Creating and compiling a custom Metal shader at initialization

Question

I am creating a class that needs differently coded shaders due to varying inputs that are known at initialization. The goal would be that when the object is initialized the shader is also initialized and compiled into a library.

My problem is I have no clue how to write a .metal file to my apps directory like I see in XCode and frankly I am not sure iOS permissions will even let me do that. I think once I have the metal file in place it becomes easy but how?

The shader I hope to edit at compile time will have a variable amount of texture inputs based on the initialization parameters.

Ken Thomases · Accepted Answer · 2018-01-03 12:50:34Z

2

You don't write a .metal file. You create a MTLLibrary object directly from a string containing the shader program text.

You call either the -newLibraryWithSource:options:error: method or the -newLibraryWithSource:options:completionHandler: method of the MTLDevice protocol.

That said, either Matthijs's answer or some other approach may be more appropriate. For example, depending on which versions of iOS and which GPU family you're targeting, you may be able to use texture arrays with function constants to accommodate a variable number of textures. You should show a real example of the sort of shader you're hoping to build and how it may vary.

answered Jan 3, 2018 at 12:50

Ken Thomases

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Matthijs Hollemans · Accepted Answer · 2018-01-03 10:14:58Z

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Please look into the concept of "function constants".

You can define a value as a function_constant in your .metal file. Then when you create the function using the MTLLibrary you can pass in the actual value for these constants and Metal will compile a specialized version of your shader on-the-fly using these constant values.

answered Jan 3, 2018 at 10:14

Matthijs Hollemans

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3 Comments

J.Doe Over a year ago

I would do this except the shader needs to support a variable amount of textures as inputs. Will update question

Matthijs Hollemans Over a year ago

Metal supports arrays of textures (and also something called texture arrays). Perhaps you could pass in this variable amount of textures using such an array?

J.Doe Over a year ago

Great idea unfortunately that only works with textures of the same size which does not work for my scenario

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