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### 3.1.4 Declaring Vertex and Fragment Programs

In order to use a vertex or fragment program in your materials (See section 3.1.5 Using Vertex and Fragment Programs in a Pass), you first have to define them. A single program definition can be used by any number of materials, the only prerequisite is that a program must be defined before being referenced in the pass section of a material.

The definition of a program can either be embedded in the .material script itself (in which case it must precede any references to it in the script), or if you wish to use the same program across multiple .material files, you can define it in an external .program script. You define the program in exactly the same way whether you use a .program script or a .material script, the only difference is that all .program scripts are guaranteed to have been parsed before all .material scripts, so you can guarantee that your program has been defined before any .material script that might use it. Just like .material scripts, .program scripts will be read from any location which is on your resource path, and you can define many programs in a single script.

Vertex and fragment programs can be low-level (i.e. assembler code written to the specification of a given low level syntax such as vs_1_1 or arbfp1) or high-level such as nVidia's Cg language (See section High-level Programs). High level languages give you a number of advantages, such as being able to write more intuitive code, and possibly being able to target multiple architectures in a single program (for example, the same Cg program might be able to be used in both D3D and GL, whilst the equivalent low-level programs would require separate techniques, each targetting a different API). High-level programs also allow you to use named parameters instead of simply indexed ones, although parameters are not defined here, they are used in the Pass.

Here is an example of a definition of a low-level vertex program:

    vertex_program myVertexProgram asm
    {
        source myVertexProgram.asm
        syntax vs_1_1
    }

As you can see, that's very simple, and defining a fragment program is exactly the same, just with vertex_program replaced with fragment_program. You give the program a name in the header, followed by the word 'asm' to indicate that this is a low-level program. Inside the braces, you specify where the source is going to come from (and this is loaded from any of the resource locations as with other media), and also indicate the syntax being used. You might wonder why the syntax specification is required when many of the assembler syntaxes have a header identifying them anyway - well the reason is that the engine needs to know what syntax the program is in before reading it, because during compilation of the material, we want to skip progams which use an unsupportable syntax quickly, without loading the program first.

The current supported syntaxes are:

vs_1_1, vs_2_0, vs_2_x, vs_3_0
  These are the DirectX vertex shader assembler syntaxes.
arbvp1
  This is the OpenGL standard assembler format for vertex programs. It's roughly equivalent to DirectX vs_1_1.
ps_1_1, ps_1_2, ps_1_3, ps_1_4, ps_2_x, ps_3_0, ps_3_x
  These are the DirectX pixel shader (ie fragment program) assembler syntaxes.
  NOTE: for ATI 8500, 9000, 9100, 9200 hardware, ps_1_1 to ps_1_4 can also be used in OpenGL. The ATI 8500 to 9200 do not support arbfp1 but do support atifs extension in OpenGL which is very similar in function to ps_1_4 in DirectX. Ogre has a built in ps_1_x to atifs compiler that is automatically invoked when ps_1_x is used in OpenGL on ATI hardware.
arbfp1
  This is the OpenGL standard assembler format for fragment programs. It's roughly equivalent to ps_2_0, which means that not all cards that support basic pixel shaders under DirectX support arbfp1 (for example neither the GeForce3 or GeForce4 support arbfp1, but they do support ps_1_1).
fp20
  This is an nVidia-specific OpenGL syntax which allows you to use the 'nvparse' format for basic fragment programs. It actually uses NV_texture_shader and NV_register_combiners to provide functionality equivalent to DirectX's ps_1_1 under GL, but only for nVidia cards. However, since ATI cards adopted arbfp1 a little earlier than nVidia, it is mainly nVidia cards like the GeForce3 and GeForce4 that this will be useful for. You can find more information about nvparse at http://developer.nvidia.com/object/nvparse.html.

You can get a definitive list of the syntaxes supported by the current card by calling GpuProgramManager::getSingleton().getSupportedSyntax().

### Default Program Parameters

While defining a vertex or fragment program, you can also specify the default parameters to be used for materials which use it, unless they specifically override them. You do this by including a nested 'default_params' section, like so:

    vertex_program Ogre/CelShadingVP cg
    {
    	source Example_CelShading.cg
    	entry_point main_vp
    	profiles vs_1_1 arbvp1

    	default_params
    	{
    		param_named_auto lightPosition light_position_object_space 0
    		param_named_auto eyePosition camera_position_object_space
    		param_named_auto worldViewProj worldviewproj_matrix
    		param_named shininess float 10
    	}
    }

The syntax of the parameter definition is exactly the same as when you define parameters when using programs, See Program Parameter Specification. Defining default parameters allows you to avoid rebinding common parameters repeatedly (clearly in the above example, all but 'shininess' are unlikely to change between uses of the program) which makes your material declarations shorter.

## High-level Programs

Support for high level vertex and fragment programs is provided through plugins; this is to make sure that an application using OGRE can use as little or as much of the high-level program functionality as they like. OGRE currently supports 3 high-level program types, Cg (an API- and card-independent, high-level language which lets you write programs for both OpenGL and DirectX for lots of cards), DirectX 9 High-Level Shader Language (HLSL), and OpenGL Shader Language (GLSL). HLSL is provided for people who only want to deploy in DirectX, or who don't want to include the Cg plugin for whatever reason. GLSL is only for the OpenGL API and can not be used with the DirectX api. To support both DirectX and OpenGL you could write your shaders in both HLSL and GLSL along with having seperate techniques in the material script. To be honest, Cg is a better bet because it lets you stay API independent - and don't be put off by the fact that it's made by nVidia, it will happily compile programs down to vendor-independent standards like DirectX and OpenGL's assembler formats, so you're not limited to nVidia cards.

### Cg programs

In order to define Cg programs, you have to have to load Plugin_CgProgramManager.so/.dll at startup, either through plugins.cfg or through your own plugin loading code. They are very easy to define:

    fragment_program myCgFragmentProgram cg
    {
        source myCgFragmentProgram.cg
        entry_point main
        profiles ps_2_0 arbfp1
    }

There are a few differences between this and the assembler program - to begin with, we declare that the fragment program is of type 'cg' rather than 'asm', which indicates that it's a high-level program using Cg. The 'source' parameter is the same, except this time it's referencing a Cg source file instead of a file of assembler.
Here is where things start to change. Firstly, we need to define an 'entry_point', which is the name of a function in the Cg program which will be the first one called as part of the fragment program. Unlike assembler programs, which just run top-to-bottom, Cg programs can include multiple functions and as such you must specify the one which start the ball rolling.
Next, instead of a fixed 'syntax' parameter, you specify one or more 'profiles'; profiles are how Cg compiles a program down to the low-level assembler. The profiles have the same names as the assembler syntax codes mentioned above; the main difference is that you can list more than one, thus allowing the program to be compiled down to more low-level syntaxes so you can write a single high-level program which runs on both D3D and GL. You are advised to just enter the simplest profiles under which your programs can be compiled in order to give it the maximum compatibility. The ordering also matters; if a card supports more than one syntax then the one listed first will be used.

### DirectX9 HLSL

DirectX9 HLSL has a very similar language syntax to Cg but is tied to the DirectX API. The only benefit over Cg is that it only requires the DirectX 9 render system plugin, not any additional plugins. Declaring a DirectX9 HLSL program is very similar to Cg. Here's an example:

    vertex_program myHLSLVertexProgram hlsl
    {
        source myHLSLVertexProgram.txt
        entry_point main
        target vs_2_0
    }

As you can see, the syntax is almost identical, except that instead of 'profiles' with a list of assembler formats, you have a 'target' parameter which allows a single assembler target to be specified - obviously this has to be a DirectX assembler format syntax code.

### OpenGL GLSL

OpenGL GLSL has a similar language syntax to HLSL but is tied to the OpenGL API. The are a few benefits over Cg in that it only requires the OpenGL render system plugin, not any additional plugins. Declaring a OpenGL GLSL program is similar to Cg but simpler. Here's an example:

    vertex_program myGLSLVertexProgram glsl
    {
        source myGLSLVertexProgram.txt
    }

In GLSL, no entry point needs to be defined since it is always 'main()' and there is no target definition since GLSL source is compiled into native GPU code and not intermediate assembly.

GLSL supports the use of modular shaders. This means you can write GLSL external functions that can be used in multiple shaders.

    vertex_program myExteranalGLSLFunction1 glsl
    {
        source myExternalGLSLfunction1.txt
    }

    vertex_program myExteranalGLSLFunction2 glsl
    {
        source myExternalGLSLfunction2.txt
    }

    vertex_program myGLSLVertexProgram1 glsl
    {
        source myGLSLfunction.txt
        attach myExteranalGLSLFunction1 myExteranalGLSLFunction2
    }

    vertex_program myGLSLVertexProgram2 glsl
    {
        source myGLSLfunction.txt
        attach myExteranalGLSLFunction1
    }

External GLSL functions are attached to the program that needs them by using 'attach' and including the names of all external programs required on the same line seperated by spaces. This can be done for both vertex and fragment programs.

### GLSL Texture Samplers

To pass texture unit index values from the material script to texture samplers in glsl use 'int' type named parameters. See the example below:

excerpt from GLSL example.frag source:

    varying vec2 UV;
    uniform sampler2D diffuseMap;

    void main(void)
    {
    	gl_FragColor = texture2D(diffuseMap, UV);
    }

In material script:

    fragment_program myFragmentShader glsl
    {
      source example.frag
    }

    material exampleGLSLTexturing
    {
      technique
      {
        pass
        {
          fragment_program_ref myFragmentShader
          {
            param_named diffuseMap int 0
          }

          texture_unit
          {
            texture myTexture.jpg 2d
          }
        }
      }
    }

An index value of 0 refers to the first texture unit in the pass, an index value of 1 refers to the second unit in the pass and so on.

### Accessing OpenGL states in GLSL

GLSL can access most of the GL states directly so you do not need to pass these states through param_named_auto in the material script. This includes lights, material state, and all the matrices used in the openGL state ie model view matrix, worldview projection matrix etc.

### Skeletal Animation in Vertex Programs

You can implement skeletal animation in hardware by writing a vertex program which uses the per-vertex blending indices and blending weights, together with an array of world matrices (which will be provided for you by Ogre if you bind the automatic parameter 'world_matrix_array_3x4'). However, you need to communicate this support to Ogre so it does not perform skeletal animation in software for you. You do this by adding the following attribute to your vertex_program definition:

    	includes_skeletal_animation true

When you do this, any skeletally animated entity which uses this material will forgo the usual animation blend and will expect the vertex program to do it, for both vertex positions and normals.

### Vertex Programs With Shadows

When using shadows (See section 7. Shadows), the use of vertex programs can add some additional complexities, because Ogre can only automatically deal with everything when using the fixed-function pipeline. If you use vertex programs, and you are also using shadows, you may need to make some adjustments.

If you use stencil shadows, then any vertex programs which do vertex deformation can be a problem, because stencil shadows are calculated on the CPU, which does not have access to the modified vertices. If the vertex program is doing standard skeletal animation, this is ok (see section above) because Ogre knows how to replicate the effect in software, but any other vertex deformation cannot be replicated, and you will either have to accept that the shadow will not reflect this deformation, or you should turn off shadows for that object.

If you use texture shadows, then vertex deformation is acceptable; however, when rendering the object into the shadow texture (the shadow caster pass), the shadow has to be rendered in a solid colour (linked to the ambient colour). You must therefore provide an alternative vertex program, so Ogre provides you with a way of specifying one to use when rendering the caster, See section Shadows and Vertex Programs.

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This document was generated by Steve Streeting on , 28 2004 using texi2html