Reading BMP file with C language

BMP Image coding

BMP namely bitmap, It's a bitmap , Generally by 4 Part of it is made up of ： Header information block 、 Image description information block 、 color table （ No color table in true color mode ） And image data area .

Before the image data , As shown in the figure , share 54 Bit data

among ,0x424d In the decimal system is 19778, Corresponding ASCII Code for BM, It means it's a bitmap file .

Windows The data is read backwards , This is a PC Computer features . If a piece of data is 42 4D, Read it backwards 4D 42, namely 0x4D42.
therefore , If bfSize The data is A2 1E 04 00, In fact, it becomes 0x00041EA2, That is to say 0x41EA2.

File header [14 byte ]

Storing file types , File size and other information

//  File header structure 
typedef struct tagBITMAPFILEHEADER{
    
    unsigned short bfType;        // Must be "BM"
    unsigned int   bfSize;        // File Bytes (2-5)
    unsigned int bfReserved;    // Bitmap file reserved word , Must be 0(6-9)
    unsigned int   bfOffBits;   // Pixel data offset  (10-13)
} bmpHeader;

Image description information [40 byte ]

#define uint unsigned int
#define ushort unsigned short
// Image header structure 
typedef struct tagBITMAPINFOHEADER{
    
    uint    biSize;          //  Structure size  (14-17)
    int     biWidth;         //  The width of the image  (18-21)
    int     biHeight;        //  Height of the image  (22-25)
    ushort  biPlanes;        //  The level of the target device , by 1(26-27)
    ushort  biBitCount;      //  Pixel digits , by 1、4、8 or 24(28-29)
    uint    biCompression;   //  Bitmap compression type ,0 For no compression 、1 by BI_RLE8、2 by BI_RLE4(30-33)
    uint    biSizeImage;     //  Single pixel data size , be equal to bfSize-bfOffBits (34-37)
    int     biXPelsPerMeter; //  Horizontal resolution , It's usually 0 (38-41)
    int     biYPelsPerMeter; //  Vertical resolution , It's usually 0 (42-45)
    uint    biClrUsed;       //  The number of colors in the bitmap color table ,0 Indicates that all palette items are used (46-49)
    uint    biClrImportant;  //  Number of important color indexes ,0 It means that everything is important (50-53)
} infoHeader;

Pixel information structure

typedef struct _PixelInfo {
    
	unsigned char rgbBlue;   // Blue weight  (0-255)
	unsigned char rgbGreen;  // Green component  (0-255)
	unsigned char rgbRed;    // Red component  (0-255)
	//unsigned char rgbReserved;//  Retain , It has to be for 0
} PixelInfo;

vs 2019 Set the structure alignment rules

The sample picture

Read images

int main()
{
    
	BITMAPFILEHEADER fileHeader;
	BITMAPINFOHEADER infoHeader;
	PixelInfo pixel;

	FILE* fp;
	fp = fopen("images/123456.bmp", "rb");
	//fp = fopen("images/b44.bmp", "rb");

	fread(&fileHeader, sizeof(fileHeader), 1, fp);
	fread(&infoHeader, sizeof(infoHeader), 1, fp);
	if (fileHeader.bfType != 19778)
	{
    
		printf("%s", "err");

	}
	printf("%d\n", fileHeader.bfType);
	printf("%d\n", fileHeader.bfSize);
	unsigned char b;
	unsigned char g;
	unsigned char r;
	unsigned char gray;
	int x, y, count = 0;
	int w = infoHeader.biWidth;
	int	h = infoHeader.biHeight;
	char info[] = "* ";
	int len = w * h + h + 1;
	char* out = (char*)malloc(len * sizeof(char));
	for (y = 0; y < h; y++)
	{
    
		for (x = 0; x < w; x++)
		{
    
			
			fread(&pixel, sizeof(pixel), 1, fp);
			//printf("%d, %d , %d \n", pixel.rgbRed, pixel.rgbGreen, pixel.rgbBlue);
			b = pixel.rgbBlue;
			g = pixel.rgbGreen;
			r = pixel.rgbRed;
			gray = (int)(r * 0.299 + g * 0.587 + b * 0.114);
			out[count++] = info[gray * strlen(info) / 256];
		}
		out[count++] = '\n';
	}
	out[count] = '\0';

	printf("%s\n", out);

	printf("helloWorld\n");
	return 0;
}

Due to different coordinate systems , You need to modify the order of reading pixels

int main3()
{
    
	BITMAPFILEHEADER fileHeader;
	BITMAPINFOHEADER infoHeader;
	PixelInfo pixel;

	FILE* fp;
	fp = fopen("images/123456.bmp", "rb");
	//fp = fopen("images/b44.bmp", "rb");

	fread(&fileHeader, sizeof(fileHeader), 1, fp);
	fread(&infoHeader, sizeof(infoHeader), 1, fp);
	if (fileHeader.bfType != 19778)
	{
    
		printf("%s", "err");

	}
	printf("%d\n", fileHeader.bfType);
	printf("%d\n", fileHeader.bfSize);
	unsigned char b;
	unsigned char g;
	unsigned char r;
	unsigned char gray;
	int x, y,count=0;
	int w = infoHeader.biWidth;
	int	h = infoHeader.biHeight;

	char info[] = "* ";
	int len = w * h + h+1;
	char* out = (char*)malloc(len * sizeof(char));

	for (y = h - 1; y >= 0; y--)
	{
    
		count = (w + 1) * y;
		for (x = 0; x < w; x++)
		{
    

			fread(&pixel, sizeof(pixel), 1, fp);
			b = pixel.rgbBlue;
			g = pixel.rgbGreen;
			r = pixel.rgbRed;
			gray = (int)(r * 0.299 + g * 0.587 + b * 0.114);
			out[count++] = info[gray * strlen(info) / 256];
		}
		
		out[count++] = '\n';
	}
	out[len-1] = '\0';


	printf("%s\n", out);

	printf("helloWorld\n");
	return 0;
}

Image distortion

Windows Specifies that the number of bytes occupied by a scan line must be 4 Multiple ( That is to long In units of ), Not enough to 0 fill .

How to calculate the number of bytes occupied by a scan line :
DataSizePerLine= (biWidth* biBitCount+31)/8;
// The number of bytes occupied by a scan line
DataSizePerLine= DataSizePerLine/44; // The number of bytes must be 4 Multiple
The size of bitmap data ( Without compression ):
DataSize= DataSizePerLine biHeight;

int main()
{
    
	BITMAPFILEHEADER fileHeader;
	BITMAPINFOHEADER infoHeader;
	PixelInfo pixel;

	FILE* fp;
	fp = fopen("images/123456.bmp", "rb");
	//fp = fopen("images/b44.bmp", "rb");

	fread(&fileHeader, sizeof(fileHeader), 1, fp);
	fread(&infoHeader, sizeof(infoHeader), 1, fp);
	if (fileHeader.bfType != 19778)
	{
    
		printf("%s", "err");

	}
	printf("%d\n", fileHeader.bfType);
	printf("%d\n", fileHeader.bfSize);
	unsigned char b;
	unsigned char g;
	unsigned char r;
	unsigned char gray;
	int x, y,count=0;
	int w = infoHeader.biWidth;
	int	h = infoHeader.biHeight;
	int DataSizePerLine = (w * infoHeader.biBitCount + 31) / 8 / 4 * 4;   //  The number of bytes must be 4 Multiple  
	char info[] = "* ";
	int len = w * h + h+1;
	char* out = (char*)malloc(len * sizeof(char));
	for (y = h - 1; y >= 0; y--)
	{
    
		count = (w + 1) * y;
		for (x = 0; x < w; x++)
		{
    

			fread(&pixel, sizeof(pixel), 1, fp);
			b = pixel.rgbBlue;
			g = pixel.rgbGreen;
			r = pixel.rgbRed;
			gray = (int)(r * 0.299 + g * 0.587 + b * 0.114);
			out[count++] = info[gray * strlen(info) / 256];
		}
		if (w % 4 != 0)
		{
    
			fseek(fp, DataSizePerLine - 3 * w,SEEK_CUR);
		}
		out[count++] = '\n';
	}
	out[len-1] = '\0';


	printf("%s\n", out);

	printf("helloWorld\n");
	return 0;
}

Reference link https://blog.csdn.net/qq_39400113/article/details/104750460
Refer to Daniel video https://www.bilibili.com/video/BV1n5411N7T4?spm_id_from=333.999.0.0