.NET Framework - 8 bit indexed bitmap object saves to jpg as 24 bit...

Asked By James Maeding

22-Jan-07 08:11 PM

I've been studying all the articles I can find on GDI+, and putting together a function to convert image colors and
types.
I am finding that I can quantize an image to 8 bit colors, and then save to png format perfectly with:
bitmap.save(filename,  ImageFormat.png)
When I do the same thing to jpg, I get a 24 bit color jpg every time.
I know jpg supports an indexed 8 bit palette, and can make them all day in other progs like corel photo-paint.

I can watch the bitmap object to inspect it just before the save, its exactly the same every time.
The only difference is I am doing bitmap.save(filename, ImageFormat.Jpeg) instead of bitmap.save(filename,
ImageFormat.png)

I tried going the bitmap.save(filename, ImageCodecInfo, EncoderParameters) in case I needed to set the quality of the
jpg to 100 or something.  didn't work.

Any suggestions?  I must be missing something simple.  I thought if I start with the exact same bitmap object with same
properties for both cases, the save method would behave but its not.
thanks

22-Jan-07 08:47 PM

Where you're going wrong is, despite what you "know", jpeg doesn't support
indexed palettes at all.  It's a lossy compression format intended for
storing photograph-type images.  It is fundamentally unsuited to indexed
images.

22-Jan-07 09:59 PM

Paint shop pro forces the image to 24 bit. Maybe this is a feature only of
corel and not generally supported?

Michael

22-Jan-07 11:00 PM

funny how we can be our own worst enemy.  I am actually hoping you are right because that would answer my question.
The reason I think jpg's can support indexed is I can open one in corel photo paint, and convert color to 8 bit
paletted.  I can load that into a bitmap object and the palete is 256 colors, indexed.
I was sold on that but maybe that is a bit special (no pun..) somehow.
I agree its a lossy format, and have no opinion on if its suited to indexed colors.
I just know the paletted ones are a lot smaller than the 24 bit.
I'll be going with png's, but I wanted to be able to go to any format with my tool.
thx

22-Jan-07 11:02 PM

it seems that might be the case - and that is fine with me.  I do not like jpg a lot as most people do not realize its
lossier with every save.  thx for the reply

22-Jan-07 11:19 PM

Interesting.  What happens if you open a jpeg, convert to 8 bit indexed,
save it as jpeg, close the file, then re-open it?

22-Jan-07 11:39 PM

wow, I wonder what else I have been missing :)
you are dead on correct, it flips back to 24 bit color in corel pp.
I guess I so rarely use jpg's (I use tif's) that I didn't notice it.  I could have sworn it worked, you should have made
a bet with me <g>.
Its like fighting with my wife now, the more I say the dumber I look....

thanks for the schooling.

22-Jan-07 11:48 PM

Betcha $50 I am right :P

Oops, too late