If you’ve used a real-time graphics package (like DirectX or OpenGL), you’ve probably noticed how pervasive transformations are. These packages use transformations for all sorts of things!

Although using transformations for all these things is mathematically elegant, it also makes reasoning about transformations a little complex. To make things easier, graphics developers invented a system for classifying and combining the different types of graphics transformations. This sytem is called the transformation pipeline. It’s accepted nearly universally, even though many people have their own twist on it.

This post will be about both of these aspects of the transformation pipeline. We’ll start with classification, and then move on to combination.

Our first topic will be vector spaces.

Vector Spaces

A vector space is just a meaning given to the coordinate axis. This meaning is chosen by convention. For example, we might define one vector space by saying the $X$ and $Z$ axes align with the ground plane, with $Y$ pointing toward the sky. We might define another vector space with $X$ and $Y$ being parallel to a camera’s lens, with $Z$ pointing into that lens.

Vector spaces work on vectors a lot like how units work on numbers. For example, the equation $5 + 2 = 7$ …

  • is correct when all the terms have the same unit: $5m + 2m = 7m$
  • makes no sense when the units don’t match: $5m + 2ft = 7(…?)$

For an equation with scalar numbers to make sense, the units attached to these numbers must match. If you have different units on your terms, you can convert some terms to a different unit of measurement. Once you’re done converting, the equation will make sense again:

$5 m + 2 ft * \dfrac{0.3048 m}{ft} = 5 m + 0.6096 m = 5.6096 m$

Vector spaces do more or less the same thing with vectors. If you have a vector equation (with vector additions, dot products, or what-have-you), the math only works if every vector is in the same vector space. Vector transformations allow you to convert a vector from one vector space to another. To continue with our analogy, these transformations are like unit conversions.

You might ask, then, how to transform vectors between vector spaces …

Transformations

You’ve probably seen vector transformations before. They allow you to scale, rotate and translate the objects in your scene.

Mathematically, each transformation is represented by a matrix. Say you have a vector in the vector space $A$ (“$A$-space” for short), and you’d like to transform it to $B$-space. If you already have the transformation matrix $M_{AB}$, you can simply multiply it with the $A$-space vector. The result is the $B$-space vector. So, $v_B = M_{AB} \times v_A$.

Often $M_{AB}$ is simple. For example, if $B$-space is the same as $A$-space, $M_{AB}$ is the identity matrix. If the origins of both spaces are the same, but one unit in $A$ space equals two units in $B$ space, then $M_{AB}$ is a scale matrix. You can build complex $M_{AB}$’s by daisy-chaining the simpler ones. They are combined via matrix multiplication:

  • If $M_{AB}$ transforms from $A$-space to $B$-space
  • And $M_{BC}$ transforms from $B$-space to $C$-space
  • Then $M_{AC} = M_{BC} \times M_{AB}$ transforms from $A$-space to $C$-space.

Intuitively, you might say $M_{AC}$ transforms from $A$-space to $B$-space to $C$-space. But keep in mind $M_{AB}$ and $M_{AC}$ have the same dimensions. Multiplying either by some vector requires an equal amount of computation.

If you have $M_{AB}$, which transforms from $A$-space to $B$-space, you can obtain $M_{BA}$, which goes from $B$-space to $A$-space, by inverting $M_{AB}$. If you had daisy-chained multiple transformations, you can obtain the inverse by reversing the order of the steps and inverting each step. For example:

  • If $M_{AC} = M_{BC} \times M_{AB}$
  • Then $M_{CA} = M_{BA} \times M_{CB} = (M_{AB})^{-1} \times (M_{BC})^{-1}$

With this background out of the way, we can get a little more concrete.

Transforming Objects

Scenes are composed of triangles. When you render your scene, you’re sending a lump of different triangles to the graphics system. While the graphics system just sees a bunch of triangles in the end, we humans prefer to subdivide these triangles into logical things. ‘These triangles form a beach ball,’ we might say, ‘while these triangles form a swing set.’

Each one of these logical things is called an object. Graphics developers, being human, like to talk about objects instead of triangles, but keep in mind an object is just a bunch of triangles. Each triangle in an object is composed of three vertices. Each vertex has a position, which is a vector in some vector space.

When we say an object is in $A$-space, we really mean the position of each vertex in the object is defined in $A$-space. When we say we transform an object form $A$-space to $B$-space, we mean we transform the position of every vertex in the object from $A$-space to $B$-space.

A natural next question is: how do we transform objects?

The Transformation Pipeline

We mentioned earlier that there are a few different things we use transformations for, and there’s a system for classifying and combining these transformations. This system is called the transformation pipeline, and it’s not to be confused with the rendering pipeline!

Here’s a rough outline of the transformation pipeline:

  1. Start with a bunch of objects (a chair, a table, etc)
  2. Transform each object so that it appears in the right part of the scene
  3. Transform all objects in the scene, so they’re positioned as they would be seen from the point of view of the camera.
  4. Transform all the objects again, this time making things closer to the camera seem bigger than things farther from the camera.
  5. Drop the $Z$ coordinate of all the vertices that have been transformed so far, and then stitch them together to create triangles on the screen.

Each of the transformation steps has a common name, as do the vector spaces each transforms between. All that’s left to do now is list them!

Vector Spaces in the Transformation Pipeline

There are five standard vector spaces in the pipeline. In theory, you can define these spaces however you like. Where possible, we’ll give the concrete definitions that OpenGL uses.

Object Space

We mentioned that the positions of all the vertices in an object reside in some vector space. Object space, then, is just a convenient coordinate system for defining those positions. Notice we said an object: each object in your scene will likely have its own object space.

Consider that hypothetical beach ball from before. We might define the object space for that beach ball like this:

  • The origin of the beach ball’s object space coincides with the center of the ball
  • The $Y$ axis aligns with the north and south poles of the beach ball
  • The unit length along any axis is the same as the radius of the beach ball

These choices were arbitrary. For example, we might instead have decided to make the origin of our object space coincide with the north pole. Or the south pole. Or a random point on the ball. Or anywhere else outside the ball. We chose the space above just to make it convenient to specify the position of each vertex in the ball.

World Space

We mentioned there were multiple object spaces (one for each object). To compose objects into the same scene, it’s useful to think of a scene-level vector space in which objects can be placed together. This space is called world space.

World space is typically defined so the center of the scene is at the origin. Sometimes the unit length in world space corresponds to physical length. This helps the scene creator reason about world-space coordinates.

Eye Space

(also known as camera space, or view space)

The camera allows us to conceptualize the viewer’s position and orientation. Eye space is simply the camera’s object space. In OpenGL, it is defined so that the camera is at the origin, looking down the negative $Z$ axis, with $Y$ pointing up. As we’ll see later, eye space is important for rendering. Although scenes are easiest set up in world space, it is easiest to render the scene after transforming all the objects in the scene to eye space. Transformations to the rescue!

Clip Space

Clip space defines vertices relative to where they appear on your screen. In OpenGL …

  • the origin of clip space is defined as the center of your monitor.
  • $X = -1$ and $X = 1$ correspond to the left and right boundaries of your monitor
  • $Y = 1$ and $Y = -1$ correspond to the top and bottom of your monitor
  • The $Z$ component determines visibility when triangles overlap. A vertex with $Z = 0$ is in front of everything; a vertex with $Z = -1$ is behind everything.

Anything outside the bounds listed above will not be rendered. The graphics pipeline will remove geometry outside these bounds, in a process called clipping (hence the name clip space).

Screen Space

Screen space is defined to make it easy to work with pixels. It’s a 2D space, with no $Z$ coordinate.

Let $w$ and $h$ be with width and height of your display, in pixels. In screen space,

  • $(0, 0)$ corresponds to the top-left corner of your display
  • $(w-1, 0)$ corresponds to the top-right
  • $(0, h-1)$ corresponds to the bottom-left
  • $(w-1, h-1)$ corresponds to the bottom-right

Thus every 2D coordinate $(i, j)$ is the index of a pixel. This is why screen-space coordinates are sometimes called pixel coordinates.

Fitting These Together

To recap, we listed five spaces above:

  • Object space: convenient for defining vertex positions of an object
  • World space: convenient for setting up those objects in one scene
  • Eye space: the same scene, but from the camera’s point of view
  • Clip space: the final space containing what will be rendered
  • Screen space: a pixel-friendly coordinate system for clip space

A typical workflow using these spaces might work as follows:

  1. An artist creates a mesh, defined in some object space that was convenient for the artist when he created the mesh.
  2. A graphics programmer writes code to transform each object into world space, creating a coherent scene.
  3. The graphics programmer writes code to let the user manipulate their viewpoint, and uses that input to transform the scene to eye space.
  4. The graphics programmer uses a transformation to foreshorten the scene, making nearby objects seem closer. This results in clip-space coordinates.
  5. The graphics programmer sends the object vertices and transformations to the graphics system.
  6. The graphics system renders these vertices on the screen, in screen space.

Matrices in the Transformation Pipeline

Now that we’ve finished naming spaces, it’s time to name the transformation matrices. There are four transformation steps in our pipeline, but only three of them are implemented using matrices.

Object $\rightarrow$ World

Called the world or model matrix. A composition of transformations that define the position, size and orientation of each object in the scene. Since there is one object space for each object, there is also one world matrix for each object.

World $\rightarrow$ Eye

Called the view matrix. This matrix is used to translate and rotate each object in the scene so that the entire scene is defined in eye space, relative to the camera. This is the opposite of the sequence of steps that place the camera into the scene (think back to matrix inversions).

Eye $\rightarrow$ Clip

Called the projection matrix. This step usually includes perspectivization, which makes closer objects seem bigger. This simulates visual perspective.

Clip $\rightarrow$ Screen

This does not have a name, because it’s usually handled by the graphics system instead of the programmer. The graphics system implements this step by dropping the $Z$ term and scaling the $X$ and $Y$ terms by the width and height of the screen, in pixels.

Implementing the Transformation Pipeline

That’s the transformation pipeline! It goes by many names, its steps are referred to by different names, and sometimes different steps are grouped in weird ways; however, the core pipeline is still pretty much the same in every instance. It’s always the same sequence of transformation steps.

Let’s close out by thinking how we could use this transformation pipeline to render the scene. Here’s a simple algorithm:

// Start in object space
orig := load_objects()

// object -> world space
objects := empty list
for i in (0 -> orig.length):
    objects[i] = orig[i].doWorldMatrix();

// world -> view space
for i in (0 -> objects.length):
    objects[i] := camera.doViewMatrix(objects[i])

// view -> clip space
for i in (0 -> objects.length):
    objects[i] := camera.doProjectionMatrix(objects[i])

// Let the graphics system do clip -> screen space and render the scene
for i in (0 -> objects.length):
    for j in (0 -> objects[i].numTriangles):
        graphicsSystem.drawTriangle(object[i].triangle[j])

No graphics system (or at least none known to the author) is set up to work this way. This is because you can speed things up by swapping the two loops. That is, the above snippet transforms objects like this:

for each transformation step
    for each object
        transform the object

Whereas we can speed things up by doing this:

for each object
    for each transformation step
        transform the object

We save space by not needing to store the intermediate object vertices. We can also have the graphics card do all our matrix-vector multiplications. The latter is significantly faster than CPU-based matrix-vector multiplications, since the GPU is massively parallel.

The snippet resulting from swapping the loops looks like this:

// Start in object space
objects := load_objects()

// Render the objects
for i in (0 -> objects.length):
    objectToClip := identityMatrix()
                  * objects[i].getWorldMatrix()
                  * camera.getViewMatrix()
                  * camera.getProjectionMatrix()

    for j in (0 -> objects[i].numTriangles);
        graphicsSystem.transformAndDrawTriangle(
            object[i].triangle[j],
            objectToClip
        )

And that’s really all there is to it! Of course, the logic behind getWorldMatrix, getViewMatrix and getProjectionMatrix is likely might be complex.

One Final Aside

We mentioned the transformation pipeline is distinct from the rendering pipeline. How do the two work together?

  • The transformation pipeline runs on the CPU. It takes in the application’s state and produces a single transformation matrix for each object in the scene. We outlined it in the pseudocode above.

  • The rendering pipeline runs on the GPU. It takes a triangle and a transformation matrix, and draws that triangle on the screen. We invoked the rendering pipeline in the pseudocode above when we called graphicsSystem.transformAndDrawTriangle.

Where to Next?

If you’re developing shaders, you may be interested in the OpenGL-based followup article Shader Transformations, which builds on what you learned in this article. Otherwise, onward to your next graphics programming adventure! :D