使用NEON指令加速RGB888和RGB565的相互转换
最近在做一个项目需要将RGB888转换为RGB565，用C语言转换的代码很简单，这是从ffmpeg中摘抄的代码
static inline void rgb24to16_c(const uint8_t *src, uint8_t *dst, int src_size)
{
uint16_t *d = (uint16_t *)dst;
const uint8_t *s = src;
const uint8_t *end = s + src_size;
while (s < end) {
const int r = *s++;
const int g = *s++;
const int b = *s++;
*d++ = (b >> 3) | ((g & 0xFC) << 3) | ((r & 0xF8) << 8);
}
}
这个项目需要转换的数据量不多，用C语言进行转换的CPU开销完全可以接受。但我并不满足于此，项目中使用的芯片支持NEON指令加速，所以为什么不用呢？
简单搜索一番发现国很少有相关的文章，最后还是去外面找了一些有用的资料，在学习NEON指令和寻找的过程中发现ARM官网已经提供了例程，那我正好可以偷懒。
这里给出ARM官网的例程链接，防止有人无法无访问，这里也给出源码
RGB888转RGB565
https://developer.arm.com/documentation/den0018/a/NEON-Code-Examples-with-Optimization/Converting-color-depth/Converting-from-RGB888-to-RGB565
uint8_t *src = image_src;
uint16_t *dst = image_dst;
int count = PIXEL_NUMBER;
while (count >= 8) {
uint8x8x3_t vsrc;
uint16x8_t vdst;
vsrc = vld3_u8(src);
vdst = vshll_n_u8(vsrc.val[0], 8);
vdst = vsriq_n_u16(vdst, vshll_n_u8(vsrc.val[1], 8), 5);
vdst = vsriq_n_u16(vdst, vshll_n_u8(vsrc.val[2], 8), 11);
vst1q_u16(dst, vdst);
dst += 8;
src += 8*3;
count -= 8;
}
uint16_t *src = image_src;
uint8_t *dst = image_dst;
int count = PIXEL_NUMBER;
while (count >= 8) {
uint16x8_t vsrc;
uint8x8x3_t vdst;
vsrc = vld1q_u16(src);
vdst.val[0] = vshrn_n_u16(vreinterpretq_u16_u8(vshrq_n_u8(vreinterpretq_u8_u16(vsrc), 3)), 5);
vdst.val[1] = vshl_n_u8(vshrn_n_u16(vsrc, 5) ,2);
vdst.val[2] = vmovn_u16(vshlq_n_u16(vsrc, 3));
vst3_u8(dst, vdst);
dst += 8*3;
src += 8;
count -= 8;
}
NEON的头文件是 arm_neon.h，编译需要加 -mcpu=cortex-a7 -mfloat-abi=softfp -mfpu=neon-vfpv4，其中-mpu参数按自己的芯片填写，我这里用的是一款A7架构芯片，主频800MHz。
接下来就是传统项目，性能测试，随机生成数据的 RGB888 图片转换为 RGB565 图片，重复1000次并计时，测试代码如下
#include <arm_neon.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
void rgb888_to_rgb565(uint8_t *in, uint8_t *out, int h, int v)
{
uint16_t *d = (uint16_t *)out;
const uint8_t *s = in;
const uint8_t *end = s + h * v * 3;
while (s < end) {
const int r = *s++;
const int g = *s++;
const int b = *s++;
*d++ = (b >> 3) | ((g & 0xFC) << 3) | ((r & 0xF8) << 8);
}
}
void rgb888_to_rgb565_neon(uint8_t *in, uint8_t *out, int h, int v)
{
uint8_t *src = in;
uint16_t *dst = (uint16_t *)out;
int count = h * v;
if (count % 8 != 0) {
printf("pixel number must align with 8\n");
return;
}
while (count >= 8) {
uint8x8x3_t vsrc;
uint16x8_t vdst;
vsrc = vld3_u8(src);
vdst = vshll_n_u8(vsrc.val[0], 8);
vdst = vsriq_n_u16(vdst, vshll_n_u8(vsrc.val[1], 8), 5);
vdst = vsriq_n_u16(vdst, vshll_n_u8(vsrc.val[2], 8), 11);
vst1q_u16(dst, vdst);
dst += 8;
src += 8*3;
count -= 8;
}
}
#define WIDTH 320
#define HEIGHT 180
// #define WIDTH 640
// #define HEIGHT 360
// #define WIDTH 960
// #define HEIGHT 540
// #define WIDTH 1280
// #define HEIGHT 720
// #define WIDTH 1600
// #define HEIGHT 900
// #define WIDTH 1920
// #define HEIGHT 1080
#define LOOP 1000
uint8_t rgb888[WIDTH * HEIGHT * 3];
uint16_t rgb565[WIDTH * HEIGHT];
int main(int argc, char **argv)
{
int32_t i;
for (i = 0; i < WIDTH * HEIGHT * 3; i++) {
rgb888[i] = rand() & 0xFF;
}
struct timeval tv1;
struct timeval tv2;
double td = 0; // ms
printf("size %d x %d, loop %d\n", WIDTH, HEIGHT, LOOP);
gettimeofday(&tv1, NULL);
for (i = 0; i < LOOP; i++) {
rgb888_to_rgb565(rgb888, (uint8_t *)rgb565, WIDTH, HEIGHT);
}
gettimeofday(&tv2, NULL);
td = ((double)tv2.tv_sec * 1000.0 + (double)tv2.tv_usec / 1000.0) - ((double)tv1.tv_sec * 1000.0 + (double)tv1.tv_usec / 1000.0);
printf("time c: %f ms\n", td);
gettimeofday(&tv1, NULL);
for (i = 0; i < LOOP; i++) {
rgb888_to_rgb565_neon(rgb888, (uint8_t *)rgb565, WIDTH, HEIGHT);
}
gettimeofday(&tv2, NULL);
td = ((double)tv2.tv_sec * 1000.0 + (double)tv2.tv_usec / 1000.0) - ((double)tv1.tv_sec * 1000.0 + (double)tv1.tv_usec / 1000.0);
printf("time neon: %f ms\n", td);
return 0;
}
来看一下耗时：
size 320 x 180, loop 1000
time c: 1049.035889 ms
time neon: 405.596924 ms
size 640 x 360, loop 1000
time c: 3948.885986 ms
time neon: 2150.033203 ms
size 960 x 540, loop 1000
time c: 9026.308838 ms
time neon: 5033.337891 ms
size 1280 x 720, loop 1000
time c: 16550.081055 ms
time neon: 8756.577881 ms
size 1600 x 900, loop 1000
time c: 25366.738037 ms
time neon: 13618.843994 ms
size 1920 x 1080, loop 1000
time c: 37058.665039 ms
time neon: 20064.520996 ms
对于RGB888转RGB565来说，NEON指令的性能大约是C语言的两倍。其他NEON指令的性能未做测试，以上测试内容仅供参考。
最后给出ARM官方的参考文档
NEON指令开发指南
https://developer.arm.com/documentation/den0018/a
