Implementing Scalable Order-Independent Transparency (OIT) in Metal

Question

arghachakraborty OP

Created Feb ’25

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Hi,

Apple’s documentation on Order-Independent Transparency (OIT) describes an approach using image blocks, where an array of size 4 is allocated per fragment to store depth and color in a tile shading compute pass.

However, when increasing the scene’s depth complexity by adding more overlapping quads, the OIT implementation fails due to the fixed array size.

Is there a way to dynamically allocate storage for fragments based on actual depth complexity encountered during rasterization, rather than using a fixed-size array? Specifically, can an adaptive array of fragments be maintained and sorted by depth, where the size grows as needed instead of being limited to 4 entries?

Any insights or alternative approaches would be greatly appreciated.

Thank you!

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Answer 1

DTS Engineer OP

Apple

Mar ’25

Hello,

We'll investigate how this value can effectively be changed and whether it's possible to do so efficiently on the GPU.

/// The number of transparent geometry layers that the app stores in image block memory.
/// Each layer consumes tile memory and increases the value of the pipeline's `imageBlockSampleLength` property.
static constexpr constant short kNumLayers = 4;

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