[Rd] Question about grid.group compositing operators in cairo

[Paul Murrell]

Hi

Thanks for the report.  It certainly sounds like I have done something 
stupid :)  For my debugging and testing could you please share the R 
code from your tests ?  Thanks!

Paul

On 26/09/22 10:27, Panagiotis Skintzos wrote:
> Hello,
> 
> I'm trying to update ggiraph package in graphic engine v15 (currently we 
> support up to v14).
> 
> I've implemented the group operators and when I compare the outputs of 
> ggiraph::dsvg with the outputs of svg/png, I noticed some weird results.
> 
> Specifically, some operators in cairo (in, out, dest.in, dest.atop) give 
> strange output, when any source element in the group has a stroke color 
> defined.
> 
> I attach three example images, where two stroked rectangles are used as 
> source (right) and destination (left).
> 
> cairo.over.png shows the result of the over operator in cairo
> 
> cairo.in.png shows the result of the in operator in cairo
> 
> dsvg.in.png shows the result of the in operator in dsvg
> 
> 
> You can see the difference between cairo.in.png and dsvg.in.png. I found 
> out why I get different results:
> 
> In dsvg implementation there is one drawing operation: Draw the source 
> element, as whole (fill and stroke) over the destination element (using 
> feComposite filter)
> 
> In cairo implementation though there are two operations: Apply the fill 
> on source and draw over the destination and then apply the stroke and 
> draw over the result of the previous operation.
> 
> I'm not sure if this is intentional or not. Shouldn't the source element 
> being drawn first as whole (fill and stroke with over operator) and then 
> apply the group operator and draw it over the destination? It would seem 
> more logical that way.
> 
> 
> Thanks,
> 
> Panagiotis
> 
> 
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

-- 
Dr Paul Murrell
Te Kura Tatauranga | Department of Statistics
Waipapa Taumata Rau | The University of Auckland
Private Bag 92019, Auckland 1142, New Zealand
64 9 3737599 x85392
paul using stat.auckland.ac.nz
www.stat.auckland.ac.nz/~paul/