biSizeImage

biSizeImage is a field in the BITMAPINFOHEADER structure used by Windows bitmap graphics (DIBs and BMP files). It records the size, in bytes, of the image data—the pixel array that stores the color information for the bitmap. This includes the data for all scanlines and any per-scanline padding added to align rows to 4-byte boundaries.

How it is used: If the bitmap uses BI_RGB (uncompressed), biSizeImage may be set to zero, and the size can be computed from the bitmap's width, height, and bit depth. For compressed formats (for example BI_JPEG or BI_PNG), biSizeImage should indicate the size of the compressed image data. In all cases, the value represents the number of bytes of the image payload described by the header.

Notes: Some programs rely on biSizeImage to allocate buffers for the pixel data; in BMP files, this field helps determine the image data length. For uncompressed images, the scanlines are padded to a 4-byte boundary, so the total image size equals the sum of the padded row sizes across all rows.

Calculation example: for a 24-bit image with width w and height h, the per-row bytes is ceil(w * 3 / 4) * 4, and the total size is perRow * h. For a 1-bit image, bits per pixel is 1 and rows are padded to a multiple of 4 bytes. biSizeImage thus reflects the memory footprint of the actual pixel data, accounting for padding.