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    <p>With a RGB888ToPlanar8 call you scatter the data and then gather it once again. This is very-very-very bad. If the memory overhead of 33% is affordable, try using the RGBA format and permute the B/R bytes in-place.</p> <p>If you want to save 33% percents, then I might suggest the following. Iterate all the pixels, but read only a multiple of 4 bytes (since lcm(3,4) is 12, that is 3 dwords).</p> <pre><code>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 &lt; 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 &gt;&gt; 8) &amp; 0xFF) | (v1 &amp; 0x00FF0000) | ((v1 &gt;&gt; 16) &amp; 0xFF) | ((v2 &gt;&gt; 24) &amp; 0xFF); nv2 = // [G2 R2 B3 G3] ... nv3 = // [R3 B4 G4 R4] ... // write 12 bytes *dst++ = nv1; *dst++ = nv2; *dst++ = nv3; } </code></pre> <p>Even better can be done with NEON intrinsics. </p> <p>See this link <a href="http://blogs.arm.com/software-enablement/161-coding-for-neon-part-1-load-and-stores/" rel="nofollow">from ARM's website</a> to see how the 24-bit swapping is done.</p> <p>The BGR-to-RGB can be done in-place like this:</p> <pre><code>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" ); } </code></pre>
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    1. POOn iOS how to quickly convert RGB24 to BGR24?
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    1. USViktor Latypov
    1. USViktor Latypov
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    1. COso sad, In my test the ** Iterate all the pixels, but read only a multiple of 4 bytes (since lcm(3,4) is 12, that is 3 dwords)** is more slow than `RGB888ToPlanar8`, which cost as twice as `RGB888ToPlanar8`.In my experiment `RGB888ToPlanar8` cost 10ms, the other need 20ms.
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    2. COThen use the assembly version, C version is _not_ a competitor to Accelerate framework
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    3. COSorry for the errors in bit-shifting. ARM is both BE/LE, but by default it is usually little-endian. And my C code is definitely a big-endian one.
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      1. USViktor Latypov
 

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