Writing BMP image in pure c/c++ without other libraries

Question

In my algorithm i need create information output. I must to write boolean matrix in bmp file. It must be monocromic image, where pixel is white if matrix on such element is true. Main problem is bmp header and how to write this.

Thanks.

Answer 1

Without the use of any other library you can look at the BMP file format. I've implemented it in the past and it can be done without too much work.

Bitmap-File Structures

Each bitmap file contains a bitmap-file header, a bitmap-information header, a color table, and an array of bytes that defines the bitmap bits. The file has the following form:

BITMAPFILEHEADER bmfh;
BITMAPINFOHEADER bmih; RGBQUAD
aColors[]; BYTE
aBitmapBits[];

... see the file format for more details

Answer 2

Note that the lines are saved from down to up and not the other way around.

Additionally, the scanlines must have a byte-length of multiples of four, you should insert fill bytes at the end of the lines to ensure this.

Answer 3

See if this works for you... In this code, i had 3 2-dimensional arrays, called red,green and blue. Each one was of size [width][height], and each element corresponded to a pixel - I hope this makes sense!

FILE *f;
unsigned char *img = NULL;
int filesize = 54 + 3*w*h;  //w is your image width, h is image height, both int
if( img )
    free( img );
img = (unsigned char *)malloc(3*w*h);
memset(img,0,sizeof(img));

for(int i=0; i<w; i++)
{
    for(int j=0; j<h; j++)
{
    x=i; y=(yres-1)-j;
    r = red[i][j]*255;
    g = green[i][j]*255;
    b = blue[i][j]*255;
    if (r > 255) r=255;
    if (g > 255) g=255;
    if (b > 255) b=255;
    img[(x+y*w)*3+2] = (unsigned char)(r);
    img[(x+y*w)*3+1] = (unsigned char)(g);
    img[(x+y*w)*3+0] = (unsigned char)(b);
}
}

unsigned char bmpfileheader[14] = {'B','M', 0,0,0,0, 0,0, 0,0, 54,0,0,0};
unsigned char bmpinfoheader[40] = {40,0,0,0, 0,0,0,0, 0,0,0,0, 1,0, 24,0};
unsigned char bmppad[3] = {0,0,0};

bmpfileheader[ 2] = (unsigned char)(filesize    );
bmpfileheader[ 3] = (unsigned char)(filesize>> 8);
bmpfileheader[ 4] = (unsigned char)(filesize>>16);
bmpfileheader[ 5] = (unsigned char)(filesize>>24);

bmpinfoheader[ 4] = (unsigned char)(       w    );
bmpinfoheader[ 5] = (unsigned char)(       w>> 8);
bmpinfoheader[ 6] = (unsigned char)(       w>>16);
bmpinfoheader[ 7] = (unsigned char)(       w>>24);
bmpinfoheader[ 8] = (unsigned char)(       h    );
bmpinfoheader[ 9] = (unsigned char)(       h>> 8);
bmpinfoheader[10] = (unsigned char)(       h>>16);
bmpinfoheader[11] = (unsigned char)(       h>>24);

f = fopen("img.bmp","wb");
fwrite(bmpfileheader,1,14,f);
fwrite(bmpinfoheader,1,40,f);
for(i=0; i<h; i++)
{
    fwrite(img+(w*(h-i-1)*3),3,w,f);
    fwrite(bmppad,1,(4-(w*3)%4)%4,f);
}
fclose(f);