How define a metal shader with dynamic buffer declaration?

Question

I have this Metal shader :

struct InVertex {
  packed_float3 pos;
  packed_uchar4 color;
};

vertex ProjectedVertex vertexShader(const device InVertex *vertexArray [[buffer(0)]],
                                    const unsigned int vertexId [[vertex_id]]){

  InVertex in = vertexArray[vertexId];
  ....

}

However I would like to make the buffer declaration "dynamic". IE I would like that my shader is able to handle buffer type like for exemple

struct InVertex1 {
  packed_float3 pos;
  packed_uchar4 color;
};

struct InVertex2 {
  float4 pos;
  flat4 color;
};

struct InVertex3 {
  float4 pos;
  float4 tangent;
  float4 color;
};

etc..

so I would like something like :

vertex ProjectedVertex vertexShader(const device ???? *vertexArray [[buffer(0)]],
                                    const unsigned int vertexId [[vertex_id]]
                                    const device int vertexType [[buffer(1)]] ){

  if vertexType = InVertex1Type {
    ... handle InVertex1 type ...
  }
  else if vertexType = InVertex2Type {  
    ... handle InVertex2 type ...
  }
  else if vertexType = InVertex3Type {  
    ... handle InVertex3 type ...
  }

}

Why exactly do you need this? Putting if-else condition in a shader may impact the performance of your app. — Hamid Yusifli
– Hamid Yusifli, Commented Nov 1, 2019 at 21:10
@0xBFE1A8 yes I know about performance :( But I can not touch the framework (based on openGL) with gave me a dynamic buffer declaration. So i must make my shader able to handle all cases — zeus
– zeus, Commented Nov 1, 2019 at 21:20

Hamid Yusifli · Accepted Answer · 2019-11-02 12:09:19Z

The Metal programming language is a C++14-based Specification with extensions and restrictions. Taking this into account you can do the following.

First create a header file called ShaderTypes.h:

//  Header containing types and enum constants shared between Metal shaders and Swift/ObjC source
#ifndef ShaderTypes_h
#define ShaderTypes_h

#ifndef __METAL_VERSION__
/// 96-bit 3 component float vector type
typedef struct __attribute__ ((packed)) packed_float3 {
    float x;
    float y;
    float z;
} packed_float3;
#endif

typedef struct
{
    packed_float3 pos;
    packed_uchar4 color;
} InVertex1;

typedef struct
{
    vector_float4 pos;
    vector_float4 color;
} InVertex2;

typedef struct
{
    vector_float4 pos;
    vector_float4 tangent;
    vector_float4 color;

} InVertex3;

enum VertexType {InVertex1Type = 0, InVertex2Type = 1, InVertex3Type = 2};

typedef struct
{
    InVertex1 InVertex1;
    InVertex2 InVertex2;
    InVertex3 InVertex3;
    VertexType vertexType;
} dynamicStruct;

#endif /* ShaderTypes_h */

In your render class add the following:

    // Include header shared between C code here, which executes Metal API commands, and .metal files
    #import "ShaderTypes.h"

    id <MTLBuffer> _dynamicBuffer;
    // Create your dynamic buffer.
    void InitBuffer(id<MTLDevice> device)
    {
        _dynamicBuffer = [device newBufferWithLength:sizeof(dynamicStruct) options:MTLResourceStorageModeShared];
    }
    // Update your dynamic buffer.
    void UpdateBuffer()
    {
        dynamicStruct* ds = (dynamicStruct*)_dynamicBuffer.contents;

        ds->InVertex1.color = {0, 0, 0, 0};
        ds->InVertex2.pos = {0, 1, 1, 1};
        ds->InVertex3.tangent = {1, 1, 1, 1};
        // Select specific struct
        ds->vertexType = VertexType::InVertex2Type;
    }

- (void)drawInMTKView:(nonnull MTKView *)view
{

    ...

     // Pass your dynamic buffer to the shader.
    [renderEncoder setVertexBuffer:_dynamicBuffer offset:0 atIndex:0];

    ...
}

And finally in your shader file (.metal):

// Including header shared between this Metal shader code and Swift/C code executing Metal API commands
#import "ShaderTypes.h"

vertex ProjectedVertex vertexShader(constant dynamicStruct & dynamicStruct[[ buffer(0) ]],
                                    const unsigned int vertexId [[vertex_id]])
{

    InVertex1 v1;
    InVertex2 v2;
    InVertex3 v3;

    if(dynamicStruct.vertexType == VertexType::InVertex1Type)
    {
        v1 = dynamicStruct.InVertex1;
    }
    else if(dynamicStruct.vertexType == VertexType::InVertex2Type)
    {
        v2 = dynamicStruct.InVertex2;
    }
    else if(dynamicStruct.vertexType == VertexType::InVertex3Type)
    {
        v3 = dynamicStruct.InVertex3;
    }
  ....

}

Did you mean for dynamicStuct to contain a union? Otherwise you're not matching the question's requirements.
Isn't this allocating all the extra memory of the typedef fields that were never used?

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