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This article mainly introduces the relevant knowledge of "how to parse bmp pictures in c language". The editor shows you the operation process through actual cases, and the operation method is simple, fast and practical. I hope this article "how to parse bmp pictures in c language" can help you solve the problem.

BMP format

The data in this format is divided into three to four parts, in turn:

The file information header (14 bytes) stores information such as file type, file size, etc.

The picture information header (40 bytes) stores information such as image size, color index, number of bits and planes, etc.

Color palette (determined by color index) [may not have this information]

Bitmap data (determined by image size) information for each pixel is stored here

General bmp images are 24-bit, that is, true color. Every 8 bits is one byte, 24 bits, that is, three bytes are used to store the information of each pixel, and the three bytes correspond to storing the data of the three primary colors. The storage range of each byte is 0-255.

So, and so on, a 32-bit map stores four kinds of data per pixel: Alpha channel, storage transparency. An 8-bit map has only grayscale information, and a binary image, which has only two colors, black or white.

File information header format

Typedef struct tagBITMAPFILEHEADER {unsigned short bfType; / / 19778, must be a BM string, corresponding to hexadecimal 0x4d42, decimal 19778 unsigned int bfSize; / / file size unsigned short bfReserved1; / / generally 0 unsigned short bfReserved2; / / generally 0 unsigned int bfOffBits; / / offset from file header to pixel data, that is, these two} BITMAPFILEHEADER

Picture message header format

Typedef struct tagBITMAPINFOHEADER {unsigned int biSize; / / the size of this structure int biWidth; / / the width of the image int biHeight; / / the number of bits occupied by the high unsigned short biPlanes; / / 1 unsigned short biBitCount; / / one pixel of the image, which is generally the size of 24 unsigned int biCompression; / / 0 unsigned int biSizeImage; / / pixel data. This value should be equal to the bfSize-bfOffBits int biXPelsPerMeter in the above file header structure. / / 0 int biYPelsPerMeter; / / 0 unsigned int biClrUsed; / / 0 unsigned int biClrImportant;// 0} BITMAPINFOHEADER

Color palette information

Here, you need to determine whether this palette information exists based on whether the bfOffBits of the file information header is equal to 54 (derived from the previous fixed 14040 bytes). If so, it does not exist; if it is greater than that, it exists.

The information can be extracted according to the demand, or moved directly to the bitmap data area to read pixel information.

This place can be expressed as a two-dimensional array unsigned char palette [N] [M], where N represents the total number of color indexes and M represents the number of bytes per pixel. For example, a 24-bit map consists of 3 bytes per pixel, M is 3, and each byte can represent 256 colors from 0 to 255, so N is 256.

What is stored in the array is the index information, that is, a mapping table that identifies the correspondence between the color index number and the color it represents

Bitmap data

All the pixel information is stored here, each pixel is one byte, and the color information is obtained by querying the color palette after reading it.

If the image is a bitmap of 24-bit or 32-bit data, the bitmap data area is not the index but the actual pixel value. As shown below, the RGB color array for each pixel in the bitmap data area is arranged:

24-bit RGB stores the values of each color channel of each pixel according to the order of BGR. After all the color component values of one pixel are saved, the next pixel is saved without interleaving storage.

32-bit data is stored in BGRA order, and the rest is stored in the same way as 24-bit bitmaps.

Note: because the image height in the bitmap information header is positive, the bitmap data in the file is arranged in the main order of behavior from the lower left corner to the upper right corner.

That is, the first thing to read is the pixel at the leftmost end of the last row from top to bottom, then the pixel in the row to the right of the same row, after reading the entire row, continue to read the penultimate row, and then continue up until all the data has been read.

This is the end of the introduction on "how to interpret bmp pictures in c language". Thank you for your reading. If you want to know more about the industry, you can follow the industry information channel. The editor will update different knowledge points for you every day.

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