Why is the triangle color a rainbow ? about learn-wgpu HOT 5 CLOSED

The triangle is a rainbow because the GPU interpolates the values between the vertices in a process known as rasterization.

As far as getting access to the pixel coordinate inside the triangle you can use the @builtin(position) decorator on a vec4 to get the pixels coordinates in screen space. It's basically the same as gl_FragPosition in OpenGL.

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when using @builtin(position) as input for the fragment shader, what's the coordinates system we're getting there ? is it possible to transform them in clip space ?

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The coordinate system of @builtin(position) is in screen space, or in other words it uses the pixel coordinates of the viewport. You can transform them back to clip space, but it's simpler to just pass the clip space coords into the fragment shader from the vertex shader.

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For a little more context in the @builtin(position), the top left corner of the viewport is (0,0) and the bottme right corner is (viewport_width, viewport_height). I believe the z component is the depth/array layer, though I'm not sure.

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Thanks so much for your explanations 🙏

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