Beginning Metal - Part 3: Triangles | Ray Wenderlich

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In this video tutorial, you'll discover why triangles are important and how to draw them using Metal.

This is a companion discussion topic for the original entry at https://www.raywenderlich.com/3537-beginning-metal/lessons/3
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Hey there!
Just a question. At the end of the tutorial when you call
commandEncoder.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: vertices.count)
doesn’t vertices.count return the number 9 in this case? But we only have 3 vertices right? Or does the function divide this number by 3 itself?

Thanks for the help and great tutorials!

1 Like
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@bababoega - you are correct :smiley: - there are 9 in the array, but only 3 vertices to be drawn.

If you change vertices.count to 3, the triangle still draws correctly.

I’m actually not sure what the GPU is doing with the other 6.

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Hi Caroline,

First of all thanks for these great tutorials :slight_smile:

I have one question though. In the fragment shader function, we have used RGBA scheme. Then why do we use .bgra8Unorm in the following line?
pipelineDescriptor.colorAttachments[0].pixelFormat = .bgra8Unorm

Thanks again,
Suvrat :slight_smile:

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It’s a great question, and I hadn’t really thought about it in depth before. bgra8Unorm is a 32 bit colour format with each component having 8 bits. So 255,255,255,1 will be fully white.

My research leads me to this paper from nVidia: http://http.download.nvidia.com/developer/Papers/2005/Fast_Texture_Transfers/Fast_Texture_Transfers.pdf

Page 5 says that the GPU prefers the BGRA format for textures. This may be for historical reasons - .tga files are in bgra format.

In addition, I’d like to clarify that the return value from the fragment function is completely distinct from the data in the Metal buffer.

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Thank you so much for the quick reply! :slight_smile:
I’ll go through the paper. I don’t know if I’ll understand it completely :stuck_out_tongue:

Thanks again for these tutorials, they are awesome!

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BGRA8UNORM is the default pixel format of the framebuffer, therefore it must match with the one you supply when creating the MTLRenderPipelineState.

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the fast forwarding of code typing in these videos is asinine, forcing me to pause and replay the video dozens of times. what made you think this was a good idea?