16bpp/15bpp to 32bpp pixel conversions — different methods

Basically this kind of conversion needs following steps:

• extract components R, G and B (using bitwise and)
• extend words from 6 or 5 bits to 8 bits (shift left)
• place components at desired places in a 32-bit word (shift, bitwise or)

R = (pixel16 and 0x001f) shl 3
G = (pixel16 and 0x07e0) shr 5
B = (pixel16 and 0xf800) shr 11

pixel32 = R or (G shl 8) or (B shl 16)

Since there aren't many pixels (32 or 64 thousand) lookup tables can be used. First approach is to use one big table indexed by pixels treated as natural numbers: this table has size 65536 * 4 bytes = 262144 bytes. Just one memory access is needed to get 32bpp pixel, however table size is large, and even if fit in L2 cache, memory latency kill performance.

pixel32 = LUT[pixel16]

Another approach needs two tables indexed by lower and higher byte of pixel, final pixel is result of bitwise or. These tables has size 2 * 256 * 4 bytes = 2048 bytes — perfectly fit in L1 cache.

pixel32 = LUT_hi[pixel16 shr 8] or LUT_lo[pixel16 and 0xff]

Sample program pixconv16bpp-32bpp.c includes different procedures:

• naive — is a straightforward implementation of algorithm described in the beginning
• lookup16 — uses single, large lookup table
• lookup8 — use two, small lookup tables; it converts one pixel per iteration
• lookup8(2) — improved lookup8 — converts two pixels per iteration, requires single fetch of 4 bytes
• MMX — naive algorithm using MMX instructions
• SSE2 — naive algorithm using SSE2 instructions; converts 8 pixels per iteration
• SSE2(2) — unrolled SSE2 version that converts 16 pixels per iteration

gcc -O3 pixconv16bpp-32bpp.c -o test

gcc -O3 -DNONTEMPORAL pixconv16bpp-32bpp.c -o testnta