I have enjoyed looking through the 'Discover Ray Tracing with Metal' video and Sample Code. While trying to recreate the Project in Swift, I have run across an issue with setting up an MTLPrimativeAccelerationStructureDescriptor.
In Renderer.mm ( Objective C ), within the createAccelerationStructures function, a loop is created:
// Create a primitive acceleration structure for each piece of geometry in the scene
	for (NSUInteger i = 0; i < _scene.geometries.count; i++) {
	Geometry *mesh = _scene.geometries[i];
	MTLAccelerationStructureGeometryDescriptor *geometryDescriptor = [mesh geometryDescriptor];
	// Assign each piece of geometry a consecutive slot in the intersection function table
geometryDescriptor.intersectionFunctionTableOffset = i;
	// Create a primitive acceleration structure descriptor to contain the single piece of acceleration structure geometry
	MTLPrimitiveAccelerationStructureDescriptor *accelDescriptor = [MTLPrimitiveAccelerationStructureDescriptor descriptor];
	accelDescriptor.geometryDescriptors = @[ geometryDescriptor ];
	// Build the acceleration structure.
	id <MTLAccelerationStructure> accelerationStructure = [self newAccelerationStructureWithDescriptor:accelDescriptor];
	// Add the acceleration structure to the array of primitive acceleration structures.
	[_primitiveAccelerationStructures addObject:accelerationStructure];
	}
In Swift, I am trying:
for i in 0..<( scene.nodes.count )
	{
	let aMesh = scene.nodes[i]
	let geometryDescriptor: MTLAccelerationStructureGeometryDescriptor = aMesh.getGeometryDescriptor()
	// Assign each piece of geometry a consecutive slot in the intersection function table
geometryDescriptor.intersectionFunctionTableOffset = i
	// Create a primitive acceleration structure descriptor to contain the single piece of acceleration structure geometry
	var accelDescriptor: MTLPrimitiveAccelerationStructureDescriptor = MTLPrimitiveAccelerationStructureDescriptor()
	accelDescriptor.geometryDescriptors.append( geometryDescriptor )
	// Build the acceleration structure.
	// To follow...
	// Add the acceleration structure to the array of primitive acceleration structures.
	 // To follow...
	}
The compiler flags appending the geometryDescriptor with "Cannot use mutating member on immutable value: 'accelDescriptor is immutable'". I have tried a couple of compiler suggested ideas to unwrap the accelDescriptor.geometryDescriptors, but end up with a nil value in the geometryDescriptors array.
It may be naive to simply append the geometryDescriptor, but I would appreciate any suggestions to fill in the accelDescriptor.
In the larger picture, it would be great to have a Swift Project sample to use as a starting point. The concepts spelled out in the video and Objective C project are very compelling!
Thanks so much - John Lobe
John Lobe
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While translating the Accelerating Ray Tracing Using Metal sample project into Swift, I have run into an issue within the createAccelerationStructures function.
I am trying to fill in the first three rows of the instance transformation matrix. In the Objective C example, it looks like:
for (int column = 0; column < 4; column++)
	for (int row = 0; row < 3; row++)
		instanceDescriptors[instanceIndex].transformationMatrix.columns[column][row] = instance.transform.columns[column][row];
The matrix targeted is a MTLPackedFloat4x3. It has defied me pulling out the needed elements from my instance transform and placing them in the descriptor. I have tried the obvious:
for column in 0..<( 4 )
	{
	for row in 0..<( 3 )
		{
		anInstanceDescriptorPointer.pointee.transformationMatrix.columns[column][row] = aNodeInstance.transform.columns[column][row]
		}
	}
I have tried more obscure ideas related to simd matrices and Swift. The compiler errors generally warn me that MTLPackedFloat4x3 has no subscripts.
Question - Is there an accepted way to assign three rows from the instance transform to the descriptor transformationMatrix in Swift?
Thank you for your earlier help with all of this. I should mention that the new intersector feature in Metal has worked really well for me when feeding it a primitiveaccelerationstructure. Very elegant! But using instances seems to offer more flexibility going forward.
Thank you very much!
I am working on implementing Image Based Lighting in a ray tracing project. In order to get the sampling to work predictably, I will need to load and process an HDR image into a MTLTexture.
While using a MTKTextureLoader to load .jpg and .png files has been super-smooth, loading a .hdr file in Swift seems to have a few barriers.
I have tried to follow the Processing HDR Images with Metal sample code [ Objective C ], but it relies on some finessing of pointers that I can't duplicate in Swift.
Is there currently a best way to move an HDR image into a Metal texture using Swift?
Thanks so much!
I have watched the Enhance Your App with Metal Ray Tracing video ( wwdc21-10149 ) a few times and am very interested in many of the upcoming features described. It seems as though there will be some tweaks ( ie. Instance IDs ) and some major additions ( ie. Keyframe Animation ).
I am wondering if there is any sense of the timing of these features becoming available ( beta or otherwise ) and also, if some of the sample code might be provided in Swift.
Thanks so much for the work that went into the video and explanations.