Read image from middle of file in ImageMagick / GraphicsMagick

Question

I have a binary file that starts off with some data. After this data, the file has a JPEG image embedded in it. After the image, the file continues with some other data. I wish to use the image as an OpenGL texture.

At the moment, the only method I know of creating an OpenGL texture with Magick is to read the image file with Magick, write it into a blob, and upload the blob.data() to opengl (from this link: http://www.imagemagick.org/pipermail/magick-developers/2005-July/002276.html).

I am trying to use Magick++, but it only allows me to specify a filename, not a C-style filehandle or filestream...Are my only options the following? :

1. Save the JPEG image portion in the binary file as a separate temporary file and get Magick++ to read that image. I don't wish to do this as writing to disk will slow my program down.

2. Read the image portion into an array, create a Blob with the array as its data, and then read the Blob to obtain an image. I don't wish to do this either because after I get the image, I will need to again write the image data to another blob, and the entire code becomes unnecessarily long.

3. Switch to another library like DevIL, which offers support for what I want. Unfortunately, DevIL is not as feature rich as Magick.

I also looked into the core C API for Magick, where I can specify a filehandle, but the documentation says that the filehandle is closed by Magick after the image is read, which is definitely not good for my program (it is going to be pretty ugly to get the rest of my program to reopen the binary file to continue its processing...

If there is a way to provide Magick with custom I/O routines, or better still, a cleaner way of using Magick with OpenGL, please enlighten me!

Answer 1

The next release of GraphicsMagick does not close the input file handle after the image is read. You can try the latest development snapshot.

You could consider using mmap() (memory mapped file) to access the data and treat it as an in-memory BLOB using Magick++. The main issue with this is you might not know how long the data was in case you need to access data following the embedded JPEG image data.

It is trivial to add FILE* support to Magick++. The only reason I did not do so was for philosophical reasons (C++ vs C).