If we have a URP shader as a .shader file, is it possible to open/convert this to a Shader Graph?
Nope. But you can copy some of the interesting functions into custom function nodes.
So if I wanted to start with a shader graph that is the same as “Simple Lit”, is that the equivalent of creating a Shader using:
Assets > Create > Shader > Universal Render Pipeline > Lit Shader Graph
Basically I want to modify the “Simple Lit” shader inside of shader graph.
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AFAICT, Shader Graph does not support Simple Lit. That menu will give you a regular Lit shader, which is probably fine. The actual difference between Simple Lit and Lit is small and won’t make much difference on modern PC/console GPUs, since both will probably end up bound by texture sample latency and fill rate. On mobile/VR it might make a difference, but if you need that level of performance for complex effects (more than Simple Lit provides), you’re probably going to want to be tweaking your shaders by hand instead of hoping Shader Graph will generate good code for you.
tl;dr: just use regular lit