jpg to yuv420sp C语言

将JPEG格式的图片转换为YUV420SP格式，可以使用libjpeg库来读取JPEG图片，然后使用以下代码将RGB格式的图片转换为YUV420SP格式：

#include <stdio.h>
#include <stdlib.h>
#include <jpeglib.h>

// Convert RGB to YUV420SP
void rgb2yuv(unsigned char *rgb, unsigned char *yuv, int width, int height)
{
    int frameSize = width * height;
    int yIndex = 0;
    int uvIndex = frameSize;

    int r, g, b, y, u, v;
    for (int j = 0; j < height; j++) {
        for (int i = 0; i < width; i++) {
            r = rgb[j * width * 3 + i * 3];
            g = rgb[j * width * 3 + i * 3 + 1];
            b = rgb[j * width * 3 + i * 3 + 2];

            y = ((66 * r + 129 * g + 25 * b + 128) >> 8) + 16;
            u = ((-38 * r - 74 * g + 112 * b + 128) >> 8) + 128;
            v = ((112 * r - 94 * g - 18 * b + 128) >> 8) + 128;

            y = (y < 16) ? 16 : ((y > 255) ? 255 : y);
            u = (u < 0) ? 0 : ((u > 255) ? 255 : u);
            v = (v < 0) ? 0 : ((v > 255) ? 255 : v);

            yuv[yIndex++] = (unsigned char)y;
            if (j % 2 == 0 && i % 2 == 0) {
                yuv[uvIndex++] = (unsigned char)u;
                yuv[uvIndex++] = (unsigned char)v;
            }
        }
    }
}

int main(int argc, char *argv[])
{
    if (argc != 3) {
        printf("Usage: %s input.jpg output.yuv\n", argv[0]);
        return 0;
    }

    char *input_file = argv[1];
    char *output_file = argv[2];

    // Read JPEG image
    FILE *file = fopen(input_file, "rb");
    if (!file) {
        printf("Error: Unable to open input file %s\n", input_file);
        return 0;
    }

    struct jpeg_decompress_struct cinfo;
    struct jpeg_error_mgr jerr;
    cinfo.err = jpeg_std_error(&jerr);
    jpeg_create_decompress(&cinfo);
    jpeg_stdio_src(&cinfo, file);
    jpeg_read_header(&cinfo, TRUE);
    jpeg_start_decompress(&cinfo);

    int width = cinfo.output_width;
    int height = cinfo.output_height;
    int numChannels = cinfo.output_components;
    int row_stride = width * numChannels;

    unsigned char *rgb = (unsigned char *)malloc(width * height * numChannels);
    unsigned char *yuv = (unsigned char *)malloc(width * height * 3 / 2);

    JSAMPARRAY buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr)&cinfo, JPOOL_IMAGE, row_stride, 1);
    int row = 0;
    while (cinfo.output_scanline < cinfo.output_height) {
        jpeg_read_scanlines(&cinfo, buffer, 1);
        for (int i = 0; i < row_stride; i += numChannels) {
            rgb[row * row_stride + i] = buffer[0][i];
            rgb[row * row_stride + i + 1] = buffer[0][i + 1];
            rgb[row * row_stride + i + 2] = buffer[0][i + 2];
        }
        row++;
    }

    jpeg_finish_decompress(&cinfo);
    jpeg_destroy_decompress(&cinfo);
    fclose(file);

    // Convert RGB to YUV420SP
    rgb2yuv(rgb, yuv, width, height);

    // Write YUV420SP image
    file = fopen(output_file, "wb");
    if (!file) {
        printf("Error: Unable to open output file %s\n", output_file);
        return 0;
    }
    fwrite(yuv, 1, width * height * 3 / 2, file);
    fclose(file);

    free(rgb);
    free(yuv);

    return 0;
}

在程序中，首先使用libjpeg库读取JPEG图片，然后将RGB格式的图片转换为YUV420SP格式，最后将YUV420SP格式的图片写入文件。注意，在YUV420SP格式中，Y分量占据前width * height个字节，U和V分量共占据(width * height) / 2个字节。