Raw RGB values to JPEG

Question

I have an array with raw RGB values in it, and I need to write these values to a JPEG file. Is there an easy way to do this?

I tried:

std::ofstream ofs("./image.JPG", std::ios::out | std::ios::binary);

for (unsigned i = 0; i < width * height; ++i) {
   ofs << (int)(std::min(1.0f, image[i].x) * 255) << (int)(std::min(1.0f, image[i].y) * 255) << (int)(std::min(1.0f, image[i].z) * 255);
}

but the format isn't recognized.

Answer 1

If you're trying to produce an image file you might look at Netpbm . You could write the intermediate format (PBM or PAM) fairly simply from what you have. There are then a large number of already written programs that will generate many types of images from your intermediate file.

Answer 2

WOH THERE!

JPEG is MUCH more complicated than raw RBG values. You are going to need to use a library, like LIBJPEG, to store the data as JPEG.

If you wrote it yourself you'd have to:

1. Convert from RGB to YCbCr
2. Sample the image
3. Divide into 8x8 blocks.
4. Perform the DCT on each block.
5. Run-length/huffman encode the values
6. Write these values in properly formatted JPEG blocks.

Answer 3

You could use Boost GIL : it's free, portable (it's part of Boost libraries), usable across a broad spectrum of operating systems (including Windows).

Popular Linux and Unix distributions such as Fedora, Debian and NetBSD include pre-built Boost packages.

The code is quite simple:

#include <boost/gil/extension/io/jpeg_io.hpp>

const unsigned width  = 320;
const unsigned height = 200;

// Raw data.
unsigned char r[width * height];  // red
unsigned char g[width * height];  // green
unsigned char b[width * height];  // blue

int main()
{
  boost::gil::rgb8c_planar_view_t view =
    boost::gil::planar_rgb_view(width, height, r, g, b, width);

  boost::gil::jpeg_write_view("out.jpg", view);

  return 0;
}

jpeg_write_view saves the currently instantiated view to a jpeg file specified by the name (throws std::ios_base::failure if it fails to create the file).

Remember to link your program with -ljpeg .