How to write frame buffer data in to a JPEG or BMP image

Question

I am new to openGl, i have a task to write the frame buffer(opengl offscreen rendering data) into JPG or Bmp image.

I google it but unable to find the right solution...

I have tried the following code,but result was an unsupported file.I couldn't able to open the file.

FILE *Out;
    unsigned char *Buff;

    // Capture a screen shot. Save as a RAW-format file.
    // First, allocate memory.
    Buff = new unsigned char[512*512*3];

   // Now, get pixels.
    glReadBuffer(GL_BACK);
    glReadPixels(0,0, 512, 512, GL_RGB, GL_UNSIGNED_BYTE, Buff);

  // Now, open to file and write pixels.
   Out = fopen(Filename, "wb");
   if(!Out) return;
   fwrite(Buff, 3, 512*512, Out);


   fclose(Out);
   delete[] Buff;

Here i used raw file but what i want is jpeg or bmp file.

Answer 1

You were or the right path.

First things first, do not declare variables (especially pointers) uninitialized it can result in the program malfunction (for more please read : https://softwareengineering.stackexchange.com/questions/223862/how-important-is-to-initialize-a-variable )

So instead of doing :

unsigned char *Buff;
Buff = new unsigned char[512*512*3];

Do :

unsigned char *Buff=new unsigned char[512*512*3];

Your buff needs to be byte*. If you want to write to a BMP file you need to have in your header the following:

#include <Windows.h>

Now you can save to bitmap files using BITMAPFILEHEADER and BITMAPINFOHEADER. Your new code should look like this.

    byte* Buff = new byte[512*512*3];
if (!Buff)
    return;

glReadBuffer(GL_BACK);
glReadPixels(0, 0, 512, 512, GL_RGB, GL_UNSIGNED_BYTE, Buff);


FILE *Out = fopen(filename, "wb");
if (!Out)
    return;
BITMAPFILEHEADER bitmapFileHeader;
BITMAPINFOHEADER bitmapInfoHeader;

bitmapFileHeader.bfType = 0x4D42;
bitmapFileHeader.bfSize = windowWidth*windowHeight * 3;
bitmapFileHeader.bfReserved1 = 0;
bitmapFileHeader.bfReserved2 = 0;
bitmapFileHeader.bfOffBits =
    sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

bitmapInfoHeader.biSize = sizeof(BITMAPINFOHEADER);
bitmapInfoHeader.biWidth = windowWidth - 1;
bitmapInfoHeader.biHeight = windowHeight - 1;
bitmapInfoHeader.biPlanes = 1;
bitmapInfoHeader.biBitCount = 24;
bitmapInfoHeader.biCompression = BI_RGB;
bitmapInfoHeader.biSizeImage = 0;
bitmapInfoHeader.biXPelsPerMeter = 0; // ?
bitmapInfoHeader.biYPelsPerMeter = 0; // ?
bitmapInfoHeader.biClrUsed = 0;
bitmapInfoHeader.biClrImportant = 0;

fwrite(&bitmapFileHeader, sizeof(BITMAPFILEHEADER), 1, Out);
fwrite(&bitmapInfoHeader, sizeof(BITMAPINFOHEADER), 1, Out);
fwrite(Buff, windowWidth*windowHeight * 3, 1, Out);
fclose(Out);

delete[] Buff;

( Code from : http://dave.thehorners.com/tech-talk/programming/124-opengl-writing-the-framebuffer-to-disk )

Answer 2

To convert to BMP, you want to create an empty bitmap the right size in 24bpp format, then copy each row of the GL_RGB data to its pixel array. (Remember that BMPs store the rows from bottom up and pad each row to a multiple of 4 bytes.) The easiest way to convert to JPEG is probably to create a BMP in memory and compress it. There are several graphics libraries that will help.