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Nick of ZA

OpenGL Transform matrix application order

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I'm building my first OpenGL demo to incorporate into the game I'm writing. I wonder if you gentlefolk might help clarify the transformation process for me. I have a basic vertex and fragment shaders up and running already.

Here's what I see as the method for getting 3D objects perspective-transformed to screen:

(pseudocode)

[CODE]

On update:

Create the camera matrix: c = camRotation * camTranslation
Create the view matrix by inverting the camera matrix: v = c^-1
Create the projection matrix using the standard perspective projection matrix terms
Create view-projection matrix: vp = p * v

For each entity
Create a model matrix using entity world position, rotation, and scale values
Multiply this specific entity's world transform matrix (the model matrix): mvp = vp * m
Set mvp as a uniform for vertex shader
glDrawElements(...);
[/CODE]

Queries:[list=1]
[*]Please offer your advice on whether or not the above structure is sensible.
[*]What options do I have to reduce the number of draw calls? Merging static geometry into a single vertex list using a common texture atlas seems to be the only option?
[*]Re the MVP matrix above, why do some sources present the final matrix value as -1, and others as 1? What should I use?
[/list]
Any other tips, suggestions on this structure welcome.

Primary references:

[url="http://www.opengl.org/wiki/Viewing_and_Transformations"]OpenGL wiki page on viewing and transformations[/url]
[url="http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Chapter-3:-3D-transformation-and-projection.html"]Joe Groff's tutorial on transformation and projection[/url] Edited by NickWiggill

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The order of the projection-view matrix is wrong. It shall be Proj*view. So the MVP matrix would be P*V*M. Some of this is done in the shader, some is not. It depends on the complexity of your design. The shader is much faster to compute this, but if a result stays the same for every pixel, then you might as well do it in the main application before sending it to the shader. You don't need a "window matrix", it is included in the projection matrix.

The vertex shader now need to compute PVM*v, for each vertex 'v'.

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Hi larspensjo,

Thanks for clarifying, I've updated the question by removing the reference to the "window" matrix.

Also could you confirm the order then, for matrix multiplication:
[source lang="java"]
modelViewMatrix = viewMatrix * modelMatrix; //??
modelViewProjectionMatrix = projectionMatrix * modelViewMatrix; //??[/source]

...Assuming matrix mul() ordering is thisMatrix * otherMatrix? Edited by NickWiggill

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[quote name='NickWiggill' timestamp='1341601398' post='4956430']
Hi larspensjo,

Thanks for clarifying, I've updated the question by removing the reference to the "window" matrix.

Also could you confirm the order then, for matrix multiplication:
[source lang="java"]
modelViewMatrix = viewMatrix * modelMatrix; //??
modelViewProjectionMatrix = projectionMatrix * modelViewMatrix; //??[/source]

...Assuming matrix mul() ordering is thisMatrix * otherMatrix?
[/quote]
Confirmed. If you are using C++, I recommend the package [url="http://glm.g-truc.net/"]glm[/url]. It is a matrix manipulation package, with a syntax close to the shader language.

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Ah, many thanks. Updated again with the multiplication order. This should allow me to proceed. I hope you have a good weekend too [img]http://public.gamedev.net//public/style_emoticons/default/smile.png[/img]

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Personally, I find this reference helpful: [url="http://www.songho.ca/opengl/gl_transform.html"]www.songho.ca/opengl/gl_transform.html[/url]

You could split up the modelview matrix into two parts, and so you would transform your vertices using your model matrix first, and then by your view matrix.

I like thinking in terms which order the transformations are applied, then afterwards rearrange things to match the multiplication order I am working with (i.e. "pre" or "post" multiplication.) Edited by scniton

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