Values for certain types of uniforms within UBOs are shown incorrectly #277

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2. mixtur commented on Aug 23, 2023

   

   mixtur

   on Aug 23, 2023

   Last edited by mixtur

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   Note layout(std140) in the shader code - it should make the layout implementation independent, but if you want, you can try it yourself https://cib3.webgears.app/3D-Engine/228e665dc1d4cd9fd12ab71a4dbcf5abd64e26e3/shadows.html (look for draw calls with PBR_MESH_SHADER in shader names) The link is only going to be alive for a few days maximum.

   Though judging by the implementation of getUboValue it is expected that there are zeroes.
   

   Spector.js/src/backend/states/drawCalls/drawCallUboInputState.ts

   Lines 50 to 75 in 35385a5

   	public getUboValue(bindingPoint: number, offset: number, size: number, type: number): any {
   	const uboType = DrawCallUboInputState.uboTypes[type];
   	if (!uboType) {
   	return undefined;
   	}
   	const destination = new uboType.arrayBufferView(size * uboType.lengthMultiplier);
   	const context2 = this.context as WebGL2RenderingContext;
   	const ownerbuffer = context2.getIndexedParameter(WebGlConstants.UNIFORM_BUFFER_BINDING.value, bindingPoint);
   	if (ownerbuffer) {
   	const startOffset = context2.getIndexedParameter(WebGlConstants.UNIFORM_BUFFER_START.value, bindingPoint);
   	const boundBuffer = context2.getParameter(WebGlConstants.UNIFORM_BUFFER_BINDING.value);
   	try {
   	context2.bindBuffer(WebGlConstants.UNIFORM_BUFFER.value, ownerbuffer);
   	context2.getBufferSubData(WebGlConstants.UNIFORM_BUFFER.value, startOffset + offset, destination);
   	}
   	catch (e) {
   	// Prevent back fromats to break the capture.
   	return undefined;
   	}
   	if (boundBuffer) {
   	context2.bindBuffer(WebGlConstants.UNIFORM_BUFFER.value, boundBuffer);
   	}
   	}
   	return Array.prototype.slice.call(destination);

   In case of float x[3] the layout is the following:

   [ 4 bytes x[0], 12 bytes to satisfy alignment,
     4 bytes x[1], 12 bytes to satisfy alignment,
     4 bytes x[2], 12 bytes padding,
     ]
   

   The implementation simply does getBufferSubdata and then seems to do nothing with it. Assuming uboType.lengthMultiplier is 1 and size is 3. It should indeed read the first float and two zero-paddings immediately following it.
4. mixtur commented on Aug 23, 2023

   

   mixtur

   on Aug 23, 2023

   Last edited by mixtur

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   According to the spec https://registry.khronos.org/OpenGL/extensions/ARB/ARB_uniform_buffer_object.txt column-major matrices are treated as arrays of columns regardless of the dimensions. And array elements are vec4-aligned. So mat4x2 is taking as much space as mat4.

   Row-major matrices are the same except they should be treated as arrays of rows.

   Although today I discovered that when on NVidia GPUs + Chrome row-major case does something else. As well as Android on Mali G68. So yeah. It seems that this stuff is still implementation-defined to some extent. Or maybe I am using the wrong spec :)
5. sebavan commented on Aug 30, 2023

   

   sebavan

   on Aug 30, 2023

   Last edited by sebavan

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   This is a really interesting one indeed :-)

   @JannikGM, thanks a lot for your proposal here, this is going in the right direction and yes it was a complete oversight on my side.