如何在iOS上快速从RGB24转换为BGRA

Question

我使用的是vImageConvert_RGB888toARGB8888和vImagePermuteChannels_ARGB8888。首先将其从RGB24更改为ARGB，因为OPENGL ES不支持ARGB，所以需要使用vImagePermuteChannels_ARGB8888将ARGB更改为BGRA。但是，当需要更改的数据很大时，时间成本很高，所以有更好的方法吗？谢谢我的代码是这样的：

- (void)transformRGBToBGRA:(const UInt8 *)pict{
rgb.data = (void *)pict;
vImage_Error error = vImageConvert_RGB888toARGB8888(&rgb,NULL,0,&argb,NO,kvImageNoFlags);
if (error != kvImageNoError) {
    NSLog(@"vImageConvert_RGB888toARGB8888 error");
}
const uint8_t permuteMap[4] = {3,2,1,0};
error = vImagePermuteChannels_ARGB8888(&argb,&bgra,permuteMap,kvImageNoFlags);
if (error != kvImageNoError) {
    NSLog(@"vImagePermuteChannels_ARGB8888 error");
}
free((void *)pict);
}

Answer 1

现在我发现最快的解决方案是组装。就像这样：

__asm__ volatile(
                 "0:                n"
                 "# load 3 64-bit regs with interleave: n"
                 "vld3.8      {d0,d1,d2}, [%0]!   n"
                 "# swap d0 and d2 - R and Bn"
                 "vswp d0, d2   n"
                 "# store 4 64-bit regs: n"
                 "vst4.8      {d0,d1,d2,d3}, [%2]!      n"
                 "subs        %1, %1, #1       n"
                 "bne         0b            n"
                 :
                 : "r"(img), "r"(numPixels24), "+r"(bgraData)
                 : "r6", "r7","r8"
                 );