Agent Skills: Metal Migration Reference

Metal Migration Reference

Complete reference for converting OpenGL/DirectX code to Metal.

When to Use This Reference

Use this reference when:

• Converting GLSL shaders to Metal Shading Language (MSL)
• Converting HLSL shaders to MSL
• Looking up GL/D3D API equivalents in Metal
• Setting up MTKView or CAMetalLayer
• Building render pipelines
• Using Metal Shader Converter for DirectX

Part 1: GLSL to MSL Conversion

Type Mappings

| GLSL | MSL | Notes | |------|-----|-------| | void | void | | | bool | bool | | | int | int | 32-bit signed | | uint | uint | 32-bit unsigned | | float | float | 32-bit | | double | N/A | Use float (no 64-bit float in MSL) | | vec2 | float2 | | | vec3 | float3 | | | vec4 | float4 | | | ivec2 | int2 | | | ivec3 | int3 | | | ivec4 | int4 | | | uvec2 | uint2 | | | uvec3 | uint3 | | | uvec4 | uint4 | | | bvec2 | bool2 | | | bvec3 | bool3 | | | bvec4 | bool4 | | | mat2 | float2x2 | | | mat3 | float3x3 | | | mat4 | float4x4 | | | mat2x3 | float2x3 | Columns x Rows | | mat3x4 | float3x4 | | | sampler2D | texture2d<float> + sampler | Separate in MSL | | sampler3D | texture3d<float> + sampler | | | samplerCube | texturecube<float> + sampler | | | sampler2DArray | texture2d_array<float> + sampler | | | sampler2DShadow | depth2d<float> + sampler | |

Built-in Variable Mappings

| GLSL | MSL | Stage | |------|-----|-------| | gl_Position | Return [[position]] | Vertex | | gl_PointSize | Return [[point_size]] | Vertex | | gl_VertexID | [[vertex_id]] parameter | Vertex | | gl_InstanceID | [[instance_id]] parameter | Vertex | | gl_FragCoord | [[position]] parameter | Fragment | | gl_FrontFacing | [[front_facing]] parameter | Fragment | | gl_PointCoord | [[point_coord]] parameter | Fragment | | gl_FragDepth | Return [[depth(any)]] | Fragment | | gl_SampleID | [[sample_id]] parameter | Fragment | | gl_SamplePosition | [[sample_position]] parameter | Fragment |

Function Mappings

| GLSL | MSL | Notes | |------|-----|-------| | texture(sampler, uv) | tex.sample(sampler, uv) | Method on texture | | textureLod(sampler, uv, lod) | tex.sample(sampler, uv, level(lod)) | | | textureGrad(sampler, uv, ddx, ddy) | tex.sample(sampler, uv, gradient2d(ddx, ddy)) | | | texelFetch(sampler, coord, lod) | tex.read(coord, lod) | Integer coords | | textureSize(sampler, lod) | tex.get_width(lod), tex.get_height(lod) | Separate calls | | dFdx(v) | dfdx(v) | | | dFdy(v) | dfdy(v) | | | fwidth(v) | fwidth(v) | Same | | mix(a, b, t) | mix(a, b, t) | Same | | clamp(v, lo, hi) | clamp(v, lo, hi) | Same | | smoothstep(e0, e1, x) | smoothstep(e0, e1, x) | Same | | step(edge, x) | step(edge, x) | Same | | mod(x, y) | fmod(x, y) | Different name | | fract(x) | fract(x) | Same | | inversesqrt(x) | rsqrt(x) | Different name | | atan(y, x) | atan2(y, x) | Different name |

Shader Structure Conversion

GLSL Vertex Shader:

#version 300 es
precision highp float;

layout(location = 0) in vec3 aPosition;
layout(location = 1) in vec2 aTexCoord;

uniform mat4 uModelViewProjection;

out vec2 vTexCoord;

void main() {
    gl_Position = uModelViewProjection * vec4(aPosition, 1.0);
    vTexCoord = aTexCoord;
}

MSL Vertex Shader:

#include <metal_stdlib>
using namespace metal;

struct VertexIn {
    float3 position [[attribute(0)]];
    float2 texCoord [[attribute(1)]];
};

struct VertexOut {
    float4 position [[position]];
    float2 texCoord;
};

struct Uniforms {
    float4x4 modelViewProjection;
};

vertex VertexOut vertexShader(
    VertexIn in [[stage_in]],
    constant Uniforms& uniforms [[buffer(1)]]
) {
    VertexOut out;
    out.position = uniforms.modelViewProjection * float4(in.position, 1.0);
    out.texCoord = in.texCoord;
    return out;
}

GLSL Fragment Shader:

#version 300 es
precision highp float;

in vec2 vTexCoord;
uniform sampler2D uTexture;

out vec4 fragColor;

void main() {
    fragColor = texture(uTexture, vTexCoord);
}

MSL Fragment Shader:

fragment float4 fragmentShader(
    VertexOut in [[stage_in]],
    texture2d<float> tex [[texture(0)]],
    sampler samp [[sampler(0)]]
) {
    return tex.sample(samp, in.texCoord);
}

Precision Qualifiers

GLSL precision qualifiers have no direct MSL equivalent — MSL uses explicit types:

| GLSL | MSL Equivalent | |------|----------------| | lowp float | half (16-bit) | | mediump float | half (16-bit) | | highp float | float (32-bit) | | lowp int | short (16-bit) | | mediump int | short (16-bit) | | highp int | int (32-bit) |

Buffer Alignment (Critical)

GLSL/C assumes:

• vec3: 12 bytes, any alignment
• vec4: 16 bytes

MSL requires:

• float3: 12 bytes storage, 16-byte aligned
• float4: 16 bytes storage, 16-byte aligned

Solution: Use simd types in Swift for CPU-GPU shared structs:

import simd

struct Uniforms {
    var modelViewProjection: simd_float4x4  // Correct alignment
    var cameraPosition: simd_float3         // 16-byte aligned
    var padding: Float = 0                   // Explicit padding if needed
}

Or use packed types in MSL (slower):

struct VertexPacked {
    packed_float3 position;  // 12 bytes, no padding
    packed_float2 texCoord;  // 8 bytes
};

Part 2: HLSL to MSL Conversion

Type Mappings

| HLSL | MSL | Notes | |------|-----|-------| | float | float | | | float2 | float2 | | | float3 | float3 | | | float4 | float4 | | | half | half | | | int | int | | | uint | uint | | | bool | bool | | | float2x2 | float2x2 | | | float3x3 | float3x3 | | | float4x4 | float4x4 | | | Texture2D | texture2d<float> | | | Texture3D | texture3d<float> | | | TextureCube | texturecube<float> | | | SamplerState | sampler | | | RWTexture2D | texture2d<float, access::read_write> | | | RWBuffer | device float* [[buffer(n)]] | | | StructuredBuffer | constant T* [[buffer(n)]] | | | RWStructuredBuffer | device T* [[buffer(n)]] | |

Semantic Mappings

| HLSL Semantic | MSL Attribute | |---------------|---------------| | SV_Position | [[position]] | | SV_Target0 | Return value / [[color(0)]] | | SV_Target1 | [[color(1)]] | | SV_Depth | [[depth(any)]] | | SV_VertexID | [[vertex_id]] | | SV_InstanceID | [[instance_id]] | | SV_IsFrontFace | [[front_facing]] | | SV_SampleIndex | [[sample_id]] | | SV_PrimitiveID | [[primitive_id]] | | SV_DispatchThreadID | [[thread_position_in_grid]] | | SV_GroupThreadID | [[thread_position_in_threadgroup]] | | SV_GroupID | [[threadgroup_position_in_grid]] | | SV_GroupIndex | [[thread_index_in_threadgroup]] |

Function Mappings

| HLSL | MSL | Notes | |------|-----|-------| | tex.Sample(samp, uv) | tex.sample(samp, uv) | Lowercase | | tex.SampleLevel(samp, uv, lod) | tex.sample(samp, uv, level(lod)) | | | tex.SampleGrad(samp, uv, ddx, ddy) | tex.sample(samp, uv, gradient2d(ddx, ddy)) | | | tex.Load(coord) | tex.read(coord.xy, coord.z) | Split coord | | mul(a, b) | a * b | Operator | | saturate(x) | saturate(x) | Same | | lerp(a, b, t) | mix(a, b, t) | Different name | | frac(x) | fract(x) | Different name | | ddx(v) | dfdx(v) | Different name | | ddy(v) | dfdy(v) | Different name | | clip(x) | if (x < 0) discard_fragment() | Manual | | discard | discard_fragment() | Function call |

Metal Shader Converter (DirectX → Metal)

Apple's official tool for converting DXIL (compiled HLSL) to Metal libraries.

Requirements:

• macOS 13+ with Xcode 15+
• OR Windows 10+ with VS 2019+
• Target devices: Argument Buffers Tier 2 (macOS 14+, iOS 17+)

Workflow:

# Step 1: Compile HLSL to DXIL using DXC
dxc -T vs_6_0 -E MainVS -Fo vertex.dxil shader.hlsl
dxc -T ps_6_0 -E MainPS -Fo fragment.dxil shader.hlsl

# Step 2: Convert DXIL to Metal library
metal-shaderconverter vertex.dxil -o vertex.metallib
metal-shaderconverter fragment.dxil -o fragment.metallib

# Step 3: Load in Swift
let vertexLib = try device.makeLibrary(URL: vertexURL)
let fragmentLib = try device.makeLibrary(URL: fragmentURL)

Key Options:

| Option | Purpose | |--------|---------| | -o <file> | Output metallib path | | --minimum-gpu-family | Target GPU family | | --minimum-os-build-version | Minimum OS version | | --vertex-stage-in | Separate vertex fetch function | | -dualSourceBlending | Enable dual-source blending |

Supported Shader Models: SM 6.0 - 6.6 (with limitations on 6.6 features)

Part 3: OpenGL API to Metal API

View/Context Setup

| OpenGL | Metal | |--------|-------| | NSOpenGLView | MTKView | | GLKView | MTKView | | EAGLContext | MTLDevice + MTLCommandQueue | | CGLContextObj | MTLDevice |

Resource Creation

| OpenGL | Metal | |--------|-------| | glGenBuffers + glBufferData | device.makeBuffer(bytes:length:options:) | | glGenTextures + glTexImage2D | device.makeTexture(descriptor:) + texture.replace(region:...) | | glGenFramebuffers | MTLRenderPassDescriptor | | glGenVertexArrays | MTLVertexDescriptor | | glCreateShader + glCompileShader | Build-time compilation → MTLLibrary | | glCreateProgram + glLinkProgram | MTLRenderPipelineDescriptor → MTLRenderPipelineState |

State Management

| OpenGL | Metal | |--------|-------| | glEnable(GL_DEPTH_TEST) | MTLDepthStencilDescriptor → MTLDepthStencilState | | glDepthFunc(GL_LESS) | descriptor.depthCompareFunction = .less | | glEnable(GL_BLEND) | pipelineDescriptor.colorAttachments[0].isBlendingEnabled = true | | glBlendFunc | sourceRGBBlendFactor, destinationRGBBlendFactor | | glCullFace | encoder.setCullMode(.back) | | glFrontFace | encoder.setFrontFacing(.counterClockwise) | | glViewport | encoder.setViewport(MTLViewport(...)) | | glScissor | encoder.setScissorRect(MTLScissorRect(...)) |

Draw Commands

| OpenGL | Metal | |--------|-------| | glDrawArrays(mode, first, count) | encoder.drawPrimitives(type:vertexStart:vertexCount:) | | glDrawElements(mode, count, type, indices) | encoder.drawIndexedPrimitives(type:indexCount:indexType:indexBuffer:indexBufferOffset:) | | glDrawArraysInstanced | encoder.drawPrimitives(type:vertexStart:vertexCount:instanceCount:) | | glDrawElementsInstanced | encoder.drawIndexedPrimitives(...instanceCount:) |

Primitive Types

| OpenGL | Metal | |--------|-------| | GL_POINTS | .point | | GL_LINES | .line | | GL_LINE_STRIP | .lineStrip | | GL_TRIANGLES | .triangle | | GL_TRIANGLE_STRIP | .triangleStrip | | GL_TRIANGLE_FAN | N/A (decompose to triangles) |

Part 4: Complete Setup Examples

MTKView Setup (Recommended)

import MetalKit

class GameViewController: UIViewController {
    var metalView: MTKView!
    var renderer: Renderer!

    override func viewDidLoad() {
        super.viewDidLoad()

        // Create Metal view
        guard let device = MTLCreateSystemDefaultDevice() else {
            fatalError("Metal not supported")
        }

        metalView = MTKView(frame: view.bounds, device: device)
        metalView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        metalView.colorPixelFormat = .bgra8Unorm
        metalView.depthStencilPixelFormat = .depth32Float
        metalView.clearColor = MTLClearColor(red: 0, green: 0, blue: 0, alpha: 1)
        metalView.preferredFramesPerSecond = 60
        view.addSubview(metalView)

        // Create renderer
        renderer = Renderer(metalView: metalView)
        metalView.delegate = renderer
    }
}

class Renderer: NSObject, MTKViewDelegate {
    let device: MTLDevice
    let commandQueue: MTLCommandQueue
    var pipelineState: MTLRenderPipelineState!
    var depthState: MTLDepthStencilState!
    var vertexBuffer: MTLBuffer!

    init(metalView: MTKView) {
        device = metalView.device!
        commandQueue = device.makeCommandQueue()!
        super.init()

        buildPipeline(metalView: metalView)
        buildDepthStencil()
        buildBuffers()
    }

    private func buildPipeline(metalView: MTKView) {
        let library = device.makeDefaultLibrary()!

        let descriptor = MTLRenderPipelineDescriptor()
        descriptor.vertexFunction = library.makeFunction(name: "vertexShader")
        descriptor.fragmentFunction = library.makeFunction(name: "fragmentShader")
        descriptor.colorAttachments[0].pixelFormat = metalView.colorPixelFormat
        descriptor.depthAttachmentPixelFormat = metalView.depthStencilPixelFormat

        // Vertex descriptor (matches shader's VertexIn struct)
        let vertexDescriptor = MTLVertexDescriptor()
        vertexDescriptor.attributes[0].format = .float3
        vertexDescriptor.attributes[0].offset = 0
        vertexDescriptor.attributes[0].bufferIndex = 0
        vertexDescriptor.attributes[1].format = .float2
        vertexDescriptor.attributes[1].offset = MemoryLayout<SIMD3<Float>>.stride
        vertexDescriptor.attributes[1].bufferIndex = 0
        vertexDescriptor.layouts[0].stride = MemoryLayout<Vertex>.stride
        descriptor.vertexDescriptor = vertexDescriptor

        pipelineState = try! device.makeRenderPipelineState(descriptor: descriptor)
    }

    private func buildDepthStencil() {
        let descriptor = MTLDepthStencilDescriptor()
        descriptor.depthCompareFunction = .less
        descriptor.isDepthWriteEnabled = true
        depthState = device.makeDepthStencilState(descriptor: descriptor)
    }

    func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {
        // Handle resize
    }

    func draw(in view: MTKView) {
        guard let drawable = view.currentDrawable,
              let descriptor = view.currentRenderPassDescriptor,
              let commandBuffer = commandQueue.makeCommandBuffer(),
              let encoder = commandBuffer.makeRenderCommandEncoder(descriptor: descriptor) else {
            return
        }

        encoder.setRenderPipelineState(pipelineState)
        encoder.setDepthStencilState(depthState)
        encoder.setVertexBuffer(vertexBuffer, offset: 0, index: 0)
        encoder.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: vertexCount)
        encoder.endEncoding()

        commandBuffer.present(drawable)
        commandBuffer.commit()
    }
}

CAMetalLayer Setup (Custom Control)

import Metal
import QuartzCore

class MetalLayerView: UIView {
    var metalLayer: CAMetalLayer!
    var device: MTLDevice!
    var commandQueue: MTLCommandQueue!
    var displayLink: CADisplayLink?

    override class var layerClass: AnyClass { CAMetalLayer.self }

    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }

    private func setup() {
        device = MTLCreateSystemDefaultDevice()!
        commandQueue = device.makeCommandQueue()!

        metalLayer = layer as? CAMetalLayer
        metalLayer.device = device
        metalLayer.pixelFormat = .bgra8Unorm
        metalLayer.framebufferOnly = true

        displayLink = CADisplayLink(target: self, selector: #selector(render))
        displayLink?.add(to: .main, forMode: .common)
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        metalLayer.drawableSize = CGSize(
            width: bounds.width * contentScaleFactor,
            height: bounds.height * contentScaleFactor
        )
    }

    @objc func render() {
        guard let drawable = metalLayer.nextDrawable(),
              let commandBuffer = commandQueue.makeCommandBuffer() else {
            return
        }

        let descriptor = MTLRenderPassDescriptor()
        descriptor.colorAttachments[0].texture = drawable.texture
        descriptor.colorAttachments[0].loadAction = .clear
        descriptor.colorAttachments[0].storeAction = .store
        descriptor.colorAttachments[0].clearColor = MTLClearColor(red: 0, green: 0, blue: 0, alpha: 1)

        guard let encoder = commandBuffer.makeRenderCommandEncoder(descriptor: descriptor) else {
            return
        }

        // Draw commands here
        encoder.endEncoding()

        commandBuffer.present(drawable)
        commandBuffer.commit()
    }
}

Compute Shader Setup

class ComputeProcessor {
    let device: MTLDevice
    let commandQueue: MTLCommandQueue
    var computePipeline: MTLComputePipelineState!

    init() {
        device = MTLCreateSystemDefaultDevice()!
        commandQueue = device.makeCommandQueue()!

        let library = device.makeDefaultLibrary()!
        let function = library.makeFunction(name: "computeKernel")!
        computePipeline = try! device.makeComputePipelineState(function: function)
    }

    func process(input: MTLBuffer, output: MTLBuffer, count: Int) {
        let commandBuffer = commandQueue.makeCommandBuffer()!
        let encoder = commandBuffer.makeComputeCommandEncoder()!

        encoder.setComputePipelineState(computePipeline)
        encoder.setBuffer(input, offset: 0, index: 0)
        encoder.setBuffer(output, offset: 0, index: 1)

        let threadGroupSize = MTLSize(width: 256, height: 1, depth: 1)
        let threadGroups = MTLSize(
            width: (count + 255) / 256,
            height: 1,
            depth: 1
        )

        encoder.dispatchThreadgroups(threadGroups, threadsPerThreadgroup: threadGroupSize)
        encoder.endEncoding()

        commandBuffer.commit()
        commandBuffer.waitUntilCompleted()
    }
}

// Compute shader
kernel void computeKernel(
    device float* input [[buffer(0)]],
    device float* output [[buffer(1)]],
    uint id [[thread_position_in_grid]]
) {
    output[id] = input[id] * 2.0;
}

Part 5: Storage Modes & Synchronization

Buffer Storage Modes

| Mode | CPU Access | GPU Access | Use Case | |------|------------|------------|----------| | .shared | Read/Write | Read/Write | Small dynamic data, uniforms | | .private | None | Read/Write | Static assets, render targets | | .managed (macOS) | Read/Write | Read/Write | Large buffers with partial updates |

// Shared: CPU and GPU both access (iOS typical)
let uniformBuffer = device.makeBuffer(length: size, options: .storageModeShared)

// Private: GPU only (best for static geometry)
let vertexBuffer = device.makeBuffer(bytes: vertices, length: size, options: .storageModePrivate)

// Managed: Explicit sync (macOS)
#if os(macOS)
let buffer = device.makeBuffer(length: size, options: .storageModeManaged)
// After CPU write:
buffer.didModifyRange(0..<size)
#endif

Texture Storage Modes

let descriptor = MTLTextureDescriptor.texture2DDescriptor(
    pixelFormat: .rgba8Unorm,
    width: 1024,
    height: 1024,
    mipmapped: true
)

// For static textures (loaded once)
descriptor.storageMode = .private
descriptor.usage = [.shaderRead]

// For render targets
descriptor.storageMode = .private
descriptor.usage = [.renderTarget, .shaderRead]

// For CPU-readable (screenshots, readback)
descriptor.storageMode = .shared  // iOS
descriptor.storageMode = .managed  // macOS
descriptor.usage = [.shaderRead, .shaderWrite]

Resources

WWDC: 2016-00602, 2018-00604, 2019-00611

Docs: /metal/migrating-opengl-code-to-metal, /metal/shader-converter, /metalkit/mtkview

Skills: axiom-metal-migration, axiom-metal-migration-diag

Last Updated: 2025-12-29 Platforms: iOS 12+, macOS 10.14+, tvOS 12+ Status: Complete shader conversion and API mapping reference