Custom Passes in 3Delight

Quite often in CG when working with multiple passes – you want  to do different things depending on what type of pass your rendering – the two more common ways to do this are to either create two different shaders and change the assignment for each pass or to write a shader which is able to figure out what type of pass it is and act appropriately.

For example in a bake pass you typically want to bake out attributes which aren’t dependant on the position or direction of the camera  – this includes things like diffuse/ambient shading, subsurface scattering and ambient occlusion data  – which can all be baked out and reused on objects. Only when the position and direction of the objects or lights changes does the scene need to be rebaked.

In order to control this in 3Delight you can use the RiMel commands to setup custom RIB commands which can be read by your shaders. The following MEL commands are placed inside a PreWorldMEL attribute on the render pass itself.

RiOption -n "user" -p "pass" "string" "bake";

RiAttribute -n "cull" -p "hidden" "integer" "0";
RiAttribute -n "cull" -p "backfacing" "integer" "0";
RiAttribute -n "dice" -p "rasterorient" "integer" "0";

This will output the following commands into the RIB file.

Option "user" "string pass" [ "bake" ]
Attribute "cull" "integer hidden" [ 0 ]
Attribute "cull" "integer backfacing" [ 0 ]
Attribute "dice" "integer rasterorient" [ 0 ]

Then within the shader I can query what pass I’m currently rendering and do something appropriate for that type of pass.

uniform string passtype = "";
option( "user:pass", passtype );
if (passtype == "bake")
    // Do something here

By default 3Delight also exports out the name of the render pass to an attribute called delight_renderpass_name – this is name of your render pass inside Maya. You can query that name using the following RSL. Obviously using this method highly depends on what you call your passes – the following shading code wouldn’t work if the render pass was called anything other than “bake” – for example if you wanted to do multiple bake passes for within the same scene like separating out animated bakes (which require multiple frames) from static bakes (things which don’t move which can be stored in one frame).

string passtype = "";
option ("user:delight_renderpass_name",passtype);
if (passtype == "bake")
	// Do something here	

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