Processing HDR images with Metal

At a glance

Project structure

Source bundle/
├── Application/
│   ├── main.m
│   ├── iOS/
│   │   ├── AAPLAppDelegate.h
│   │   ├── AAPLAppDelegate.m
│   │   ├── WindowSceneDelegate.h
│   │   ├── WindowSceneDelegate.m
│   │   ├── AAPLViewControllerIOS.h
│   │   └── AAPLViewControllerIOS.m
│   └── macOS/
│       ├── AAPLWindowController.m
│       └── AAPLWindowController.h
└── Renderer/
    ├── AAPLRenderer.m
    ├── AAPLShaders.metal
    └── AAPLRenderer.h

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: C/Objective-C header, Metal, Objective-C, Objective-C++.
• The verified tree contains 9 project/configuration file(s) and 14 source declaration(s).

Overall architecture

flowchart LR
    N1["main"]
    N2["AAPLWindowController"]
    N3["AAPLRenderer"]
    N4["MetalKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

Application/main.m:18 — architecture anchor

#if defined(TARGET_IOS) || defined(TARGET_TVOS)
int main(int argc, char * argv[]) {
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AAPLAppDelegate class]));
    }
}
#endif

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into Metal.

Ownership and state

classDiagram
    WindowSceneDelegate o-- UIWindow : window
    AAPLRenderer o-- float : bloomIntensity
    AAPLRenderer o-- float : bloomThreshold
    AAPLRenderer o-- float : bloomRange

Ownership evidence

Application/iOS/WindowSceneDelegate.h:13 — stored dependency or nearest verified ownership anchor

@interface WindowSceneDelegate : UIResponder <UIWindowSceneDelegate>

@property (nonatomic, strong) UIWindow *window;

@end

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Concurrency, scheduling, and thread safety

Evidence limit: actor isolation, async/await, or Task creation does not by itself prove background-thread execution; Sendable conformance alone does not prove thread-safe mutation.

No source-visible execution, scheduling, or synchronization boundary was found in the indexed source.

@MainActor/MainActor.run, DispatchQueue.main, and RunLoop.main are reported as distinct isolation, queue, and event-loop mechanisms. A plain Task is kept separate from Task.detached; neither is labeled as a background thread.

State propagation, frameworks, and dependencies

Evidence limit: an import proves a source-level compilation dependency at the cited line; it does not prove runtime use, architectural adoption, or whether a Swift package is a direct application dependency.

receive(on:) describes downstream delivery scheduling, while subscribe(on:) describes upstream subscription/request/cancel scheduling. An import Combine alone establishes neither behavior nor a Store, reducer, Redux, or other application architecture.

Class and protocol design

Application/iOS/AAPLAppDelegate.h:10 — representative type boundary

@interface AAPLAppDelegate : UIResponder <UIApplicationDelegate>

@end

No local protocol conformance is claimed as protocol-oriented design; external framework conformances are listed only as dependencies.

Access control

Reference code

Application/iOS/AAPLViewControllerIOS.h:12 — representative boundary

@interface AAPLViewControllerIOS : UIViewController <UIPickerViewDataSource, UIPickerViewDelegate>
// For menu bar interaction
@property (nonatomic) BOOL isUIDisplayed;
@end

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Main application flow

sequenceDiagram
    participant AAPLRenderer
    participant _commandQueue
    participant NSString
    participant averageLuminanceCommandBuffer
    participant MTLRenderPassDescriptor
    participant rce
    participant bce
    AAPLRenderer->>_commandQueue: commandBuffer()
    AAPLRenderer->>NSString: stringWithFormat()
    AAPLRenderer->>averageLuminanceCommandBuffer: addCompletedHandler()
    AAPLRenderer->>MTLRenderPassDescriptor: renderPassDescriptor()
    AAPLRenderer->>averageLuminanceCommandBuffer: renderCommandEncoderWithDescriptor()
    AAPLRenderer->>rce: setDepthStencilState()
    AAPLRenderer->>bce: generateMipmapsForTexture()
    AAPLRenderer->>averageLuminanceCommandBuffer: commit()

Reference code

Renderer/AAPLRenderer.m:1022 — encodeSceneExposureCalculationWithCommandBuffer()

- (void)encodeSceneExposureCalculationWithCommandBuffer:(id<MTLCommandBuffer>)commandBuffer
{
    // ...
    if (_exposureType == kExposureControlTypeKey)
    {
        id<MTLCommandBuffer> averageLuminanceCommandBuffer = [_commandQueue commandBuffer];
        averageLuminanceCommandBuffer.label = [NSString stringWithFormat:@"Avg Luminance CommandBuffer %lu", _cameraAnimationFrameIndex];
        __weak AAPLRenderer * weakSelf = self;
        [averageLuminanceCommandBuffer addCompletedHandler:^(id<MTLCommandBuffer> cb)
        {
            AAPLRenderer * strongSelf = weakSelf;
            strongSelf->_averageLuminanceDuration = cb.GPUEndTime - cb.GPUStartTime;
            return;
        }];
        MTLRenderPassDescriptor * rpd = [MTLRenderPassDescriptor renderPassDescriptor];
        rpd.colorAttachments[0].texture = _logLuminanceTexture;
        rpd.colorAttachments[0].loadAction = MTLLoadActionClear;
        rpd.colorAttachments[0].storeAction = MTLStoreActionStore;
        rpd.colorAttachments[0].clearColor = MTLClearColorMake(0.0, 0.0, 0.0, 1.0);
        id<MTLRenderCommandEncoder> rce = [averageLuminanceCommandBuffer renderCommandEncoderWithDescriptor:rpd];
        rce.label = @"Log Luminance";
        [rce setDepthStencilState:_depthStateDisabled];
        [rce setCullMode:MTLCullModeBack];
        [rce setRenderPipelineState:_logLuminancePipeline];
        [rce setFragmentTexture:_sceneLinearColorTexture atIndex:0];
        [rce drawPrimitives:MTLPrimitiveTypeTriangle vertexStart:0 vertexCount:3];
        [rce endEncoding];
        id<MTLBlitCommandEncoder> bce = [averageLuminanceCommandBuffer blitCommandEncoder];
        bce.label = @"Mipmap Gen: Avg Luminance";
        [bce generateMipmapsForTexture:_logLuminanceTexture];
        [bce endEncoding];
        [averageLuminanceCommandBuffer commit];
    }
}

Naming conventions

• Types: Controller: AAPLWindowController; Delegate: AAPLAppDelegate, WindowSceneDelegate; Renderer: AAPLRenderer.
• Protocols: no local protocol declaration in the scanned source.
• Methods: initWithMetalKitView, setTonemapWhitepoint, cameraAnimationStepCount, minimumResolutionScale, maximumResolutionScale, setResolutionScale, updateWithDevice, updateWithSize.
• Files: Application/iOS/AAPLAppDelegate.h, Renderer/AAPLRenderer.m, Application/macOS/AAPLWindowController.m, Application/iOS/AAPLAppDelegate.m, Application/iOS/WindowSceneDelegate.h, Application/iOS/WindowSceneDelegate.m.

Architecture takeaways

• main is the main source-visible entry or composition anchor for this sample.
• Framework work reaches simd, MetalKit, UIKit, Cocoa through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• The source does not justify labeling the design protocol-oriented.

Source map

• Official Apple documentation