ImageIO correlation between format and mimetype

Question

ImageIO has method for getting readers by both mimetype & format, but writing only by format. Is there a defined mapping between ImageFormat and Mimetype. or if you read an image with :
ImageIO.read(new File(path));
or
ImageIO.read(new DataInputStream(new ByteArrayInputStream(imageData)));
how do you find out the format of the BufferedImage
thanks paul

Answer 1

It's not that strict. A BufferedImage doesn't remember its input's mimetype. The closest connection would
be that a BufferedImage has a ColorModel. For example, it could be an IndexColorModel with 256 opaque
colors -- GIF, you claim, but the imput could have come from a PNG or even a JPEG. (Or I could have created
it directly in memory). What you have to be mindful of is that not every ImageWriter and image format works
well with every type of ColorModel. For example, if your BufferedImage has an alpha channel (it is either
transluscent or a bit mask) you can successfully write it to a jpeg file, but most tools (apart from ImageIO!)
will be unable to read it. A couple of posters have mentioned that the ACME GifEncoder fails when
the ColorModel has more than 256 colors, but other GIF encoders are able to succeed. There may be other
concerns as well. TIFF format accepts multiple images in a single file, and again you can do this with
JPEG, but most other tools will only find one image in the file. If you think JPEG is being wimpy, ImageIO with
TIFF lets me compress multiple images with JPEG's compression algorithm, but the tool I'm using to
view the results can't handle that, sigh.
The bottom line is if you're interested in writing out one specific image, try it and see, and tweak it until it works.
If you're trying to implement code that works across a variety of input mime types, be ready for lots of
testing, to discover the special cases.