How define a metal shader with dynamic buffer declaration? - ios

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 ...
}
}

Answer 1

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;
}
....
}