# OpenGL Having trouble with OpenGL matrices...

## Recommended Posts

bryanedds    119
Hello all, I just started learning some OpenGL coming from XNA. I'm trying to get my Matrix struct to be aligned how glLoadMatrixf expects. Here's the relevant parts -
	struct Matrix
{
float
M11, M12, M13, M14,
M21, M22, M23, M24,
M31, M32, M33, M34,
M41, M42, M43, M44;

operator float*();
};

I know that the fields are supposed to be in column-major order, but I'm not sure if this particular order is column-major or not. I think it's not, but when I transpose them, my rendering no longer works. Additionally, I'm not sure if I'm pushing the model, view, and projection matrices in the proper order while rendering. It works when I use glTranslatef, but not when I push my own world matrix instead. Here's my current attempt -
	glPushMatrix();
{
glPushMatrix();
{
glPushMatrix();
{
glTranslatef(0, 0, -10);
// render sphere - works, but probably shouldn't!

// render sphere - doesn't work...
}
glPopMatrix();
}
glPopMatrix();
}
glPopMatrix();

This seems to work, but like I said, when I try transposing the Matrix so that it's seemingly in column-major order, everything breaks down. Anyone have any clues as to what all is going wrong here? Thanks!

##### Share on other sites
yngvedh    122
I suspect you want to use glMultMatrix instead of glLoadMatrix.
You also don't need to use glPushMatrix unless you want to "rewind" the stack to a previous state.

##### Share on other sites
bryanedds    119
Ok, I have things reformed to what I believe was the suggestion.

Here is a snipped of my current Matrix code -

	struct Matrix	{		operator float*();		float			M11, M21, M31, M41,			M12, M22, M32, M42,			M13, M23, M33, M43,			M14, M24, M34, M44;	};

And here is a snippet of my drawing code -

	glMatrixMode(GL_MODELVIEW);	glLoadIdentity();	glMultMatrixf(Matrix::Transpose(camera.GetProjection()));	glMultMatrixf(Matrix::Transpose(camera.GetView()));	glPushMatrix();	{		glMultMatrixf(Matrix::Transpose(sphereView.GetTransform().GetWorldTransform()));		sphereView.Draw(timeElapsed);	}	glPopMatrix();

My question is: why do I have to transpose each matrix while drawing? My struct now has the fields in column-major order. Any ideas?

[Edited by - bryanedds on November 23, 2008 1:05:39 AM]

##### Share on other sites
NumberXaero    2624
In the second post, you simply changed the matrix element names, opengl doesnt refer to these. Transpose, GetProjection() and friends are still probably copying row1 in the first 4, row2 into the next 4, etc. What opengl understands is 16 elements with col1 in the first 4, etc.

float gl[16];
gl[0] = m.r1c1; gl[4] = m.r1c2; gl[8] = m.r1c3; gl[12] = m.r1c4;
gl[1] = m.r2c1; gl[5] = m.r2c2; gl[9] = m.r2c3; gl[13] = m.r2c4;
gl[2] = m.r3c1; gl[6] = m.r3c2; gl[10] = m.r3c3; gl[14] = m.r3c4;
gl[3] = m.r4c1; gl[7] = m.r4c2; gl[11] = m.r4c3; gl[15] = m.r4c4;

thats why the transpose is still needed, opengl doesnt care what you call you matrix class elements. Best to leave them named correctly like the first post, then do the above;

##### Share on other sites
bryanedds    119
Consider this part of the implementation -

	Matrix::operator float*()	{		return reinterpret_cast<float*>(this);	}

Consider that I'm passing the result of this reinterpret_cast to the gl function directly. From my eye, I didn't change the names of the fields inasmuch as I changed their order inside the struct. To my thinking, therefore, the order in which the fields are declared in the struct do matter. That is, if I do m[1][2] on this reinterpret_casted pointer, it should come out in the expected column-major alignment.

Please let me know if this is not the case :)

##### Share on other sites
snoutmate    362
Quote:
 This seems to work, but like I said, when I try transposing the Matrix so that it's seemingly in column-major order, everything breaks down.

Column-major vs. row-major is simply semantics. Te important part is that you are consistent in all operations. If your code works with opengl without transposing that means that internally you use column-major matrices and operations (withhout realizing it), and simply have mis-named the values.

##### Share on other sites
haegarr    7372
1) Supplying OpenGL with matrices is usually done in 2 steps. The projection is set-up usually like here:
glMatrixMode(GL_PROJECTION);
It is often done once, since most application need not change the projection afterwards. The model-view matrix is composed from the view matrix and the model matrices, and often varies from frame to frame, so it is set-up each time at the beginning of the rendering loop:
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glMultMatrix(someModel->worldFrame());
glPopMatrix();
where the latter 3 lines are repeated for each model that is rendred (it may be more complex, e.g. if deeper frame nesting is used).

2) Mathematically OpenGL uses column vectors with the homogeneous component at the 4th place. A matrix looks like:
[ Rxx Ryx Rzx Tx ][ Rxy Ryy Rzy Ty ][ Rxz Ryz Rzz Ty ][  0   0   0   1 ]
what can be interpreted as [Rxx Rxy Rxz]t denoting the x direction vector, [Ryx Ryy Ryz]t denoting the y direction vector, [Rzx Rzy Rzz]t denoting the z direction vector, and [Tx Ty Tz]t denoting the position. (Here the super-posed t denotes the transpose operator, since I wrote the column vectors as rows; its just for a convenient writing.)

Such a 2D construct has to be mapped to the 1D (linear) computer memory. OpenGL uses the column major order, what means that it walks down a column before it goes to the next one. In summary that means a layout like
Rxx Rxy Rxz 0 Ryx Ryy Ryz 0 Rzx Rzy Rzz 0 Tx Ty Tz 1

Now, if you remember back, D3D uses row vectors as well as row-major order. Notice please that both kinds of handling are changed! That means that the same matrix as above has to be transposed to yield in the mathematical representation:
[ Rxx Rxy Rxz 0 ][ Rxy Ryy Ryz 0 ][ Rzx Rzy Rzz 0 ][ Tx  Ty  Tz  1 ]

When mapping this in row major order to a linear memory, you'll get
Rxx Rxy Rxz 0 Ryx Ryy Ryz 0 Rzx Rzy Rzz 0 Tx Ty Tz 1
what is unsurprisingly the same as with OpenGL. I write "unsurprisingly" because both the vector kind as well as the order kind are changed so that both changes annul each other.

Hence, it is senseful to use the above layout. Notice please that this is nevertheless not the only possible layout. E.g. Collada uses an incompatible layout.

I hope that has cleared the issue.

EDIT: 2 syntax errors corrected.

[Edited by - haegarr on November 24, 2008 6:07:56 AM]

##### Share on other sites
bryanedds    119
haegar,

Wonderful post! Thank you so much for the time you put it into it! I have things working perfectly now. Thank you as well snoutmate and yngvedh!

Here's my final Matrix snippet -
	struct Matrix	{		float			M11, M12, M13, M14,			M21, M22, M23, M24,			M31, M32, M33, M34,			M41, M42, M43, M44;		operator float*();	};

Here's my final rendering code -
	glMatrixMode(GL_PROJECTION);	glLoadMatrixf(camera.GetProjection());	glMatrixMode(GL_MODELVIEW);	glLoadMatrixf(camera.GetView());	glPushMatrix();	{		glMultMatrixf(sphereView.GetTransform().GetWorldTransform());		sphereView.Draw(timeElapsed);	}	glPopMatrix();

Thank you again to everyone!

## Create an account or sign in to comment

You need to be a member in order to leave a comment

## Create an account

Sign up for a new account in our community. It's easy!

Register a new account

• ### Similar Content

• I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.

• There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window.
I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
anyone, please help me .. how to go further... to create an application like VR CAVE.

Thanks
• By cebugdev
hi all,

i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
thank you, and looking forward to positive replies.

• I have a few beginner questions about tesselation that I really have no clue.
The opengl wiki doesn't seem to talk anything about the details.

What is the relationship between TCS layout out and TES layout in?
How does the tesselator know how control points are organized?
e.g. If TES input requests triangles, but TCS can output N vertices.
What happens in this case?
http://www.informit.com/articles/article.aspx?p=2120983
the isoline example TCS out=4, but TES in=isoline.
And gl_TessCoord is only a single one.
So which ones are the control points?
How are tesselator building primitives?
• By Orella
I've been developing a 2D Engine using SFML + ImGui.
Here you can see an image
The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine.
I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui.
3D Editor preview
But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
If you can provide code will be better. And if you want me to provide any specific code tell me.
Thanks!

• 14
• 24
• 37
• 13
• 39