3' s light map generator and bsp format ?
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