Metal off-screen drawing with Multi-Sampling - ios

Question

How do I render primitives into off screen texture, not directly into the screen?
I have a set of triangles and corresponding color, I just want to draw them the same way I do to screen, but into off screen texture, that I can save into a file.
Can anybody show me a code sample of that?

Answer 1

Ok, I realized it myself. This code does the job, with only exception that it draw too huge triangles, but that is a different topic for Vertex function.
Here is my code:
let fragmentProgram = defaultLibrary.newFunctionWithName("image_fragmentT")
let vertexProgram = defaultLibrary.newFunctionWithName("image_vertexT")
struct VertexT {
var x, y, z, w : Float
var r, g, b, a : Float
}
let vertexDescriptor = MTLVertexDescriptor()
vertexDescriptor.attributes[0].offset = 0
vertexDescriptor.attributes[0].format = .Float4
vertexDescriptor.attributes[0].bufferIndex = 0
vertexDescriptor.attributes[1].offset = 0
vertexDescriptor.attributes[1].format = .Float4
vertexDescriptor.attributes[1].bufferIndex = 0
vertexDescriptor.layouts[0].stepFunction = .PerVertex
vertexDescriptor.layouts[0].stride = sizeof(VertexT)
let pipelineStateDescriptor = MTLRenderPipelineDescriptor()
pipelineStateDescriptor.vertexDescriptor = vertexDescriptor
pipelineStateDescriptor.vertexFunction = vertexProgram
pipelineStateDescriptor.fragmentFunction = fragmentProgram
pipelineStateDescriptor.colorAttachments[0].pixelFormat = .RGBA8Unorm;
pipelineStateDescriptor.colorAttachments[0].blendingEnabled = true
pipelineStateDescriptor.sampleCount = 4
pipelineStateDescriptor.colorAttachments[0].rgbBlendOperation = MTLBlendOperation.Add
pipelineStateDescriptor.colorAttachments[0].alphaBlendOperation = MTLBlendOperation.Add
pipelineStateDescriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactor.SourceAlpha
pipelineStateDescriptor.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactor.SourceAlpha
pipelineStateDescriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactor.OneMinusSourceAlpha
pipelineStateDescriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactor.OneMinusSourceAlpha
let sampleDesc = MTLTextureDescriptor()
sampleDesc.textureType = MTLTextureType.Type2DMultisample
sampleDesc.width = inTexture.width
sampleDesc.height = inTexture.height
sampleDesc.sampleCount = 4
sampleDesc.pixelFormat = .RGBA8Unorm
sampleDesc.storageMode = .Private
sampleDesc.usage = .RenderTarget
let sampletex = device.device.newTextureWithDescriptor(sampleDesc)
let renderPassDescriptor = MTLRenderPassDescriptor()
renderPassDescriptor.colorAttachments[0].texture = sampletex
renderPassDescriptor.colorAttachments[0].resolveTexture = outTexture
renderPassDescriptor.colorAttachments[0].loadAction = .Clear
renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 0.0)
renderPassDescriptor.colorAttachments[0].storeAction = .MultisampleResolve
let renderCB = commandQueue.commandBuffer()
let renderCommandEncoder = renderCB.renderCommandEncoderWithDescriptor(renderPassDescriptor)
let pipelineState = try! device.device.newRenderPipelineStateWithDescriptor(pipelineStateDescriptor)
renderCommandEncoder.setRenderPipelineState(pipelineState)
let vertexBuf = device.device.newBufferWithLength(triangles.count * 3 * sizeof(VertexT), options: .CPUCacheModeDefaultCache)
var vBufPointer = [VertexT]()
for i in 0..<triangles.count {
// create buffer here
}
memcpy(vertexBuf.contents(), &vBufPointer, triangles.count * 3 * sizeof(VertexT))
renderCommandEncoder.setVertexBuffer(vertexBuf, offset: 0, atIndex: 0)
renderCommandEncoder.drawPrimitives(.Triangle, vertexStart: 0, vertexCount: triangles.count * 3)
renderCommandEncoder.endEncoding()
renderCB.commit()
renderCB.waitUntilCompleted()
You image now is in outTexture variable.