Hello. In the iOS app i'm working on we are very tight on memory budget and I was looking at ways to reduce our texture memory usage. However I noticed that comparing ASTC8x8 to ASTC12x12, there is no actual difference in allocated memory for most of our textures despite ASTC12x12 having less than half the bpp of 8x8. The difference between the two only becomes apparent for textures 1024x1024 and larger, and even in that case the actual texture data is sometimes only 60% of the allocation size. I understand there must be some alignment and padding going on, but this seems extreme. For an example scene in my app with astc12x12 for most textures there is over a 100mb difference in astc size on disk versus when loaded, so I would love to be able to recover even a portion of that memory.
Here is some test code with some measurements i've taken using an iphone 11:
for(int i = 0; i < 11; i++) {
MTLTextureDescriptor *texDesc = [[MTLTextureDescriptor alloc] init];
texDesc.pixelFormat = MTLPixelFormatASTC_12x12_LDR;
int dim = 12;
int n = 2 << i;
int mips = i+1;
texDesc.width = n;
texDesc.height = n;
texDesc.mipmapLevelCount = mips;
texDesc.resourceOptions = MTLResourceStorageModeShared;
texDesc.usage = MTLTextureUsageShaderRead;
// Calculate the equivalent astc texture size
int blocks = 0;
if(mips == 1) {
blocks = n/dim + (n%dim>0? 1 : 0);
blocks *= blocks;
} else {
for(int j = 0; j < mips; j++) {
int a = 2 << j;
int cur = a/dim + (a%dim>0? 1 : 0);
blocks += cur*cur;
}
}
auto tex = [objCObj newTextureWithDescriptor:texDesc];
printf("%dx%d, mips %d, Astc: %d, Metal: %d\n", n, n, mips, blocks*16, (int)tex.allocatedSize);
}
MTLPixelFormatASTC_12x12_LDR
128x128, mips 7, Astc: 2768, Metal: 6016
256x256, mips 8, Astc: 10512, Metal: 32768
512x512, mips 9, Astc: 40096, Metal: 98304
1024x1024, mips 10, Astc: 158432, Metal: 262144
128x128, mips 1, Astc: 1936, Metal: 4096
256x256, mips 1, Astc: 7744, Metal: 16384
512x512, mips 1, Astc: 29584, Metal: 65536
1024x1024, mips 1, Astc: 118336, Metal: 147456
MTLPixelFormatASTC_8x8_LDR
128x128, mips 7, Astc: 5488, Metal: 6016
256x256, mips 8, Astc: 21872, Metal: 32768
512x512, mips 9, Astc: 87408, Metal: 98304
1024x1024, mips 10, Astc: 349552, Metal: 360448
128x128, mips 1, Astc: 4096, Metal: 4096
256x256, mips 1, Astc: 16384, Metal: 16384
512x512, mips 1, Astc: 65536, Metal: 65536
1024x1024, mips 1, Astc: 262144, Metal: 262144
I also tried using MTLHeaps (placement and automatic) hoping they might be better, but saw nearly the same numbers.
Is there any way to have metal allocate these textures in a more compact way to save on memory?
