[英]How to convert 16-bit unsigned short to 8-bit unsigned char using scaling efficiently?
我正在嘗試使用某些縮放功能將16位unsigned short
數據轉換為8位unsigned char
。 目前,我正在通過轉換為float並按比例縮小然后飽和為8位來實現此目的。 有沒有更有效的方法來做到這一點?
int _tmain(int argc, _TCHAR* argv[])
{
float Scale=255.0/65535.0;
USHORT sArr[8]={512,1024,2048,4096,8192,16384,32768,65535};
BYTE bArr[8],bArrSSE[8];
//Desired Conventional Method
for (int i = 0; i < 8; i++)
{
bArr[i]=(BYTE)(sArr[i]*Scale);
}
__m128 vf_scale = _mm_set1_ps(Scale),
vf_Round = _mm_set1_ps(0.5),
vf_zero = _mm_setzero_ps();
__m128i vi_zero = _mm_setzero_si128();
__m128i vi_src = _mm_loadu_si128(reinterpret_cast<const __m128i*>(&sArr[0]));
__m128 vf_Src_Lo=_mm_cvtepi32_ps(_mm_unpacklo_epi16(vi_src, _mm_set1_epi16(0)));
__m128 vf_Src_Hi=_mm_cvtepi32_ps(_mm_unpackhi_epi16(vi_src, _mm_set1_epi16(0)));
__m128 vf_Mul_Lo=_mm_sub_ps(_mm_mul_ps(vf_Src_Lo,vf_scale),vf_Round);
__m128 vf_Mul_Hi=_mm_sub_ps(_mm_mul_ps(vf_Src_Hi,vf_scale),vf_Round);
__m128i v_dst_i = _mm_packus_epi16(_mm_packs_epi32(_mm_cvtps_epi32(vf_Mul_Lo), _mm_cvtps_epi32(vf_Mul_Hi)), vi_zero);
_mm_storel_epi64((__m128i *)(&bArrSSE[0]), v_dst_i);
for (int i = 0; i < 8; i++)
{
printf("ushort[%d]= %d * %f = %.3f ,\tuChar[%d]= %d,\t SSE uChar[%d]= %d \n",i,sArr[i],Scale,(float)(sArr[i]*Scale),i,bArr[i],i,bArrSSE[i]);
}
return 0;
}
請注意,可能需要將縮放因子設置為其他值,例如255.0/512.0
255.0/1024.0
或255.0/2048.0
,因此任何解決方案都不應硬編碼為255.0/65535.0
。
如果代碼中的比例是固定的,則可以使用以下算法進行縮放
使用_mm_srli_epi16可以輕松實現第一部分
第二個比較棘手,但基本上是取每個單詞的bit7(低字節的較高位),將其復制到整個單詞,然后取反。
我使用了另一種方法:通過將向量與自身進行比較以得出相等性,我創建了一個值為-1的單詞向量。
然后我隔離了每個源單詞的bit7並將其添加到-1個單詞中。
#include <stdio.h>
#include <emmintrin.h>
int main(int argc, char* argv[])
{
float Scale=255.0/65535.0;
unsigned short sArr[8]={512,1024,2048,4096,8192,16384,32768,65535};
unsigned char bArr[8], bArrSSE[16];
//Desired Conventional Method
for (int i = 0; i < 8; i++)
{
bArr[i]=(unsigned char)(sArr[i]*Scale);
}
//Values to be converted
__m128i vi_src = _mm_loadu_si128((__m128i const*)sArr);
//This computes 8 words (16-bit) that are
// -1 if the low byte of relative word in vi_src is less than 0x80
// 0 if the low byte of relative word in vi_src is >= than 0x80
__m128i vi_off = _mm_cmpeq_epi8(vi_src, vi_src); //Set all words to -1
//Add the bit15 of each word in vi_src to each -1 word
vi_off
= _mm_add_epi16(vi_off, _mm_srli_epi16(_mm_slli_epi16(vi_src, 8), 15));
//Shift vi_src word right by 8 (move hight byte into low byte)
vi_src = _mm_srli_epi16 (vi_src, 8);
//Add the offsets
vi_src = _mm_add_epi16(vi_src, vi_off);
//Pack the words into bytes
vi_src = _mm_packus_epi16(vi_src, vi_src);
_mm_storeu_si128((__m128i *)bArrSSE, vi_src);
for (int i = 0; i < 8; i++)
{
printf("%02x %02x\n", bArr[i],bArrSSE[i]);
}
return 0;
}
這是使用_mm_mulhi_epu16
執行定點縮放操作的實現和測試工具。
scale_ref
是您的原始標量代碼, scale_1
是您(當前已刪除)答案中的浮點SSE實現,而scale_2
是我的定點實現。
我將各種實現分解為單獨的函數,還添加了一個size參數和一個循環,以便它們可用於任何大小數組(盡管當前,對於SSE實現, n
必須是8的倍數)。
有一個編譯時標志ROUND
,它控制定點實現是截斷(如標量代碼)還是舍入(最接近)。 截斷略快。
還要注意, scale
是一個運行時參數,在下面的測試工具中當前被硬編碼為255(相當於255.0/65535.0
),但是它可以是任何合理的值。
#include <stdio.h>
#include <stdint.h>
#include <limits.h>
#include <xmmintrin.h>
#define ROUND 1 // use rounding rather than truncation
typedef uint16_t USHORT;
typedef uint8_t BYTE;
static void scale_ref(const USHORT *src, BYTE *dest, const USHORT scale, const size_t n)
{
const float kScale = (float)scale / (float)USHRT_MAX;
for (size_t i = 0; i < n; i++)
{
dest[i] = src[i] * kScale;
}
}
static void scale_1(const USHORT *src, BYTE *dest, const USHORT scale, const size_t n)
{
const float kScale = (float)scale / (float)USHRT_MAX;
__m128 vf_Scale = _mm_set1_ps(kScale);
__m128 vf_Round = _mm_set1_ps(0.5f);
__m128i vi_zero = _mm_setzero_si128();
for (size_t i = 0; i < n; i += 8)
{
__m128i vi_src = _mm_loadu_si128((__m128i *)&src[i]);
__m128 vf_Src_Lo = _mm_cvtepi32_ps(_mm_unpacklo_epi16(vi_src, _mm_set1_epi16(0)));
__m128 vf_Src_Hi = _mm_cvtepi32_ps(_mm_unpackhi_epi16(vi_src, _mm_set1_epi16(0)));
__m128 vf_Mul_Lo = _mm_mul_ps(vf_Src_Lo, vf_Scale);
__m128 vf_Mul_Hi = _mm_mul_ps(vf_Src_Hi, vf_Scale);
//Convert -ive to +ive Value
vf_Mul_Lo = _mm_max_ps(_mm_sub_ps(vf_Round, vf_Mul_Lo), vf_Mul_Lo);
vf_Mul_Hi = _mm_max_ps(_mm_sub_ps(vf_Round, vf_Mul_Hi), vf_Mul_Hi);
__m128i v_dst_i = _mm_packus_epi16(_mm_packs_epi32(_mm_cvtps_epi32(vf_Mul_Lo), _mm_cvtps_epi32(vf_Mul_Hi)), vi_zero);
_mm_storel_epi64((__m128i *)&dest[i], v_dst_i);
}
}
static void scale_2(const USHORT *src, BYTE *dest, const USHORT scale, const size_t n)
{
const __m128i vk_scale = _mm_set1_epi16(scale);
#if ROUND
const __m128i vk_round = _mm_set1_epi16(scale / 2);
#endif
for (size_t i = 0; i < n; i += 8)
{
__m128i v = _mm_loadu_si128((__m128i *)&src[i]);
#if ROUND
v = _mm_adds_epu16(v, vk_round);
#endif
v = _mm_mulhi_epu16(v, vk_scale);
v = _mm_packus_epi16(v, v);
_mm_storel_epi64((__m128i *)&dest[i], v);
}
}
int main(int argc, char* argv[])
{
const size_t n = 8;
const USHORT scale = 255;
USHORT src[n] = { 512, 1024, 2048, 4096, 8192, 16384, 32768, 65535 };
BYTE dest_ref[n], dest_1[n], dest_2[n];
scale_ref(src, dest_ref, scale, n);
scale_1(src, dest_1, scale, n);
scale_2(src, dest_2, scale, n);
for (size_t i = 0; i < n; i++)
{
printf("src = %u, ref = %u, test_1 = %u, test_2 = %u\n", src[i], dest_ref[i], dest_1[i], dest_2[i]);
}
return 0;
}
Ok參照此找到了解決方案。
這是我的解決方案:
int _tmain(int argc, _TCHAR* argv[])
{
float Scale=255.0/65535.0;
USHORT sArr[8]={512,1024,2048,4096,8192,16384,32768,65535};
BYTE bArr[8],bArrSSE[8];
//Desired Conventional Method
for (int i = 0; i < 8; i++)
{
bArr[i]=(BYTE)(sArr[i]*Scale);
}
__m128 vf_scale = _mm_set1_ps(Scale),
vf_zero = _mm_setzero_ps();
__m128i vi_zero = _mm_setzero_si128();
__m128i vi_src = _mm_loadu_si128(reinterpret_cast<const __m128i*>(&sArr[0]));
__m128 vf_Src_Lo=_mm_cvtepi32_ps(_mm_unpacklo_epi16(vi_src, _mm_set1_epi16(0)));
__m128 vf_Src_Hi=_mm_cvtepi32_ps(_mm_unpackhi_epi16(vi_src, _mm_set1_epi16(0)));
__m128 vf_Mul_Lo=_mm_mul_ps(vf_Src_Lo,vf_scale);
__m128 vf_Mul_Hi=_mm_mul_ps(vf_Src_Hi,vf_scale);
//Convert -ive to +ive Value
vf_Mul_Lo=_mm_max_ps(_mm_sub_ps(vf_zero, vf_Mul_Lo), vf_Mul_Lo);
vf_Mul_Hi=_mm_max_ps(_mm_sub_ps(vf_zero, vf_Mul_Hi), vf_Mul_Hi);
__m128i v_dst_i = _mm_packus_epi16(_mm_packs_epi32(_mm_cvtps_epi32(vf_Mul_Lo), _mm_cvtps_epi32(vf_Mul_Hi)), vi_zero);
_mm_storel_epi64((__m128i *)(&bArrSSE[0]), v_dst_i);
for (int i = 0; i < 8; i++)
{
printf("ushort[%d]= %d * %f = %.3f ,\tuChar[%d]= %d,\t SSE uChar[%d]= %d \n",i,sArr[i],Scale,(float)(sArr[i]*Scale),i,bArr[i],i,bArrSSE[i]);
}
return 0;
}
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