在iOS上如何快速将RGB24转换为BGR24

Question

我使用Accelerate.framework中的vImageConvert_RGB888toPlanar8和vImageConvert_Planar8toRGB888将RGB24转换为BGR24，但当需要转换的数据非常大时，例如3M或4M，需要花费的时间约为10ms。所以有人知道一些足够快的想法？。我的代码是这样的：

- (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格式并在适当的位置排列B/R字节。

如果你想节省33%，那么我可能会建议如下。迭代所有像素，但只读取4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;
}

NEON内部函数可以做得更好。

请参阅ARM网站上的链接，了解24位交换是如何完成的。

BGR到RGB可以这样进行：

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 Bn"
        "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"
     );
}

Answer 2

只需将通道-BGRA切换为RGBA

- (void)convertBGRAFrame:(const CLPBasicVideoFrame &)bgraFrame toRGBA:(CLPBasicVideoFrame &)rgbaFrame
{
    vImage_Buffer bgraImageBuffer = {
        .width = bgraFrame.width,
        .height = bgraFrame.height,
        .rowBytes = bgraFrame.bytesPerRow,
        .data = bgraFrame.rawPixelData
    };
    vImage_Buffer rgbaImageBuffer = {
        .width = rgbaFrame.width,
        .height = rgbaFrame.height,
        .rowBytes = rgbaFrame.bytesPerRow,
        .data = rgbaFrame.rawPixelData
    };
    const uint8_t byteSwapMap[4] = { 2, 1, 0, 3 };
    vImage_Error error;
    error = vImagePermuteChannels_ARGB8888(&bgraImageBuffer, &rgbaImageBuffer, byteSwapMap, kvImageNoFlags);
    if (error != kvImageNoError) {
        NSLog(@"%s, vImage error %zd", __PRETTY_FUNCTION__, error);
    }
}