Currently I'm loading them and then setting the
transparency index using gif header, but can't get them
to save correctly at all, even if I rewrite the
background color in the header.

Thanks for you posting in the group!

BTW, If you use the CImage class to handle the GIF image, I suggest a
sample project on the usage of that class:
http://www.codeproject.com/bitmap/trans_cimage.asp?target=gif%7Ctrans...

Best regards,

Gary Chang
Microsoft Online Partner Support

Get Secure! - www.microsoft.com/security
This posting is provided "AS IS" with no warranties, and confers no rights.
--------------------
Third-Party Link Disclaimer

I've been through the ATL code, and the bits are copied
for the palette, but the alpha bits are not used in the
GDI bitmap, and no transparency index initialized.

I'm using:

CImage img;
img.Load(picPath);

to load the bitmap. If it's a gif, I grab the header and
info block from the file and use img.SetTransparentColor
(...) set to the background color field value if the
packed field indicates it's valid.

To save I was trying to do essentially the same, after
the img.save(tempPath), I write the packed field to
indicate the tranparency is valid by using | 0x80, and
set the background color field, but it doesn't work. I've
confirmed the changes are written. At present I'm
attempting to use Gdiplus::Bitmap.Save(), to see if it
supports writing the transparent values, but I need to
figure out the encoder parameters first. More header
reading.

Thanks,
Scott

The shortest example would be:

CImage img;
img.Load("C:\\some.gif");
LONG tcolor = img.GetTransparentColor();

tcolor will always receive -1 even if the GIF file is set
for a transparent background. The problem is also found
in PNG files.

The problem is in the ATL CImage Load and Save code. In
the Load, it makes a GDI palette without initializing any
transparency info, and in the Save, it uses the GDI
bitmap's palette which doesn't contain any transparency
info. I've bypassed both using my own code based on the
GDI+ Image Load and Save and it works fine, so it's
obviously supported in the encoder/decoder, just not the
ATL CImage code.

This should either be classified as a bug, or ammended in
the docs somewhere for the ATL CImage class, since the
docs currently note transparency is supported for 4 or 8
bit indexed images, but fail to note it can't load or
save them. If you would like the code that fixes it, let
me know and I'll make it available on the net.

Thanks and Happy New Year to you too.
Scott

Thanks for your quickly reply!

It seems a problem to the CImage.Load()/Save() method, now I have reported
this issue to the product team and waiting for their feedback, perhaps it
will need some time.
If I get any response, I will reply to you as soon as possible.

And meantime, we will very appreciate if you can share your valuable
revised Load()/Save() function code with the community members.

Thanks!

Best regards,

Gary Chang
Microsoft Online Partner Support

Thanks for your feedback on it.

After looking through the atlimage.h file, it appears that the
m_iTransparentColor member (which is what is returned after the call to
GetTransparentColor() ) is automatically initialized to -1 when the CImage
object is created. It The only way we can change that is by calling the
SetTransparentColor() function.

So yes, I agree with you that we should notify the product group about
this. I will contact them on this problem to see how to document this
behavior.

Thanks again for your feedback. If you feel there is any we can do, please
feel free to post here.

Best regards,
Yanhong Huang
Microsoft Community Support