Hi,
As I've introduced my self previously here on comp.compression, the one's
who've read it, know that I'm busy writing a JPEG decoder.
I want you to take a look at the following images. The first one is the
input jpeg image and the second one is the output I get in a .bmp file.
http://home.scarlet.be/~ad049105/eindwerk/testorig.jpg
http://home.scarlet.be/~ad049105/eindwerk/output.bmp
As you can see most of the image is correct (the output image), but still in
the leaves of the rose/tulip (not really a botanicus) have these blue like
distortions, also in the white below there are some yellow/green dots.
Does anyone know a probable cause of these artifacts and a solution to
remove them?
Many thanks in advance
Arne Demmers

"Arne Demmers" <arne.demmers@scarlet.bewrote in message
@scarlet.biz
Hi,

As I've introduced my self previously here on comp.compression, the one's
who've read it, know that I'm busy writing a JPEG decoder.

I want you to take a look at the following images. The first one is the
input jpeg image and the second one is the output I get in a .bmp file.

http://home.scarlet.be/~ad049105/eindwerk/testorig.jpg
http://home.scarlet.be/~ad049105/eindwerk/output.bmp

As you can see most of the image is correct (the output image), but still
in the leaves of the rose/tulip (not really a botanicus) have these blue
like distortions, also in the white below there are some yellow/green
dots.

Does anyone know a probable cause of these artifacts and a solution to
remove them?

I'm not sure what's going on, but it looks like some sort of overflow
in your arithmetic. Perhaps your matrix or idct doesn't have an
adequate range to support the output, or perhaps a limiter is not
working correctly. Have you tried it with a variety of simple horizontal
gradients (both black to white, and various colors). If you can get
it to break on these it might make debugging easier.

You only mentioned a decoder. If you're also working on an encoder
you could try matrix -matrix, hbf -hbf, then dct -idct and so on.

Arne Demmers wrote:
http://home.scarlet.be/~ad049105/eindwerk/testorig.jpg
http://home.scarlet.be/~ad049105/eindwerk/output.bmp

It looks like the blue channel of the RGB image is wrapping around from
0->255 (red->purple) and from 255->0 (white->yellow). Check that you're
handling saturation correctly when you convert from YCbCr to RGB.
-- Ben

Thanks both for your help.
The blue channel indeed was the problem -I had forgotten to change a
variable to the right one after copy pasting a piece of code.
Greets
Arne Demmers