[cairo] White seams (lines) appearing between objects

Kristof Van Landschoot kristof at coin-c.com
Tue Jan 15 00:55:27 PST 2008


Thanks for the observations so far.  However, I'm not sure I'm with  
you on this all...

On 14 Jan 2008, at 18:33, MenTaLguY wrote:

> On Mon, 14 Jan 2008 16:03:19 +0100, Kristof Van Landschoot  
> <kristof at coin-c.com> wrote:
>> Hi,
>> We've isolated a problem where we seem to get white seams between two
>> objects that really shouldn't have them.  A black rectangle and a red
>> image should touch seamlessly, but using these parameters for
>> transformation matrix and image size they do generate a pixel-wide
>> white line between them.
>> Is there anything we can do about this?
> The simplest solution is to render without antialiasing and then  
> decimate
> the output with an NxN averaging filter to get an antialiased result 
> [1].
> In the general case, the fringing is impossible to avoid when using
> "incremental" antialiasing.  In some specific cases, you can use the
> SATURATE compositing mode, but it only works when the shapes are  
> entirely
> opaque and/or do not actually overlap anywhere.
> -mental
> [1] i.e. NxN supersampling

Isn't what we're doing in the code sample turning antialiasing off?  
I.e., the call to


is what should take care of it, no?  But we still get the white line?

And then I don't see how supersampling could solve the issue since  
even from supersampling a fine white line you would still get a  
lighter color than the red or the black on either side of the line,  
wouldn't you?


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