Voyez si cela fonctionne pour vous... Dans ce code, j'avais 3 tableaux en 2 dimensions, appelés rouge,vert et bleu. Chacun était de taille [largeur][hauteur], et chaque élément correspond à un pixel - j'espère que cela a du sens!

FILE *f;
unsigned char *img = NULL;
int filesize = 54 + 3*w*h;  //w is your image width, h is image height, both int

img = (unsigned char *)malloc(3*w*h);
memset(img,0,3*w*h);

for(int i=0; i<w; i++)
{
    for(int j=0; j<h; j++)
    {
        x=i; y=(h-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(int i=0; i<h; i++)
{
    fwrite(img+(w*(h-i-1)*3),3,w,f);
    fwrite(bmppad,1,(4-(w*3)%4)%4,f);
}

free(img);
fclose(f);

c'est le meilleur exemple de bas niveau que je connaisse, écrit par Evercat. copié à partir de https://en.wikipedia.org/wiki/User:Evercat/Buddhabrot.c

void drawbmp (char * filename) {

unsigned int headers[13];
FILE * outfile;
int extrabytes;
int paddedsize;
int x; int y; int n;
int red, green, blue;

extrabytes = 4 - ((WIDTH * 3) % 4);                 // How many bytes of padding to add to each
                                                    // horizontal line - the size of which must
                                                    // be a multiple of 4 bytes.
if (extrabytes == 4)
   extrabytes = 0;

paddedsize = ((WIDTH * 3) + extrabytes) * HEIGHT;

// Headers...
// Note that the "BM" identifier in bytes 0 and 1 is NOT included in these "headers".

headers[0]  = paddedsize + 54;      // bfSize (whole file size)
headers[1]  = 0;                    // bfReserved (both)
headers[2]  = 54;                   // bfOffbits
headers[3]  = 40;                   // biSize
headers[4]  = WIDTH;  // biWidth
headers[5]  = HEIGHT; // biHeight

// Would have biPlanes and biBitCount in position 6, but they're shorts.
// It's easier to write them out separately (see below) than pretend
// they're a single int, especially with endian issues...

headers[7]  = 0;                    // biCompression
headers[8]  = paddedsize;           // biSizeImage
headers[9]  = 0;                    // biXPelsPerMeter
headers[10] = 0;                    // biYPelsPerMeter
headers[11] = 0;                    // biClrUsed
headers[12] = 0;                    // biClrImportant

outfile = fopen(filename, "wb");

//
// Headers begin...
// When printing ints and shorts, we write out 1 character at a time to avoid endian issues.
//

fprintf(outfile, "BM");

for (n = 0; n <= 5; n++)
{
   fprintf(outfile, "%c", headers[n] & 0x000000FF);
   fprintf(outfile, "%c", (headers[n] & 0x0000FF00) >> 8);
   fprintf(outfile, "%c", (headers[n] & 0x00FF0000) >> 16);
   fprintf(outfile, "%c", (headers[n] & (unsigned int) 0xFF000000) >> 24);
}

// These next 4 characters are for the biPlanes and biBitCount fields.

fprintf(outfile, "%c", 1);
fprintf(outfile, "%c", 0);
fprintf(outfile, "%c", 24);
fprintf(outfile, "%c", 0);

for (n = 7; n <= 12; n++)
{
   fprintf(outfile, "%c", headers[n] & 0x000000FF);
   fprintf(outfile, "%c", (headers[n] & 0x0000FF00) >> 8);
   fprintf(outfile, "%c", (headers[n] & 0x00FF0000) >> 16);
   fprintf(outfile, "%c", (headers[n] & (unsigned int) 0xFF000000) >> 24);
}

//
// Headers done, now write the data...
//

for (y = HEIGHT - 1; y >= 0; y--)     // BMP image format is written from bottom to top...
{
   for (x = 0; x <= WIDTH - 1; x++)
   {

      red = reduce(redcount[x][y] + COLOUR_OFFSET) * red_multiplier;
      green = reduce(greencount[x][y] + COLOUR_OFFSET) * green_multiplier;
      blue = reduce(bluecount[x][y] + COLOUR_OFFSET) * blue_multiplier;

      if (red > 255) red = 255; if (red < 0) red = 0;
      if (green > 255) green = 255; if (green < 0) green = 0;
      if (blue > 255) blue = 255; if (blue < 0) blue = 0;

      // Also, it's written in (b,g,r) format...

      fprintf(outfile, "%c", blue);
      fprintf(outfile, "%c", green);
      fprintf(outfile, "%c", red);
   }
   if (extrabytes)      // See above - BMP lines must be of lengths divisible by 4.
   {
      for (n = 1; n <= extrabytes; n++)
      {
         fprintf(outfile, "%c", 0);
      }
   }
}

fclose(outfile);
return;
}


drawbmp(filename);

notez que les lignes sont sauvegardées de bas en haut et non l'inverse.

de plus, les lignes de balayage doivent avoir un byte-longueur de multiples de quatre, vous devez insérer des octets de remplissage à la fin des lignes pour assurer cela.

Voici une variante C++ du code qui fonctionne pour moi. Remarque, j'ai dû changer la taille de calcul pour tenir compte de la ligne de remplissage.

// mimeType = "image/bmp";

unsigned char file[14] = {
    'B','M', // magic
    0,0,0,0, // size in bytes
    0,0, // app data
    0,0, // app data
    40+14,0,0,0 // start of data offset
};
unsigned char info[40] = {
    40,0,0,0, // info hd size
    0,0,0,0, // width
    0,0,0,0, // heigth
    1,0, // number color planes
    24,0, // bits per pixel
    0,0,0,0, // compression is none
    0,0,0,0, // image bits size
    0x13,0x0B,0,0, // horz resoluition in pixel / m
    0x13,0x0B,0,0, // vert resolutions (0x03C3 = 96 dpi, 0x0B13 = 72 dpi)
    0,0,0,0, // #colors in pallete
    0,0,0,0, // #important colors
    };

int w=waterfallWidth;
int h=waterfallHeight;

int padSize  = (4-(w*3)%4)%4;
int sizeData = w*h*3 + h*padSize;
int sizeAll  = sizeData + sizeof(file) + sizeof(info);

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

info[ 4] = (unsigned char)( w   );
info[ 5] = (unsigned char)( w>> 8);
info[ 6] = (unsigned char)( w>>16);
info[ 7] = (unsigned char)( w>>24);

info[ 8] = (unsigned char)( h    );
info[ 9] = (unsigned char)( h>> 8);
info[10] = (unsigned char)( h>>16);
info[11] = (unsigned char)( h>>24);

info[20] = (unsigned char)( sizeData    );
info[21] = (unsigned char)( sizeData>> 8);
info[22] = (unsigned char)( sizeData>>16);
info[23] = (unsigned char)( sizeData>>24);

stream.write( (char*)file, sizeof(file) );
stream.write( (char*)info, sizeof(info) );

unsigned char pad[3] = {0,0,0};

for ( int y=0; y<h; y++ )
{
    for ( int x=0; x<w; x++ )
    {
        long red = lround( 255.0 * waterfall[x][y] );
        if ( red < 0 ) red=0;
        if ( red > 255 ) red=255;
        long green = red;
        long blue = red;

        unsigned char pixel[3];
        pixel[0] = blue;
        pixel[1] = green;
        pixel[2] = red;

        stream.write( (char*)pixel, 3 );
    }
    stream.write( (char*)pad, padSize );
}

Clean C Code for Bitmap (BMP) Image Generation""

Image Générée:

---

_{le code n'utilise pas de bibliothèque autre que stdio.h . Donc, le code peut être facilement incorporé dans d'autres langues de C, C++, C#, Java.}

---

#include <stdio.h>

const int bytesPerPixel = 3; /// red, green, blue
const int fileHeaderSize = 14;
const int infoHeaderSize = 40;

void generateBitmapImage(unsigned char *image, int height, int width, char* imageFileName);
unsigned char* createBitmapFileHeader(int height, int width);
unsigned char* createBitmapInfoHeader(int height, int width);


int main(){
    int height = 541;
    int width = 800;
    unsigned char image[height][width][bytesPerPixel];
    char* imageFileName = "bitmapImage.bmp";

    int i, j;
    for(i=0; i<height; i++){
        for(j=0; j<width; j++){
            image[i][j][2] = (unsigned char)((double)i/height*255); ///red
            image[i][j][1] = (unsigned char)((double)j/width*255); ///green
            image[i][j][0] = (unsigned char)(((double)i+j)/(height+width)*255); ///blue
        }
    }

    generateBitmapImage((unsigned char *)image, height, width, imageFileName);
}


void generateBitmapImage(unsigned char *image, int height, int width, char* imageFileName){

    unsigned char* fileHeader = createBitmapFileHeader(height, width);
    unsigned char* infoHeader = createBitmapInfoHeader(height, width);
    unsigned char padding[3] = {0, 0, 0};
    int paddingSize = (4-(width*bytesPerPixel)%4)%4;

    FILE* imageFile = fopen(imageFileName, "wb");

    fwrite(fileHeader, 1, fileHeaderSize, imageFile);
    fwrite(infoHeader, 1, infoHeaderSize, imageFile);

    int i;
    for(i=0; i<height; i++){
        fwrite(image+(i*width*bytesPerPixel), bytesPerPixel, width, imageFile);
        fwrite(padding, 1, paddingSize, imageFile);
    }

    fclose(imageFile);
}

unsigned char* createBitmapFileHeader(int height, int width){
    int fileSize = fileHeaderSize + infoHeaderSize + bytesPerPixel*height*width;

    static unsigned char fileHeader[] = {
        0,0, /// signature
        0,0,0,0, /// image file size in bytes
        0,0,0,0, /// reserved
        0,0,0,0, /// start of pixel array
    };

    fileHeader[ 0] = (unsigned char)('B');
    fileHeader[ 1] = (unsigned char)('M');
    fileHeader[ 2] = (unsigned char)(fileSize    );
    fileHeader[ 3] = (unsigned char)(fileSize>> 8);
    fileHeader[ 4] = (unsigned char)(fileSize>>16);
    fileHeader[ 5] = (unsigned char)(fileSize>>24);
    fileHeader[10] = (unsigned char)(fileHeaderSize + infoHeaderSize);

    return fileHeader;
}

unsigned char* createBitmapInfoHeader(int height, int width){
    static unsigned char infoHeader[] = {
        0,0,0,0, /// header size
        0,0,0,0, /// image width
        0,0,0,0, /// image height
        0,0, /// number of color planes
        0,0, /// bits per pixel
        0,0,0,0, /// compression
        0,0,0,0, /// image size
        0,0,0,0, /// horizontal resolution
        0,0,0,0, /// vertical resolution
        0,0,0,0, /// colors in color table
        0,0,0,0, /// important color count
    };

    infoHeader[ 0] = (unsigned char)(infoHeaderSize);
    infoHeader[ 4] = (unsigned char)(width    );
    infoHeader[ 5] = (unsigned char)(width>> 8);
    infoHeader[ 6] = (unsigned char)(width>>16);
    infoHeader[ 7] = (unsigned char)(width>>24);
    infoHeader[ 8] = (unsigned char)(height    );
    infoHeader[ 9] = (unsigned char)(height>> 8);
    infoHeader[10] = (unsigned char)(height>>16);
    infoHeader[11] = (unsigned char)(height>>24);
    infoHeader[12] = (unsigned char)(1);
    infoHeader[14] = (unsigned char)(bytesPerPixel*8);

    return infoHeader;
}

j'ai modifié le code HTP DE ralf pour qu'il soit compilé (sur gcc, avec ubuntu 16.04 lts). C'était juste une question d'initialisation des variables.

    int w = 100; /* Put here what ever width you want */
    int h = 100; /* Put here what ever height you want */
    int red[w][h]; 
    int green[w][h];
    int blue[w][h];


    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));
    int x;
    int y;
    int r;
    int g;
    int b;

    for(int i=0; i<w; i++)
    {
            for(int j=0; j<h; j++)
            {
                    x=i; y=(h-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(int 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);

si vous recevez des changements de couleurs étranges au milieu de votre image en utilisant la fonction C++ ci-dessus. Assurez-vous d'ouvrir l'outstream en mode binaire: imgFile.open(filename, std::ios_base::out | std::ios_base::binary);

Sinon windows insère des caractères indésirables au milieu de votre fichier! (à se taper la tête sur ce problème pendant des heures)

voir la question connexe ici: pourquoi ofstream insère-t-il un octet 0x0D avant 0x0A?