MetalKit - Drawable.texture assertion error - swift

Question

I am new to MetalKit and trying to convert this tutorial from playground back to OSX app:
import MetalKit
public class MetalView: MTKView {
var queue: MTLCommandQueue! = nil
var cps: MTLComputePipelineState! = nil
required public init(coder: NSCoder) {
super.init(coder: coder)
device = MTLCreateSystemDefaultDevice()
registerShaders()
}
override public func drawRect(dirtyRect: NSRect) {
super.drawRect(dirtyRect)
if let drawable = currentDrawable {
let command_buffer = queue.commandBuffer()
let command_encoder = command_buffer.computeCommandEncoder()
command_encoder.setComputePipelineState(cps)
command_encoder.setTexture(drawable.texture, atIndex: 0)
let threadGroupCount = MTLSizeMake(8, 8, 1)
let threadGroups = MTLSizeMake(drawable.texture.width / threadGroupCount.width, drawable.texture.height / threadGroupCount.height, 1)
command_encoder.dispatchThreadgroups(threadGroups, threadsPerThreadgroup: threadGroupCount)
command_encoder.endEncoding()
command_buffer.presentDrawable(drawable)
command_buffer.commit()
}
}
func registerShaders() {
queue = device!.newCommandQueue()
do {
let library = device!.newDefaultLibrary()!
let kernel = library.newFunctionWithName("compute")!
cps = try device!.newComputePipelineStateWithFunction(kernel)
} catch let e {
Swift.print("\(e)")
}
}
}
I got an error at the line:
command_encoder.setTexture(drawable.texture, atIndex: 0)
failed assertion `frameBufferOnly texture not supported for compute.'
How can I resolve this?

Answer 1

If you want to write to a drawable's texture from a compute function, you'll need to tell the MTKView that it should configure its layer not to be framebuffer-only:
metalView.framebufferOnly = false
With this value set to false, your drawable will give you a texture with the shaderWrite usage flag set, which is required when writing a texture from a shader function.

Answer 2

I have the same issue even I set the frameBufferOnly to false and print the boolean for double check before using the drawable.
But when I turn off the GPU frame capture, no more assertion occurs (for now at least). Still don't know why.
Menu > Scheme > Edit Scheme > Run > Options > GPU Frame Capture: Disabled