在iOS上如何快速将RGB24转换为BGR24？

Question

我使用vImageConvert_RGB888toPlanar8和vImageConvert_Planar8toRGB888来自Accelerate.framework将RGB24转换为BGR24,但是当需要转换的数据非常大时,例如3M或4M,需要花费的时间大约是10ms.所以有人知道一些足够快的想法吗？.我的代码如下:

- (void)transformRGBToBGR:(const UInt8 *)pict{
rgb.data = (void *)pict;

vImage_Error error = vImageConvert_RGB888toPlanar8(&rgb,&red,&green,&blue,kvImageNoFlags);
if (error != kvImageNoError) {
    NSLog(@"vImageConvert_RGB888toARGB8888 error");
}

error = vImageConvert_Planar8toRGB888(&blue,&green,&red,&bgr,kvImageNoFlags);
if (error != kvImageNoError) {
    NSLog(@"vImagePermuteChannels_ARGB8888 error");
}

free((void *)pict);
}

Answer 1

使用RGB888ToPlanar8调用,您可以分散数据,然后再次收集数据.这非常非常糟糕.如果33%的内存开销是可以承受的,请尝试使用RGBA格式并就地置换B/R字节.

如果你想节省33%的百分比,那么我可能会建议如下.迭代所有像素,但只读取4个字节的倍数(因为lcm(3,4)是12,即3个双字).

uint8_t* src_image;
uint8_t* dst_image;

uint32_t* src = (uint32_t*)src_image;
uint32_t* dst = (uint32_t*)dst_image;

uint32_t v1, v2, v3;
uint32_t nv1, nv2, nv3;
for(int i = 0 ; i < num_pixels / 12 ; i++)
{
     // read 12 bytes
     v1 = *src++;
     v2 = *src++;
     v3 = *src++;
     // shuffle bits in the pixels
     // [R1 G1 B1 R2 | G2 B2 R3 G3 | B3 R4 G4 B4]
     nv1 = // [B1 G1 R1 B2]
      ((v1 >> 8) & 0xFF) | (v1 & 0x00FF0000) | ((v1 >> 16) & 0xFF) | ((v2 >> 24) & 0xFF);
     nv2 = // [G2 R2 B3 G3]
       ...
     nv3 = // [R3 B4 G4 R4]
       ...
     // write 12 bytes
     *dst++ = nv1;
     *dst++ = nv2;
     *dst++ = nv3;
}

使用NEON内在函数可以做得更好.

请参阅ARM网站上的链接,了解24位交换是如何完成的.

BGR-to-RGB可以就地完成,如下所示:

void neon_asm_convert_BGR_TO_RGB(uint8_t* img, int numPixels24)
{
    // numPixels is divided by 24
    __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 B\n"
        "vswp d0, d2   \n"
        "# store 3 64-bit regs: \n"
        "vst3.8      {d0,d1,d2}, [%0]!      \n"
        "subs        %1, %1, #1       \n"
        "bne         0b            \n"
        :
        : "r"(img), "r"(numPixels24)
        : "r4", "r5"
     );
}