Types sharing between Metal and App

19 Jan 2019

One of the best things about Metal is the Metal Shading Language which is a based on C++. What this means is that we can share data structures between the shader and app code. There are a few cases that we have to look out for though, which is what this post is about.

Using C++

Lets say we want to define a data structure that represents vertex data:

// common.h

#include <simd/simd.h>

typedef struct
{
    simd::float4 position;
    simd::float4 color;
} Vertex;

The first problem we would have to tackle is that even though the simd types are the same in both C++ code and the Metal code but in C++ the types are part of simd namespace.

One way to fix this issue is to add using namespace simd when calling from C++.

// common.h

#ifdef __METAL_VERSION__
#else
#include <simd/simd.h>
using namespace simd;
#endif

typedef struct
{
    float4 position;
    float4 color;
} Vertex;

Now we can reuse the data structure in our C++ code, which looks even better thanks to the C99 style member initialization.

// renderer.cpp

#include "common.h"

 Vertex data[] = {
    {
        .position = { 0.0f, 1.0f, 0.0f, 1.0f },
        .color = { 1.0f, 0.0f, 0.0f, 1.0f }
    },
    {
        .position = { -1.0f, -1.0f, 0.0f, 1.0f },
        .color = { 0.0f, 1.0f, 0.0f, 1.0f }
    },
    {
        .position = { 1.0f, -1.0f, 0.0f, 1.0f },
        .color = { 0.0f, 0.0f, 1.0f, 1.0f }
    }
};

And on the shading side

// shader.metal

#include <metal_stdlib>
#include "common.h"

using namespace metal;

vertex Vertex vertexShader(device Vertex *data [[buffer(0)]], uint vid [[vertex_id]])
{
    return data[vid];
}


fragment float4 fragShader(Vertex in [[stage_in]])
{
    return in.color;
}

This has a problem. Since we are returning a struct from the vertex shader function, we need to have an element with attribute [[position]]. From the official docs:

If the return type of a vertex function is not void, it must include the vertex position. If the vertex return type is float4, then it always refers to the vertex position, and the [[position]] attribute must not be specified. If the vertex return type is a struct, it must include an element declared with the [[position]] attribute.

The fix is actually pretty simple, we can simply add the missing attribute to the Vertex

// common.h

typedef struct
{
    float4 position [[position]];
    float4 color;
} Vertex;

This should solve most of the errors except that compiler might emit a warning when compiling the C++ code:

Unknown attribute ‘position’ ignored

This could again be fixed with our beautiful preprocessor check

// common.h

#ifdef __METAL_VERSION__
#define ATTRIB_POSITION [[position]]
#else
#include <simd/simd.h>
#define ATTRIB_POSITION
using namespace simd;
#endif

typedef struct
{
    float4 position ATTRIB_POSITION;
    float4 color;
} Vertex;

Not the prettiest looking code out there, but gets the job done.

Using Objective-C

With Objective-C, the same trick can be applied, since for all simd types a C variant is available.

// common.h

#ifdef __METAL_VERSION__
#define ATTRIB_POSITION [[position]]
#else
#define ATTRIB_POSITION
#endif

typedef struct _Vertex
{
    vector_float4 position ATTRIB_POSITION;
    vector_float4 color;
} Vertex;

Nothing changes on the Metal side.

Whacky Labs

Types sharing between Metal and App

Using C++

Using Objective-C