#metal#volume-rendering #ray-marching#ios #macos#swift #transfer-function#dicom

Indigo

Metal-based 3D volume renderer for macOS and iOS — ray marching through MTLTexture3D with gradient lighting and a configurable transfer function pipeline.

PlatformsmacOS · iOS

LanguageSwift · Metal · Metal Shader Language (MSL)

DataJPEG, PNG slices · DICOM series · NIfTI volume (.nii file)

RendererGPU ray marching · MTLTexture3D

Textures Transfer Function is a 1D RGBA control-point texture

SourceClosed Source, Unpublished Work

Overview

Indigo is a volume rendering engine written in Swift and Metal integrated into applications for macOS and iOS. The core rendering engine performs GPU-resident ray marching through a MTLTexture3D populated from either image slices or synthetic floating-point phantoms. Gradient-based lighting and a configurable Transfer Function (TF) pipeline map volumetric density to colour and opacity.

Screenshots

macOS Application

iOS Application

Architecture

Component Graph

  Application System [User Input]
        │
        ▼
  ┌──────────────────────────────────────────┐ 
  │  VolumeRenderView  ──►  DialToolbar      │
  │  MetalVolumeView   ──►  SelectorPanel    │
  │  VolumeRenderViewModel                   │
  └─────────────┬───────────────────────┬────┘
                │ render()              │ load()
                ▼                       ▼
  ╔══════════════════════════════════════════════╗  IndigoVolumeEngine 
  ║  VolumeRenderer ──► VolumeShaders.metal      ║
  ║       │                                      ║
  ║       ├──────────────► TransferFunction      ║
  ║       └──────────────► VolumeData            ║
  ║                             ▲                ║
  ║              VolumeSliceLoader               ║
  ╚══════════════════════════════════════════════╝
                │ rgba32Float MTLTexture2D
                ▼
  ┌──────────────────────────┐
  │   TextureBlitRenderer    │
  └─────────────┬────────────┘
                │ bgra8Unorm → CAMetalDrawable
                ▼
              Display

Components

Component	Role
`VolumeRenderView`	Root SwiftUI scene; hosts gesture recognizers (drag · pinch · rotate), three collapsible trays, and the `MetalVolumeView` canvas
`VolumeRenderViewModel`	Main-actor `ObservableObject`; owns the renderer and device; mediates load requests, render triggers, and all published UI state
`DialToolbar` / `SelectorPanel`	Pure-SwiftUI instrument panels — `AnalogKnob`, `ManettinoSelector`, `TFSwatchGrid` — with no Metal coupling
`MetalVolumeView` + `TextureBlitRenderer`	Platform-bridged `MTKView` wrapper; blits the compute-output `rgba32Float` texture to the drawable each frame
`VolumeRenderer`	Manages three `MTLComputePipelineState`s (DVR · MIP · Debug); assembles `VolumeUniforms` (208 bytes, 16-byte aligned) and dispatches compute threads; exposes clip-plane axis and distance
`VolumeShaders.metal`	`volumeRayMarch`: front-to-back ray accumulation with gradient lighting, window/level transform, TF texture sample, and axis-aligned clip-plane discard. `volumeMIP`: max-intensity projection. `volumeDebug`: hit/density/ray-length visualisation
`TransferFunction`	Interpolates RGBA control points into a 256-sample `rgba32Float MTLTexture1D`; bound at `texture(2)`; nine named presets (CT Bone, CT Angio, MRI Brain, …)
`VolumeData`	Owns the `r32Float MTLTexture3D`; exposes `volumeScale` = dims × `voxelSpacing` normalised to unit cube; provides synthetic generators (sphere, Shepp-Logan phantom, noise)
`VolumeSliceLoader`	Ingests PNG/JPEG/TIFF slices (8-bit and 16-bit), raw binary volumes, and NIfTI-1 files (.nii / .nii.gz); applies slope/intercept and percentile window/level; emits per-slice progress callbacks

Data Flows

Connection	Carries	Transport
User → ViewModel	Gesture deltas, knob values, preset picks, file URLs	SwiftUI bindings / `@ObservedObject` didSet
ViewModel → VolumeSliceLoader	Directory / file URL, extension filter	`Task.detached` → `throws VolumeData`; progress `(Float) → Void` callback dispatched to main thread
VolumeSliceLoader → VolumeData	Normalised `[Float]`, width × height × depth, voxel spacing	`init(device:data:…)` — `MTLTexture.replace(region:…)` CPU upload
ViewModel → VolumeRenderer	Render parameters, `TransferFunction`, `VolumeData`, clip-plane axis + distance	Property writes + `render(width:height:transferFunction:)` call
VolumeRenderer → GPU kernel	`VolumeUniforms` (208 B), `MTLTexture3D` at binding 1, `MTLTexture1D` TF at binding 2, output `MTLTexture2D` at binding 0	`MTLComputeCommandEncoder.setBytes / setTexture` → `dispatchThreadgroups`
GPU kernel → TextureBlitRenderer	`rgba32Float MTLTexture2D` (compute output)	`MTLTexture` reference passed through `VolumeRenderViewModel.currentTexture`
TextureBlitRenderer → Display	`bgra8Unorm` pixel data	Fragment-shader blit → `CAMetalDrawable` → `MTLCommandBuffer.present`