Hi,
Materialx nodes seemed still to be compiled to vex code and Karma CPU did know Vex Vop.
Why no result when we merge them?

The mechanism by which they are translated to vex is different. MaterialX is JIT converted from the graph while rendering, whereas vex vops generate code directly when generating the USD. MaterialX vops could theoretically generate vex code too and be mixed with other vex vops, but this would generate a vex shader and someone defeat the purpose of using an open interchange format. It would also be incompatible with XPU.

Vex VOPs represent explicit VEX code snippets. When translating to USD, all the snippets need to be combined into one shader function, which is stroed as sourceCode attribute on the USD Shader primitive. That sourceCode can only have valid VEX function and syntax. It can't refer to foreign MaterialX entities.

MaterialX are abstracted shader functions, and in theory some of them could be incorporated as VEX code snippets into a VEX shader. But not all of them (eg, MaterialX has some data types like VDF and EDF that are not handled in VEX).

In the end they are two separate ways of building shaders, and combining them in a coherent way presents some challenges.