#### 3.1.6 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 main 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.

Important Matrix Ordering Note: One thing to bear in mind is that HLSL allows you to use 2 different ways to multiply a vector by a matrix - mul(v,m) or mul(m,v). The only difference between them is that the matrix is effectively transposed. You should use mul(m,v) with the matrices passed in from Ogre - this agrees with the shaders produced from tools like RenderMonkey, and is consistent with Cg too, but disagrees with the Dx9 SDK and FX Composer which use mul(v,m) - you will have to switch the parameters to mul() in those shaders.

Note that if you use the float3x4 / matrix3x4 type in your shader, bound to an OGRE auto-definition (such as bone matrices) you should use the column_major_matrices = false option (discussed below) in your program definition. This is because OGRE passes float3x4 as row-major to save constant space (3 float4’s rather than 4 float4’s with only the top 3 values used) and this tells OGRE to pass all matrices like this, so that you can use mul(m,v) consistently for all calculations. OGRE will also to tell the shader to compile in row-major form (you don’t have to set the /Zpr compile option or #pragma pack(row-major) option, OGRE does this for you). Note that passing bones in float4x3 form is not supported by OGRE, but you don’t need it given the above.

Advanced options

preprocessor_defines <defines>

  This allows you to define symbols which can be used inside the HLSL shader code to alter the behaviour (through #ifdef or #if clauses). Definitions are separated by ’;’ or ’,’ and may optionally have a ’=’ operator within them to specify a definition value. Those without an ’=’ will implicitly have a definition of 1.

column_major_matrices <true|false>

  The default for this option is ’true’ so that OGRE passes matrices auto-bound matrices in a form where mul(m,v) works. Setting this option to false does 2 things - it transpose auto-bound 4x4 matrices and also sets the /Zpr (row-major) option on the shader compilation. This means you can still use mul(m,v), but the matrix layout is row-major instead. This is only useful if you need to use bone matrices (float3x4) in a shader since it saves a float4 constant for every bone involved.

optimisation_level <opt>

  Set the optimisation level, which can be one of ’default’, ’none’, ’0’, ’1’, ’2’, or ’3’. This corresponds to the /O parameter of fxc.exe, except that in ’default’ mode, optimisation is disabled in debug mode and set to 1 in release mode (fxc.exe uses 1 all the time). Unsurprisingly the default value is ’default’. You may want to change this if you want to tweak the optimisation, for example if your shader gets so complex that it will not longer compile without some minimum level of optimisation.