c - 如何在 4 字节边界对齐扫描线

Question

我正在使用 libjpeg 解压缩 JPEG 图像并将其写入 BMP 文件。假设图像宽度设置为 2550 像素，每像素 24 位(3 字节)，则生成的行宽度将不是 4 的倍数。如何在 4 字节边界对齐每行？

struct jpeg_decompress_struct cinfo;
unsigned int bytesPerRow = cinfo.output_width * cinfo.num_components;
unsigned int colColor;
FILE *bmpFile = NULL;

while (cinfo.output_scanline < cinfo.image_height) {

    JSAMPROW row_pointer[1];
    row_pointer[0] = raw_image
            + cinfo.output_scanline * bytesPerRow;
    jpeg_read_scanlines(&cinfo, row_pointer, 1);
    for (colColor = 0; colColor < cinfo.image_width; colColor++) {

        /* BMP scanlines should be aligned at 4-byte boundary */

    }

    /* write each row to bmp file */
    fwrite(row_pointer[0], 1, bytesPerRow, bmpFile);
}

Answer 1

bytesPerRow = 4 * ((cinfo.output_width * cinfo.num_components + 3) / 4); – Hans Passant

就四舍五入到 4 的倍数而言，该公式是正确的，但使用被乘数 cinfo.num_components 时不正确；根据USING THE IJG JPEG LIBRARY :

You will need output_width * output_components JSAMPLEs per scanline in your output buffer…

The output arrays are required to be output_width * output_components JSAMPLEs wide.

所以，是

unsigned int bytesPerRow = (cinfo.output_width * cinfo.output_components + 3) / 4 * 4;