Resize 8-bit image by 2 with ARM NEON

Question

I have an 8-bit 640x480 image that I would like to shrink to a 320x240 image:

void reducebytwo(uint8_t *dst, uint8_t *src)
//src is 640x480, dst is 320x240

What would be the best way to do that using ARM SIMD NEON? Any sample code somewhere?

As a starting point, I simply would like to do the equivalent of:

for (int h = 0; h < 240; h++)
    for (int w = 0; w < 320; w++)
        dst[h * 320 + w] = (src[640 * h * 2 + w * 2] + src[640 * h * 2 + w * 2 + 1] + src[640 * h * 2 + 640 + w * 2] + src[640 * h * 2 + 640 + w * 2 + 1]) / 4;

**Best** needs to be defined. Fastest, highest quality, minimum size, etc? For *highest quality*, there are different tradeoffs in image reduction. Preserving low frequency content is important is some cases and high frequency in others. What is *8-bit*? A gray scale, colour mapped, or something else? — artless noise, Jul 23 '13 at 17:20

score 4 · Answer 1 · edited Oct 08 '13 at 15:28

This is a one to one translation of your code to arm NEON intrinsics:

#include <arm_neon.h>
#include <stdint.h>

static void resize_line (uint8_t * __restrict src1, uint8_t * __restrict src2, uint8_t * __restrict dest)
{
  int i;
  for (i=0; i<640; i+=16)
  {
    // load upper line and add neighbor pixels:
    uint16x8_t a = vpaddlq_u8 (vld1q_u8 (src1));

    // load lower line and add neighbor pixels:
    uint16x8_t b = vpaddlq_u8 (vld1q_u8 (src2));

    // sum of upper and lower line: 
    uint16x8_t c = vaddq_u16 (a,b);

    // divide by 4, convert to char and store:
    vst1_u8 (dest, vshrn_n_u16 (c, 2));

    // move pointers to next chunk of data
    src1+=16;
    src2+=16;
    dest+=8;
   }
}   

void resize_image (uint8_t * src, uint8_t * dest)
{
  int h;    
  for (h = 0; h < 240 - 1; h++)
  {
    resize_line (src+640*(h*2+0), 
                 src+640*(h*2+1), 
                 dest+320*h);
  }
}

It processes 32 source-pixels and generates 8 output pixels per iteration.

I did a quick look at the assembler output and it looks okay. You can get better performance if you write the resize_line function in assembler, unroll the loop and eliminate pipeline stalls. That would give you an estimated factor of three performance boost.

It should be a lot faster than your implementation without assembler changes though.

Note: I haven't tested the code...

Great! Do you think that having the whole resize_image function in assembler would be much faster or do you think that with your suggestion, I have already 90% of the time saving? — gregoiregentil, Jul 24 '13 at 17:40

score 2 · Answer 2 · answered Aug 07 '13 at 12:09

If you're not too concerned with precision then this inner loop should give you twice the compute throughput compared to the more accurate algorithm:

for (i=0; i<640; i+= 32)
{
    uint8x16x2_t a, b;
    uint8x16_t c, d;

    /* load upper row, splitting even and odd pixels into a.val[0]
     * and a.val[1] respectively. */
    a = vld2q_u8(src1);

    /* as above, but for lower row */
    b = vld2q_u8(src2);

    /* compute average of even and odd pixel pairs for upper row */
    c = vrhaddq_u8(a.val[0], a.val[1]);
    /* compute average of even and odd pixel pairs for lower row */
    d = vrhaddq_u8(b.val[0], b.val[1]);

    /* compute average of upper and lower rows, and store result */
    vst1q_u8(dest, vrhaddq_u8(c, d));

    src1+=32;
    src2+=32;
    dest+=16;
}

It works by using the vhadd operation, which has a result the same size as the input. This way you don't have to shift the final sum back down to 8-bit, and all of the arithmetic throughout is eight-bit, which means you can perform twice as many operations per instruction.

However it is less accurate, because the intermediate sum is quantised, and GCC 4.7 does a terrible job of generating code. GCC 4.8 does just fine.

The whole operation has a good chance of being I/O bound, though. The loop should be unrolled to maximise separation between loads and arithmetic, and __builtin_prefetch() (or PLD) should be used to hoist the incoming data into caches before it's needed.

score 1 · Accepted Answer · answered Oct 08 '13 at 12:50

Here is the asm version on reduce_line that @Nils Pipenbrinck suggested

static void reduce2_neon_line(uint8_t* __restrict src1, uint8_t* __restrict src2, uint8_t* __restrict dest, int width) {
    for(int i=0; i<width; i+=16) {
        asm (
             "pld [%[line1], #0xc00]     \n"
             "pld [%[line2], #0xc00]     \n"
             "vldm %[line1]!, {d0,d1}  \n"
             "vldm %[line2]!, {d2,d3}  \n"
             "vpaddl.u8 q0, q0         \n"
             "vpaddl.u8 q1, q1         \n"
             "vadd.u16  q0, q1         \n"
             "vshrn.u16 d0, q0, #2     \n"

             "vst1.8 {d0}, [%[dst]]! \n"

             :
             : [line1] "r"(src1), [line2] "r"(src2), [dst] "r"(dest)
             : "q0", "q1", "memory"
             );
    }
}

It is about 4 times faster then C version (tested on iPhone 5).

Resize 8-bit image by 2 with ARM NEON

3 Answers3