Understanding OpenGL’s Matrices

I often get questions related to OpenGL’s matrices: how do they work, how do they get built, and so forth. This is a topic that I have been frequently confused by, myself, and I feel that it warrants further explanation.

To better understand OpenGL’s matrices, and how and why we use them, we first need to understand the OpenGL coordinate space.

Normalized device coordinates

At the heart of things, OpenGL 2.0 doesn’t really know anything about your coordinate space or about the matrices that you’re using. OpenGL only requires that when all of your transformations are done, things should be in normalized device coordinates.

These coordinates range from -1 to +1 on each axis, regardless of the shape or size of the actual screen. The bottom left corner will be at (-1, -1), and the top right corner will be at (1, 1). OpenGL will then map these coordinates onto the viewport that was configured with glViewport. The underlying operating system’s window manager will then map that viewport to the appropriate place on the screen.

Adjusting to the screen’s aspect ratio

While OpenGL wants things to be in normalized device coordinates, it’s hard to work with these directly. The first problem is that they always range from -1 to +1, so if you use these coordinates directly, your image might be stretched when switching from portrait mode to landscape mode.

The first thing you can do to get around this problem is to define an orthographic projection. Android has the orthoM method; other platforms will have something similar. Let’s take a closer look at Android’s method:

orthoM(float[] m, int mOffset, float left, float right, float bottom, float top, float near, float far)

To define a simple matrix that adjusts things for the screen’s aspect ratio, we might call orthoM as follows:

float aspectRatio = (float) width / (float) height;
orthoM(projectionMatrix, 0, -aspectRatio, aspectRatio, -1, 1, -1, 1);

Let’s say that the screen dimensions are 800×600. The call would proceed as follows:

orthoM(projectionMatrix, 0, -1.333, 1.333, -1, 1, -1, 1);

Although the screen is wider than it’s tall, we automatically adjust the coordinate space to match by mapping -(800/600) to the left side and (800/600) to the right side.

This also works when we switch to portrait mode:

orthoM(projectionMatrix, 0, -0.75, 0.75, -1, 1, -1, 1);

We shrink the width in order to compensate for the smaller screen.

At the heart of things, the orthographic projection matrix will still convert things to the [-1, 1] range, since that’s what OpenGL expects. It just provides a way to adjust our coordinate space, so that we can see more of our scene if the screen is wider, and less if the screen is narrower.

3D projections

What about 3D projections? For those, we can use frustumM:

frustumM(float[] m, int offset, float left, float right, float bottom, float top, float near, float far)

We could define a simple 3D projection as follows:

frustumM(projectionMatrix, 0, -aspectRatio, aspectRatio, -1, 1, 1, 100);

The near & far range are handled differently: both have to be positive, and far has to be greater than near. We also have to watch out for the Z axis: frustumM will actually invert it, so that the negative Z points into the distance!

This has to do with convention: normalized device coordinates are in a left-handed coordinate system, while by convention, when we use a projection matrix, we work in a right-handed coordinate system.

Below is a good image illustrating the situation:

Left-handed and right-handed coordinate systems. Source: http://viz.aset.psu.edu/gho/sem_notes/3d_fundamentals/html/3d_coordinates.html
Left-handed and right-handed coordinate systems. Source: http://viz.aset.psu.edu/gho/sem_notes/3d_fundamentals/html/3d_coordinates.html

The perspective divide

The perspective projection doesn’t actually create the 3D effect; for that, we need to do something called the perspective divide. Each coordinate in OpenGL actually has four components, X, Y, Z, and W. The projection matrix sets things up so that after multiplying with the projection matrix, each coordinate’s W will increase the further away the object is. OpenGL will then divide by w: X, Y, Z will be divided by W. The further away something is, the more it will be pulled towards the center of the screen.

This PDF goes into more detail: http://www.terathon.com/gdc07_lengyel.pdf

This image shows how the same coordinate gets closer to the center of the screen as the W value increases:

Perspective divide by W
Perspective divide by W

Say you have three XYZ source positions of the following:

(3, 3,    -3)
(3, 3,    -6)
(3, 3, -1000)

The second point is a little bit further, or more “into the screen” than the first, and the third point is much further away than the second point. An infinite projection matrix would convert the coordinates as follows:

(3, 3,   1,    3)
(3, 3,   4,    6)
(3, 3, 998, 1000)

That last component is W. Now, OpenGL will divide everything by W, so you get something like this:

(1,     1,     0.33...)
(0.5,   0.5,   0.66...)
(0.003, 0.003, 0.998  )

There are two side effects of this division by W:

  1. The depth buffer becomes non-linear. There is a lot of Z precision up close, but less further away.
  2. If you try to do translations, etc… on your vertices after the perspective projection, then you won’t get the results you expect. This is because many of these operations depend on the W being 1, while after perspective projection it can be something else.

Here is an example of the perspective divide: imagine that we have a perspective projection matrix that looks as follows:

 1.5, 0,  0,    0,
 0,   1,  0,    0,
 0,   0, -1.2, -2.2,
 0,   0, -1,    0

This will transform coordinates as follows:

(1, 1, -1, 1) --> (1.5, 1, -1,   1)
(1, 1, -2, 1) --> (1.5, 1,  0.2, 2)
(2, 2, -2, 1) --> (3,   2,  0.2, 2)

After division by W, we get this:

(1.5, 1, -1,   1) --> (1.5,  1,   -1  )
(1.5, 1,  0.2, 2) --> (0.75, 0.5,  0.1)
(3,   2,  0.2, 2) --> (1.5,  1,    0.1)

Notice that the projection matrix just sets up the W, and it’s the actual divide by OpenGL that does the perspective effect. To verify this, try it out with a matrix calculator:


The view and model matrices

In OpenGL, we commonly use two additional matrices: the view and model matrices:

The model matrix

This matrix is used to move a model somewhere in the world. For example, let’s say we have a car model, and it’s defined such that it is centered around (0, 0, 0). We can place one car at (5, 5, 5) by setting up a model matrix that translates everything by (5, 5, 5), and drawing the car model with this matrix. We can then easily add a second car to the scene by just adjusting the translation. The model matrix helps us to push stuff out into the world.

The view matrix

The view matrix is functionally equivalent to a camera. It does the same thing as a model matrix, but it applies the same transformations equally to every object in the scene. Moving the whole world 5 units towards us is the same as if we had walked 5 units forwards.

Order of operations

These matrices all have to be multiplied in a specific way, if we want our results to be correct. Let’s start with some basic definitions:

This is an original vertex, as defined inside one of our object models.
This is a vertex in world coordinates. We get to here by using a model matrix to push the model out into the world.
This is a vertex in eye coordinates. We get here by using a view matrix to move the entire scene around.
This is a vertex in clip coordinates (also known as homogeneous coordinates): this is the coordinate space after projection, but before the perspective divide.
This is a vertex in normalized device coordinates, and this is what we end up with after the perspective divide.

As we can see, getting to vertexndc is just a matter of applying each transformation in order. Let’s try to formulate this as an expression:

vertexndc = PerspectiveDivide(ProjectionMatrix * vertexeye)
vertexndc = PerspectiveDivide(ProjectionMatrix * ViewMatrix * vertexworld)
vertexndc = PerspectiveDivide(ProjectionMatrix * ViewMatrix * ModelMatrix * vertexmodel)

OpenGL takes care of the perspective division for us, so we don’t actually need to worry about that. All we need to worry about is the order of operations; since matrix multiplication is non commutative, we’ll get a different result depending on the order.

Column-major versus row-major order

A final point of confusion is often the layout of matrices in memory. OpenGL follows column-major order, meaning that the array offsets are specified like this:

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

m[0] … m[3] refer to the first column of the matrix.

Please let me know your questions, comments, and feedback!

Enhanced by Zemanta

About the book

Android is booming like never before, with millions of devices shipping every day. In OpenGL ES 2 for Android: A Quick-Start Guide, you’ll learn all about shaders and the OpenGL pipeline, and discover the power of OpenGL ES 2.0, which is much more feature-rich than its predecessor.

It’s never been a better time to learn how to create your own 3D games and live wallpapers. If you can program in Java and you have a creative vision that you’d like to share with the world, then this is the book for you.


Author: Admin

Kevin is the author of OpenGL ES 2 for Android: A Quick-Start Guide. He also has extensive experience in Android development.

22 thoughts on “Understanding OpenGL’s Matrices”

  1. Thanks a lot, there are just people like you who love to share their knowledge and experience make the world better.

  2. Thank you very much for all of these tutorials, I will be spending some time getting to know them and learning the wonders of OpenGL =). Keep it up, it’s very much appreciated.

  3. 10qvm, Some times when I see that kind of non-selfish-selfpromoting-money-chasing work I imidiatelly have a graete desire to do stuff just like that.

    thanks for making the wrold a better place =]

  4. First, thanks for sharing the concept of OpenGL’s Matrix. But I would like to ask some questions before I misunderstood your explanations or just to make sure I got it correctly.

    1. Although the ndc coordinates system are a left-handed coordinate system, we still do the modelling transformation according to right-handed coordinate system right? and everthing will be inverted automatically according to left-handed coordinate system.

    2. What the difference between Projection matrix (we create by calling frustum) and Perspective projection matrix? are they the same matrix? In Augmented Reality case, we have camera parameter (perpective projection matrix) and we need not to create it again.


    1. The right-handed / left-handed conversion is actually done by the projection matrix. You don’t have to do things this way, and you could use left-handed throughout. By default, Android’s Matrix class will set things up so that you work with a right-handed coordinate space.

      For the second question, there’s no difference between a frustum and a perspective projection matrix — they both do the same job and look similar. The difference is the parameters you use to set them up; for the frustum, you set up the near, far, left, right, etc… directly, while for the perspective call, you use the “field of vision” to set up the frustum. At the end, you still end up with a frustum with both calls.

  5. (3, 3, -3, 3)
    (3, 3, -6, 6)

    That last component is W. Now, OpenGL will divide everything by W, so you get something like this:

    (1, 1, -1)
    (0.5, 0.5, -0.5)

    The last point doesn’t should be (0.5, 0.5, -1) ?

    1. Thanks, good catch. Somehow it made sense to me at the time, but I guess it doesn’t. 😉 I rewrote using actual results from an “infinite projection matrix”.

  6. Hi, Im having a hard time understanding the W component. what does it really do. and does it only requires 1 and 0 as value?

  7. Can you please tell me, from where the variable “projectionMatrix” come. you didn’t declared such variable but you are using it like “orthoM(projectionMatrix, 0, -aspectRatio, aspectRatio, -1, 1, -1, 1)”. waiting for reply.
    thanks in advance

  8. Thank you for your kind reply. I’m new in openGL and I’m feeling a bit difficult in it, if you can please suggest me any openGL tutorial for beginners.

  9. vertexndc = PerspectiveDivide(ProjectionMatrix * vertexeye)
    vertexndc = PerspectiveDivide(ProjectionMatrix * ViewMatrix * vertexworld)
    vertexndc = PerspectiveDivide(ProjectionMatrix * ViewMatrix * ModelMatrix * vertexmodel)

    Thanks for sharing…
    Are all these therefore equal? like
    PerspectiveDivide(ProjectionMatrix * vertexeye) = PerspectiveDivide(ProjectionMatrix * ViewMatrix * vertexworld) = PerspectiveDivide(ProjectionMatrix * ViewMatrix * ModelMatrix * vertexmodel)? and all yield vertexndc? or did you not mean to have vertexndc on the left side of each operation or is it that no matter which the perspective divide is applied to we still arrive at the vertexndc, or are there more that one vertexndc? or are there ndc’s for the vertex, and the eye and the world?

    Please, what Am I missing?


    1. So these are all intended to be equal, and what it’s showing is how we can get from a vertex in model space to a vertex in normalized device coordinates, by multiplying it by each successive matrix.

      For example, to get the vertex in world coordinates, we multiply the position in model space by the model matrix:
      vertexworld = ModelMatrix * vertexmodel

      Now that we have it in world coordinates, we can get it in eye coordinates (relative to the camera) like this:
      vertexeye = ViewMatrix * vertexworld

      Which is really the same thing as this:
      vertexeye = ViewMatrix * ModelMatrix * vertexmodel

      And so on up to ndc coordinates.

  10. Hi, thank you so very much for explaining all these fundamental stuff which a lot of other sites actually is missing making them hard to understand. It’s tempting to skip the real basics as there’s so much other timeconsuming stuff in the area of 3D graphics and OpenGL, but knowing the matrices and coordinate systems is an absolutely must, so thank you again.

    I do have two questions regarding one of your paragraphs, or more like two statements, I hope you can confirm:
    “At the heart of things, OpenGL 2.0 doesn’t really know anything about your coordinate space or about the matrices that you’re using. OpenGL only requires that when all of your transformations are done, things should be in normalized device coordinates.”
    – Does this mean that you could input coordinates from your own coordinate system (not restricted to -1,1), and output will always be in NDC?
    – Is it true that NDC could be outside -1,1, but when they are, it means they are invisible and should be clipped in some way?

Leave a Reply

Your email address will not be published. Required fields are marked *