GLSL Programming/Unity/Shading in World Space

This tutorial introduces uniform variables. It is based on, , and.

In this tutorial we will look at a shader that changes the fragment color depending on its position in the world. The concept is not too complicated; however, there are extremely important applications, e.g. shading with lights and environment maps. We will also have a look at shaders in the real world; i.e., what is necessary to enable non-programmers to use your shaders?

Transforming from Object to World Space
As mentioned in, the attribute  specifies object coordinates, i.e. coordinates in the local object (or model) space of a mesh. The object space (or object coordinate system) is specific to each game object; however, all game objects are transformed into one common coordinate system — the world space.

If a game object is put directly into the world space, the object-to-world transformation is specified by the Transform component of the game object. To see it, select the object in the Scene View or the Hierarchy View and then find the Transform component in the Inspector View. There are parameters for “Position”, “Rotation” and “Scale” in the Transform component, which specify how vertices are transformed from object coordinates to world coordinates. (If a game object is part of a group of objects, which is shown in the Hierarchy View by means of indentation, then the Transform component only specifies the transformation from object coordinates of a game object to the object coordinates of the parent. In this case, the actual object-to-world transformation is given by the combination of the transformation of a object with the transformations of its parent, grandparent, etc.) The transformations of vertices by translations, rotations and scalings, as well as the combination of transformations and their representation as 4×4 matrices are discussed in.

Back to our example: the transformation from object space to world space is put into a 4×4 matrix, which is also known as “model matrix” (since this transformation is also known as “model transformation”). This matrix is available in the uniform variable (in Unity 5, maybe   in elder version), which is defined and used in the following shader:

Note that this shader makes sure that the definition of the uniform is included in both the vertex and the fragment shader (although this particular fragment shader doesn't need it). This is similar to the definition of varyings discussed in.

Usually, an OpenGL application has to set the value of uniform variables; however, Unity takes care of always setting the correct value of predefined uniform variables such as ; thus, we don't have to worry about it.

This shader transforms the vertex position to world space and gives it to the fragment shader in a varying. For the fragment shader the varying variable contains the interpolated position of the fragment in world coordinates. Based on the distance of this position to the origin of the world coordinate system, one of two colors is set. Thus, if you move an object with this shader around in the editor it will turn green near the origin of the world coordinate system. Farther away from the origin it will turn dark grey.

More Unity-Specific Uniforms
There are, in fact, several predefined uniform variables similar to. Here is a short list (including ), which appears in the shader codes of several tutorials: As the comments suggest, instead of defining all these uniforms (except  and  ), you could also include the file. However, for some unknown reason  and   are not included in this file; thus, we have to define them separately: Unity does not always update all of these uniforms. In particular,,  , and   are only set correctly for shader passes that are tagged appropriately, e.g. with   as the first line in the   block; see also.

More OpenGL-Specific Uniforms
Another class of built-in uniforms are defined for the OpenGL compability profile, for example the  matrix , which is equivalent to the matrix product   of two other built-in uniforms. The corresponding transformations are described in detail in.

As you can see in the shader above, these uniforms don't have to be defined; they are always available in GLSL shaders in Unity. If you had to define them, the definitions would look like this: In fact, the compability profile of OpenGL defines even more uniforms; see Chapter 7 of the “OpenGL Shading Language 4.10.6 Specification” available at Khronos' OpenGL page. Unity supports many of them but not all.

Some of these uniforms are arrays, e.g. In fact, an array of matrices ,, ...,   is available, where   is a built-in integer.

Computing the View Matrix
Traditionally, it is customary to do many computations in view space, which is just a rotated and translated version of world space (see for the details). Therefore, OpenGL offers only the product of the model matrix $$\mathrm{M}_{\text{object}\to\text{world}}$$ and the view matrix $$\mathrm{M}_{\text{world}\to\text{view}}$$, i.e. the model-view matrix $$\mathrm{M}_{\text{object}\to\text{view}}$$, which is available in the uniform. The view matrix is not available. Unity also doesn't provide it.

However,  is just the model matrix and   is the inverse model matrix. (Except that all but the bottom-right element have to be scaled by .) Thus, we can easily compute the view matrix. The mathematics looks like this:

$$\mathrm{M}_{\text{object}\to\text{view}} = \mathrm{M}_{\text{world}\to\text{view}} \mathrm{M}_{\text{object}\to\text{world}}\,$$ $$\Rightarrow \mathrm{M}_{\text{world}\to\text{view}} = \mathrm{M}_{\text{object}\to\text{view}} \mathrm{M}_{\text{object}\to\text{world}}^{-1} $$

In other words, the view matrix is the product of the model-view matrix and the inverse model matrix (which is  except for the bottom-right element, which is 1). Assuming that we have defined the uniforms  and , we can compute the view matrix this way in GLSL:

User-Specified Uniforms: Shader Properties
There is one more important type of uniform variables: uniforms that can be set by the user. Actually, these are called shader properties in Unity. You can think of them as parameters of the shader. A shader without parameters is usually used only by its programmer because even the smallest necessary change requires some programming. On the other hand, a shader using parameters with descriptive names can be used by other people, even non-programmers, e.g. CG artists. Imagine you are in a game development team and a CG artist asks you to adapt your shader for each of 100 design iterations. It should be obvious that a few parameters, which even a CG artist can play with, might save you a lot of time. Also, imagine you want to sell your shader: parameters will often dramatically increase the value of your shader.

Since the description of shader properties in Unity's ShaderLab reference is quite OK, here is only an example, how to use shader properties in our example. We first declare the properties and then define uniforms of the same names and corresponding types.

With these parameters, a non-programmer can modify the effect of our shader. This is nice; however, the properties of the shader (and in fact uniforms in general) can also be set by scripts! For example, a JavaScript attached to the game object that is using the shader can set the properties with these lines: Use  if you want to change the parameters for all objects that use this material and just   if you want to change the parameters only for one object. With scripting you could, for example, set the  to the position of another object (i.e. the   of its Transform component). In this way, you can specify a point just by moving another object around in the editor. In order to write such a script, select Create > JavaScript in the Project View and copy & paste this code: Then, you should attach the script to the object with the shader and drag & drop another object to the  variable of the script in the Inspector View.

Summary
Congratulations, you made it! (In case you wonder: yes, I'm also talking to myself here. ;) We discussed:
 * How to transform a vertex into world coordinates.
 * The most important Unity-specific uniforms that are supported by Unity.
 * The most important OpenGL-specific uniforms that are supported by Unity.
 * How to make a shader more useful and valuable by adding shader properties.