Function queryMipLevelMixWeightsForDevice

In an attempt to pass on more devices without lowering the tolerances so low they are meaningless, we ask the hardware to tell us, for a given gradient, level, what mix weights are being used.

This is done by drawing instanced quads and using instance_index to write out results into an array. We sample a 2x2 pixel texture with 2 mip levels and set the 2nd mip level to white. This means the value we get back represents the weight used to mix the 2 mip levels.

Just as a record of some differences across GPUs

level weights: mapping from the mip level parameter of textureSampleLevel to the mix weight used by the GPU

+--------+--------+--------+--------+ | | | intel | amd | | | m1 | gen-9 | rna-1 | | level | mac | mac | mac | +--------+--------+--------+--------+ | 0.0000 | 0.0000 | 0.0000 | 0.0000 | | 0.0313 | 0.0314 | 0.0313 | 0.0000 | | 0.0625 | 0.0625 | 0.0625 | 0.0000 | | 0.0938 | 0.0939 | 0.0938 | 0.0000 | | 0.1250 | 0.1250 | 0.1250 | 0.0313 | | 0.1563 | 0.1564 | 0.1563 | 0.0703 | | 0.1875 | 0.1875 | 0.1875 | 0.1094 | | 0.2188 | 0.2189 | 0.2188 | 0.1484 | | 0.2500 | 0.2500 | 0.2500 | 0.1875 | | 0.2813 | 0.2814 | 0.2813 | 0.2266 | | 0.3125 | 0.3125 | 0.3125 | 0.2656 | | 0.3438 | 0.3439 | 0.3438 | 0.3047 | | 0.3750 | 0.3750 | 0.3750 | 0.3438 | | 0.4063 | 0.4064 | 0.4063 | 0.3828 | | 0.4375 | 0.4375 | 0.4375 | 0.4219 | | 0.4688 | 0.4689 | 0.4688 | 0.4609 | | 0.5000 | 0.5000 | 0.5000 | 0.5000 | | 0.5313 | 0.5314 | 0.5313 | 0.5391 | | 0.5625 | 0.5625 | 0.5625 | 0.5781 | | 0.5938 | 0.5939 | 0.5938 | 0.6172 | | 0.6250 | 0.6250 | 0.6250 | 0.6563 | | 0.6563 | 0.6564 | 0.6563 | 0.6953 | | 0.6875 | 0.6875 | 0.6875 | 0.7344 | | 0.7188 | 0.7189 | 0.7188 | 0.7734 | | 0.7500 | 0.7500 | 0.7500 | 0.8125 | | 0.7813 | 0.7814 | 0.7813 | 0.8516 | | 0.8125 | 0.8125 | 0.8125 | 0.8906 | | 0.8438 | 0.8439 | 0.8438 | 0.9297 | | 0.8750 | 0.8750 | 0.8750 | 0.9688 | | 0.9063 | 0.9064 | 0.9063 | 1.0000 | | 0.9375 | 0.9375 | 0.9375 | 1.0000 | | 0.9688 | 0.9689 | 0.9688 | 1.0000 | | 1.0000 | 1.0000 | 1.0000 | 1.0000 | +--------+--------+--------+--------+

grad weights: mapping from ddx value passed into textureSampleGrad to the mix weight used by the GPU

+--------+--------+--------+--------+ | | | intel | amd | | | m1 | gen-9 | rna-1 | | ddx | mac | mac | mac | +--------+--------+--------+--------+ | 0.5000 | 0.0000 | 0.0000 | 0.0000 | | 0.5109 | 0.0390 | 0.0430 | 0.0000 | | 0.5221 | 0.0821 | 0.0859 | 0.0000 | | 0.5336 | 0.1211 | 0.1289 | 0.0352 | | 0.5453 | 0.1600 | 0.1719 | 0.0898 | | 0.5572 | 0.2032 | 0.2109 | 0.1328 | | 0.5694 | 0.2422 | 0.2461 | 0.1797 | | 0.5819 | 0.2814 | 0.2852 | 0.2305 | | 0.5946 | 0.3203 | 0.3203 | 0.2773 | | 0.6076 | 0.3554 | 0.3594 | 0.3164 | | 0.6209 | 0.3868 | 0.3906 | 0.3633 | | 0.6345 | 0.4218 | 0.4258 | 0.4063 | | 0.6484 | 0.4532 | 0.4609 | 0.4492 | | 0.6626 | 0.4882 | 0.4922 | 0.4883 | | 0.6771 | 0.5196 | 0.5234 | 0.5273 | | 0.6920 | 0.5507 | 0.5547 | 0.5664 | | 0.7071 | 0.5860 | 0.5859 | 0.6055 | | 0.7226 | 0.6132 | 0.6133 | 0.6406 | | 0.7384 | 0.6407 | 0.6445 | 0.6797 | | 0.7546 | 0.6679 | 0.6719 | 0.7148 | | 0.7711 | 0.6953 | 0.6992 | 0.7461 | | 0.7880 | 0.7225 | 0.7266 | 0.7813 | | 0.8052 | 0.7500 | 0.7539 | 0.8164 | | 0.8229 | 0.7814 | 0.7813 | 0.8516 | | 0.8409 | 0.8086 | 0.8086 | 0.8828 | | 0.8593 | 0.8321 | 0.8320 | 0.9141 | | 0.8781 | 0.8554 | 0.8594 | 0.9492 | | 0.8974 | 0.8789 | 0.8828 | 0.9766 | | 0.9170 | 0.9025 | 0.9063 | 1.0000 | | 0.9371 | 0.9297 | 0.9297 | 1.0000 | | 0.9576 | 0.9532 | 0.9531 | 1.0000 | | 0.9786 | 0.9765 | 0.9766 | 1.0000 | | 1.0000 | 1.0000 | 1.0000 | 1.0000 | +--------+--------+--------+--------+