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

Wed Sep 28 23:17:12 CEST 2022

Thank you for the very thorough explanation Paul.

To answer your question on 11: The dsvg device, simply defines svg 
elements with their attributes (rect with fill & stroke in my examples). 
It does not do any internal image processing like cairo.

My concern is how to proceed with the implementation in dsvg.

If I leave it as it is now, they're will be cases where it will give 
different results from cairo (and perhaps other devices that will 
implement group compositing in similar way).

On the other hand It would be quite challenging in practice to simulate 
the cairo implementation and apply first the fill and then the stroke 
with the active operator, on the element itself.

Any suggestions? :-)

Panagiotis

On 28/9/22 02:56, Paul Murrell wrote:
> Hi
>
> Thanks for the code (and for the previous attachments).
>
> Some thoughts so far (HTML version with images attached) ...
>
> <1>
> As you have pointed out, the Cairo device draws a stroked-and-filled 
> shape with two separate drawing operations: the path is filled and 
> then the path is stroked.  I do not believe that there is any 
> alternative in Cairo graphics (apart from filling and stroking as an 
> isolated group and then drawing the group, which we will come back to).
>
> <2>
> This fill-then-stroke approach is easy to demonstrate just with a thick
> semitransparent border ...
>
> library(grid)
> overlapRect <- rectGrob(width=.5, height=.5,
>                         gp=gpar(fill="green", lwd=20,
>                                 col=rgb(1,0,0,.5)))
> grid.newpage()
> grid.draw(overlapRect)
>
> <3>
> This fill-then-stroke approach is what happens on many (most?) 
> graphics devices, including, for example, the core windows() device, 
> the core quartz() device, the 'ragg' devices, and 'ggiraph'.  The 
> latter is true because this is actually the defined behaviour for SVG ...
>
> https://www.w3.org/TR/SVG2/render.html#Elements
> https://www.w3.org/TR/SVG2/render.html#PaintingShapesAndText
>
> <4>
> There are exceptions to the fill-then-stroke approach, including the 
> core pdf() device, but I think they are in the minority.  The PDF 
> language supports a "B" operator that only fills within the border (no 
> overlap between fill and border).  Demonstrating this is complicated 
> by the fact that not all PDF viewers support this correctly (e.g., 
> evince and Firefox do not;  ghostscript and chrome do)!
>
> <5>
> Forcing all R graphics devices to change the rendering of 
> filled-and-stroked shapes to match the PDF definition instead of 
> fill-then-stroke is unlikely to happen because it would impact a lot 
> of graphics devices, it would break existing behaviour, it may be 
> difficult/impossible for some devices, and it is not clear that it is 
> the best approach anyway.
>
> <6>
> Finally getting back to your example, the fill-then-stroke approach 
> produces some interesting results when applying compositing operators 
> because the fill is drawn using the compositing operator to combine it 
> with previous drawing and then the stroke is drawn using the 
> compositing operator to combine it with *the result of combining the 
> fill with previous drawing*. The result makes sense in terms of how 
> the rendering works, but it probably fails the "principle of least 
> surprise".
>
> srcRect <- rectGrob(2/3, 1/3, width=.6, height=.6,
>                     gp=gpar(lwd = 5, fill=rgb(0, 0, 0.9, 0.4)))
> dstRect <- rectGrob(1/3, 2/3, width=.6, height=.6,
>                     gp=gpar(lwd = 5, fill=rgb(0.7, 0, 0, 0.8)))
> grid.newpage()
> grid.group(srcRect, "in", dstRect)
>
> <7>
> This issue is not entirely unanticipated because it can arise 
> slightly-less-unintentionally if we combine a 'src' and/or 'dst' that 
> draw more than one shape, like this ...
>
> src <- circleGrob(3:4/5, r=.2, gp=gpar(col=NA, fill=2))
> dst <- circleGrob(1:2/5, r=.2, gp=gpar(col=NA, fill=3))
> grid.newpage()
> grid.group(src, "xor", dst)
>
> This was discussed in the Section "Compositing and blend modes" in the 
> original technical report about groups and compositing ...
>
> https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/groups/groups.html#userdetails 
>
>
> <8>
> A solution to the problem of drawing more than one shape (above) is to 
> take explicit control of how shapes are combined, *using explicit 
> groups* ...
>
> grid.newpage()
> grid.group(groupGrob(src), "xor", dst)
>
> <9>
> Explicit groups can be used to solve the problem of overlapping fill 
> and stroke (here we specify that the rectangle border should be 
> combined with the rectangle fill using the "source" operator) ...
>
> grid.newpage()
> grid.group(overlapRect, "source")
>
> <10>
> Explicit groups can also be used to get the result that we might have 
> originally expected for the "in" operator example (here we isolate the 
> 'src' rectangle so that the border and the fill are combined together 
> [using the default "over" operator] and then combined with the other 
> rectangle using the "in" operator) ...
>
> grid.newpage()
> grid.group(groupGrob(srcRect), "in", dstRect)
>
> <11>
> A possible change would be to force an implicit group (with op=OVER) 
> on the 'src' and 'dst' in dev->group().  I believe this is effectively 
> what you are doing with your dsvg() device (?).
>
> Currently, dev->group() does this ...
>
> [OVER] shape shape shape OP shape shape shape
>
> ... and an implicit group on 'src' and 'dst' would do this ...
>
> ([OVER] shape shape shape) OP ([OVER] shape shape shape)
>
> An implicit (OVER) group would make it easier to combine multiple 
> shapes with OVER (though only slightly) ...
>
> grid.group(src, OP, dst)
>
> ... instead of ...
>
> grid.group(groupGrob(src), OP, dst)
>
> On the other hand, an implicit (OVER) group would make it harder to 
> combine multiple shapes with an operator other than OVER (by quite a 
> lot?) ...
>
> grid.group(groupGrob(shape, OP, groupGrob(shape, OP, shape)), OP, dst)
>
> ... instead of ...
>
> grid.group(src, OP, dst)
>
> The complicating factor is that what constitutes more than one shape 
> (or drawing operation) can be unexpected ...
>
> gList(rectGrob(), rectGrob())  ## obvious
> rectGrob(width=1:2/2)          ## less obvious
> rectGrob(gp=gpar(col=, fill=)) ## a bit of a surprise
>
> <12>
> In summary, while there is some temptation to add an implicit group 
> around 'src' and 'dst' in a group, there are also reasons not to.
>
> Happy to hear further arguments on this.
>
> Paul
>
> On 28/09/2022 8:04 am, Panagiotis Skintzos wrote:
>> Here is the code again in text:
>>
>>
>> src <- rectGrob(2/3, 1/3, width=.6, height=.6, gp=gpar(lwd = 5,
>> fill=rgb(0, 0, 0.9, 0.4)))
>> dst <- rectGrob(1/3, 2/3, width=.6, height=.6, gp=gpar(lwd = 5,
>> fill=rgb(0.7, 0, 0, 0.8)))
>>
>> svg("cairo.in.svg", width = 5, height = 5)
>> grid.group(src, "in", dst)
>> dev.off()
>>
>>
>>
>> On 27/9/22 04:44, Paul Murrell wrote:
>>  >
>>  > Could you also please send me the SVG code that your device is
>>  > generating for your example.  Thanks!
>>  >
>>  > Paul
>>  >
>>  > On 27/09/22 08:50, Paul Murrell wrote:
>>  >> 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 
>> <https://stat.ethz.ch/mailman/listinfo/r-devel>
>>  >>
>>  >
>