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

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