[英]Converting RGB8 to to NV12 with libav/ffmpeg
我正在嘗試使用 libav 將輸入的 RGB8 圖像轉換為 NV12,但 sws_scale 引發了讀取訪問沖突。 我一定是飛機或步幅不對,但我不明白為什么。
在這一點上,我相信我會從一雙新鮮的眼睛中受益。 我錯過了什么?
void convertRGB2NV12(unsigned char *rgb_in, width, height) {
struct SwsContext* sws_context = nullptr;
const int in_linesize[1] = {3 * width}; // RGB stride
int out_linesize[2] = {width, width}; // NV12 stride
// NV12 data is separated in two
// planes, one for the intensity (Y) and another one for
// the colours(UV) interleaved, both with
// the same width as the frame but the UV plane with
// half of its height.
uint8_t* out_planes[2];
out_planes[0] = new uint8_t[width * height];
out_planes[1] = new uint8_t[width * height/2];
sws_context = sws_getCachedContext(sws_context, width, height,
AV_PIX_FMT_RGB8, width, height,
AV_PIX_FMT_NV12, 0, 0, 0, 0);
sws_scale(sws_context, (const uint8_t* const*)rgb_in, in_linesize,
0, height, out_planes, out_linesize);
// (.....)
}
正如Rotem在原始帖子的第一條評論中所注意到的,輸入格式平面必須采用指針數組格式。
主要有兩個問題:
用AV_PIX_FMT_RGB8
替換AV_PIX_FMT_RGB24
。
rgb_in
應該用指針數組“包裝”:
const uint8_t* in_planes[1] = {rgb_in}; sws_scale(sws_context, in_planes, ...)
使用 FFmpeg 命令行工具創建 RGB24 像素格式的二進制輸入:
ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1 -vcodec rawvideo -pix_fmt rgb24 -frames 1 -f rawvideo rgb_image.bin
使用 C 代碼讀取輸入圖像:
const int width = 192;
const int height = 108;
unsigned char* rgb_in = new uint8_t[width * height * 3];
FILE* f = fopen("rgb_image.bin", "rb");
fread(rgb_in, 1, width * height * 3, f);
fclose(f);
執行convertRGB2NV12(rgb_in, width, height);
.
在函數結束前,添加將輸出寫入二進制文件的臨時代碼:
FILE* f = fopen("nv12_image.bin", "wb");
fwrite(out_planes[0], 1, width * height, f);
fwrite(out_planes[1], 1, width * height/2, f);
fclose(f);
將 nv12_image.bin 作為灰度輸入轉換為 PNG 圖像文件(用於查看結果):
ffmpeg -y -f rawvideo -s 192x162 -pix_fmt gray -i nv12_image.bin -pix_fmt rgb24 nv12_image.png
完整的代碼示例:
#include <stdio.h>
#include <string.h>
#include <stdint.h>
extern "C"
{
#include <libswscale/swscale.h>
}
void convertRGB2NV12(const unsigned char *rgb_in, int width, int height)
{
struct SwsContext* sws_context = nullptr;
const int in_linesize[1] = {3 * width}; // RGB stride
const int out_linesize[2] = {width, width}; // NV12 stride
// NV12 data is separated in two
// planes, one for the intensity (Y) and another one for
// the colours(UV) interleaved, both with
// the same width as the frame but the UV plane with
// half of its height.
uint8_t* out_planes[2];
out_planes[0] = new uint8_t[width * height];
out_planes[1] = new uint8_t[width * height/2];
sws_context = sws_getCachedContext(sws_context, width, height,
AV_PIX_FMT_RGB24, width, height,
AV_PIX_FMT_NV12, SWS_BILINEAR, nullptr, nullptr, nullptr);
const uint8_t* in_planes[1] = {rgb_in};
int response = sws_scale(sws_context, in_planes, in_linesize,
0, height, out_planes, out_linesize);
if (response < 0)
{
printf("Error: sws_scale response = %d\n", response);
return;
}
// (.....)
//Write NV12 output image to binary file (for testing)
////////////////////////////////////////////////////////////////////////////
FILE* f = fopen("nv12_image.bin", "wb");
fwrite(out_planes[0], 1, width * height, f);
fwrite(out_planes[1], 1, width * height/2, f);
fclose(f);
////////////////////////////////////////////////////////////////////////////
delete[] out_planes[0];
delete[] out_planes[1];
sws_freeContext(sws_context);
}
int main()
{
//Use ffmpeg for building raw RGB image (used as input).
//ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1 -vcodec rawvideo -pix_fmt rgb24 -frames 1 -f rawvideo rgb_image.bin
const int width = 192;
const int height = 108;
unsigned char* rgb_in = new uint8_t[width * height * 3];
//Read input image for binary file (for testing)
////////////////////////////////////////////////////////////////////////////
FILE* f = fopen("rgb_image.bin", "rb");
fread(rgb_in, 1, width * height * 3, f);
fclose(f);
////////////////////////////////////////////////////////////////////////////
convertRGB2NV12(rgb_in, width, height);
delete[] rgb_in;
return 0;
}
將 NV12 轉換為 RGB:
ffmpeg -y -f rawvideo -s 192x108 -pix_fmt nv12 -i nv12_image.bin -pix_fmt rgb24 rgb_output_image.png
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