CODEWITHSUNDEEP
image-processingcompiler-optimizationneon
bhamadicharef
bhamadicharef

Reputation: 388

NEON acceleration for 12-bit to 8-bit

I have a buffer of 12-bit data (stored in 16-bit data) and need to converts into 8-bit (shift by 4)

How can the NEON accelerate this processing ?

Thank you for your help

Brahim

Upvotes: 0

Views: 483

Answers (2)

mbrenon
mbrenon

Reputation: 4941

Took the liberty to assume a few things explained below, but this kind of code (untested, may require a few modifications) should provide a good speedup compared to naive non-NEON version:

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

void convert(const restrict *uint16_t input, // the buffer to convert
             restrict *uint8_t output,       // the buffer in which to store result
             int sz) {                       // their (common) size

  /* Assuming the buffer size is a multiple of 8 */
  for (int i = 0; i < sz; i += 8) {
    // Load a vector of 8 16-bit values:
    uint16x8_t v = vld1q_u16(buf+i);
    // Shift it by 4 to the right, narrowing it to 8 bit values.
    uint8x8_t shifted = vshrn_n_u16(v, 4);
    // Store it in output buffer
    vst1_u8(output+i, shifted);
  }

}

Things I assumed here:

  • that you're working with unsigned values. If it's not the case, it will be easy to adapt anyway (uint* -> int*, *_u8->*_s8 and *_u16->*_s16)
  • as the values are loaded 8 by 8, I assumed the buffer length was a multiple of 8 to avoid edge cases. If that's not the case, you should probably pad it artificially to a multiple of 8.

Finally, the 2 resource pages used from the NEON documentation:

Hope this helps!

Upvotes: 3

Jake &#39;Alquimista&#39; LEE
Jake &#39;Alquimista&#39; LEE

Reputation: 6354

prototype : void dataConvert(void * pDst, void * pSrc, unsigned int count);
    1:
    vld1.16 {q8-q9}, [r1]!
    vld1.16 {q10-q11}, [r1]!
    vqrshrn.u16 d16, q8, #4
    vqrshrn.u16 d17, q9, #4
    vqrshrn.u16 d18, q10, #4
    vqrshrn.u16 d19, q11, #4
    vst1.16 {q8-q9}, [r0]!
    subs r2, #32
    bgt 1b

q flag : saturation

r flag : rounding

change u16 to s16 in case of signed data.

Upvotes: 1

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