rendering
7 min read
Draped at the wrong tile: a deferred write turned a shared uniform pool into a data race
A draped landcover polygon rendered hundreds of kilometres offshore — but only on WebGPU. Every vector slice of one source shares one Material uniform pool, and each slice resets the pool cursor to 0, so a later slice's tile i overwrote an earlier slice's tile-i buffer. WebGPU's queue.writeBuffer defers to submit (last-writer-wins), so every draw read the final bytes; WebGL2's immediate bufferSubData never showed it — and the differential test was structurally blind, because both frames aliased identically.