Reading JPEGs: ImageIO.read() messes up color spac

问题:

I'm trying to read, rescale and save images within a Servlet. That's the relevant code:

BufferedImage image = ImageIO.read(file);

BufferedImage after = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
AffineTransform at = AffineTransform.getScaleInstance(factor, factor);
AffineTransformOp scaleOp = new AffineTransformOp(at, AffineTransformOp.TYPE_BILINEAR);
after = scaleOp.filter(image, null);

ImageIO.write(after, "JPG", file));

The original file is a plain RGB-Jpeg, but when I open and save the file it comes out as a CMYK-Jpeg. That happens even if i don't rescale the image, just opening and closing the image causes the problem.

When I open PNGs or GIFs everything is fine. Does anybody know what to do here? I would expect ImageIO's read-Method to retain the original colorspace.

If there's another way comfortable way of reading jpeg's?

Thanks for any suggestions!

回答1:

You create after and then overwrite it with scaleOp.filter. Is this correct? So your after image may not be RGB even though you think it is? If you want after to be RGB then you may need to 'draw' image onto after before you do the transform.

回答2:

I have had the same problem, and found this page.

I tried the suggestion above of creating a BufferedImage with the right type and using it as the after image instead of null in the filter call; and that did indeed resolve the problem.

回答3:

ImageIO.read ignores all embedded metadata, including an embedded color profile, which defines how RBG values map to physical devices such as screens or printers.

You could read the metadata separately and pass it to ImageIO.write, but it's easier to just convert the image to the (default) sRGB color space and ignore the metadata.

If you don't mind losing the metadata, replace

after = scaleOp.filter(image, null);

with

after = scaleOp.filter(image, after);

From the documentation of AffineTransformOp.filter:

If the color models for the two images do not match, a color conversion into the destination color model is performed.