Question

glViewport ( 0, 0, 320, 480 );
byte* memory 
glDrawElements ( GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, 1 );
glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, memory);

I want to convert the byte array, which is read from the frame buffer using glreadpixel(), to any image of PNG/JPEG:

The printf statement shows that glreadpixel() is reading the memory correctly.
In what way can I write the image into to a file, e.g: “C:\image.jpg”?

Answer 1

OpenGL (ES included) accepts only the RAW texture data (that is, RGB triples, RGBA quads or some other predefined format) or some data with s3tc (S3 Texture Compression), but not the file formats. Same with the output.

EDIT:

To be sure that the elements actually appear in the framebuffer, glFlush() call must be done right before the glReadPixels() call. It will force the update of the buffer.

Addition:

If all you need is to dump the buffer contents to the file, I would suggest you to use the .bmp file format. It’s only a 54-byte header at the beginning and then the data which is read directly from the OpenGL (watch for GL_UNPACK_ALIGNMENT though).

Here’s a WriteBMP() function for you:

void WriteBMP(const char *fname, int w,int h,unsigned char *img)
{
    FILE *f = fopen(fname,"wb");

    unsigned char bfh[54] = {0x42, 0x4d,
    /* bfSize [2]*/ 54, 0, 0, 0, /**/
    /* reserved [6]*/ 0, 0, 0, 0, /**/
    /* biOffBits [10]*/ 54, 0, 0, 0, /**/
    /* biSize [14]*/ 40, 0, 0, 0, /**/
    /* width [18]*/ 0, 0, 0, 0, /**/
    /* height [22]*/ 0, 0, 0, 0, /**/
    /* planes [26]*/ 1, 0, /**/
    /* bitcount [28]*/ 24, 0,/**/
    /* compression [30]*/ 0, 0, 0, 0, /**/
    /* size image [34]*/ 0, 0, 0, 0, /**/
    /* xpermeter [38]*/ 0, 0, 0, 0, /**/
    /* ypermeter [42]*/ 0, 0, 0, 0, /**/
    /* clrused [46]*/ 0, 0, 0, 0, /**/
    /* clrimportant [50]*/ 0, 0, 0, 0 /**/};
    int realw = w * 3, rem = w % 4, isz = (realw + rem) * h, fsz = isz + 54;
    //bfh.bfSize = fsz;
    bfh[2] = (fsz & 0xFF); bfh[3] = (fsz >> 8) & 0xFF; bfh[4] = (fsz >> 16) & 0xFF; bfh[5] = (fsz >> 24) & 0xFF;
    //bfh.biSize = isz
    bfh[34] = (isz & 0xFF); bfh[35] = (isz >> 8) & 0xFF; bfh[36] = (isz >> 16) & 0xFF; bfh[37] = (isz >> 24) & 0xFF;
    //bfh.biWidth = w;
    bfh[18] = (w & 0xFF); bfh[19] = (w >> 8) & 0xFF; bfh[20] = (w >> 16) & 0xFF; bfh[21] = (w >> 24) & 0xFF;
    //bfh.biHeight = h;
    bfh[22] = (h & 0xFF); bfh[23] = (h >> 8) & 0xFF; bfh[24] = (h >> 16) & 0xFF; bfh[25] = (h >> 24) & 0xFF;

    // xpels/ypels
    // bfh[38] = 19; bfh[39] = 11;
    // bfh[42] = 19; bfh[43] = 11;

    fwrite((void*)bfh, 54, 1, f);

    unsigned char* bstr = new unsigned char[realw], *remstr = 0; 
    if(rem != 0) { remstr = new unsigned char[rem]; memset(remstr,0,rem); }

    for(int j = h-1 ; j > -1 ; j--){
            for(int i = 0 ; i < w ; i++)
                    for(int k = 0 ; k < 3 ; k++) { bstr[i*3+k] = img[(j*realw+i*3)+(2-k)]; }
            fwrite(bstr,realw,1,f); if (rem != 0) { fwrite(remstr,rem,1,f); }
    }

    delete [] bstr; if(remstr) delete [] remstr;

    fclose(f);
}

Then just do the call:

WriteBMP("C:\\image.bmp", 320, 480, memory);

If you really need the .jpg/.png file, you might convert the resulting .bmp file or use the FreeImage library or the libjpng/libpng themselves. They all deal with specific file formats.