How to read embed thumbnail from jpeg images?

Question

I am writing a thumbnail viewer in c++.

I first make use of EXIF information to retrive an image's thumbnail, but the thumbnail in the EXIF is bad, with black bands. So I want to get the jpeg's embedded thumbnail, how can I do this?

Another question: does jpeg's embeded thumbnail equal to EXIF thumbnial?

Answer 1

If the EXIF thumbnail is bad, you can generate your own from the JPEG itself, without needing to completely decode the JPEG.

Look for the source code for EPEG . It's part of the Enlightenment project on SourceForge, and was part of the old EFL a year ago. You can still dig it up from some old SVN commits or from a source tarball that might be floating around.

Basically, what EPEG does, is it collects the DCT coefficients from the image, and performs a rescaling operation on them. The DCT coefficient is normally used as the base coefficient for an 8x8 block of pixels. You can treat it as one pixel. As a result, you have a (computationally free) thumbnail exactly 1/8th the size of the original image. Rescale it as you would any image data to the desired dimensions.

Answer 2

In the majority of cases where people talk about JPEG files having thumbnails, they mean the file is JPEG-EXIF and the EXIF data contains a thumbnail image.

However, it is true to say that JFIF 1.02 supports a thumbnail image. The thumbnail may be stored in the JFIF APP0 marker or an official JFXX APP0 extension marker; the thumbnail may be uncompressed RGB, JPEG or palletted. See http://en.wikipedia.org/wiki/JPEG_File_Interchange_Format#JFIF_segment_format for details.

Answer 3

The fastest and recommended ways are libraries.

For example, www.exiv2.org can provide you with the library for embedded thumbnail and EXIF info extraction. There is a lot of alternatives, but I think this one is a good solution.

Answer 4

www.exiv2.org in my eyes is a bad solution. There are just too many deps to other projects. Expat and zlib for instance. If you want to come up with a lightweight solution that you can support on your own, I would recommend to write your own EXIF decoder.