How to draw a line with variable endpoints with Metal on macOS?

Question

I need to draw a line (light-source/light-target) with Metal on macOS within a 60 FPS animation while the two endpoints coordinates of the line change on each frame.

Actually I can properly draw a static line with Metal using a MTLBuffer containing just two fixed endpoints, but any transformation I apply on the line matrix affects both the endpoints, not each endpoint independently.

So how can a vary each of the two endpoints coordinates at each render pass? Is a way to modify the MTLBuffer at each frame? Should I pass the two coordinates to the shader through setVertexBytes: then use a different pipeline to draw this line?

Answer 1

I have found the solution. I modify the line MTLBuffer content at each render pass. I just set the 2 endpoints coordinates value then I set the vertexBuffer and draw the line.

Vertex *vertex = mLineVertexBuffer.contents;
vertex[0].position = lightSourcePos3D.xyz;
vertex[1].position = lightTargetPos3D.xyz;
[cmdEncoder setVertexBuffer:mLineVertexBuffer offset:0 atIndex:0];
[cmdEncoder drawPrimitives:MTLPrimitiveTypeLine vertexStart:0 vertexCount:2];

The strange think is that my vertexBuffer is described as immutable while it seems that I can still flawlessly modify it.

pipelineDescriptor.vertexBuffers[0].mutability = MTLMutabilityImmutable;

Answer 2

The strange think is that my vertexBuffer is described as immutable while it seems that I can still flawlessly modify it.

Metal can improve perfomance if you declare that you won't modify a buffer's contents between when you set the buffer in an encoder's argument table and when the associated command buffer finishes executing . Neither the CPU or GPU can update the buffer during this time interval. For better performance, use immutable buffers whenever possible.