Bad performance with Canvas and extensive clipping

Jim Graham james.graham at
Tue May 27 22:54:50 UTC 2014

Hi Tom,

There are 2 upgrades to consider.  One involves new API, but is probably 
best in the long run.

Without API, we'd have to detect if the path were rectangular in the 
processing of the CLIP command in  If the 4 coordinates 
are an axis aligned rectangle on integer coordinates then we could 
special case that with g.setClipRect().  There are other considerations, 
such as:

- If there is already a soft non-rect clip, then it should probably not 
bother with the special case since it won't simplify anything.
- If we have special cased the rectangle, then we must track that across 
save/restore properly.
- If we have a special case cliprect and then we get a non-special case 
rect as the argument of a CLIP command, then we need to resolve it into 
a singular case (most likely default back to soft clipping).
- The processing that tries to detect "are they clearing the entire 
buffer" needs to be aware of any clip in effect - those tests are done 
at the javafx.scene.canvas.GraphicsContext level.

We could put that fix in with no new API so it could go in as soon as we 
are satisfied with its stability.

If we want to add new API, so that you don't have to construct a path 
every time you want to do clipRect() and we don't have to decipher your 
path to figure out that it is a rectangle, then we would have to wait 
for the next opportunity to add API (FX can add API in between major JDK 
releases, but there is a process to go through and I don't think we can 
do it for 8u20 any more).  The process for that would be:

javafx.scene.canvas.GraphicsContext would need a new method that would 
take the rectangular clipping parameters and put them into the buffer. 
The existing fillRect() method would provide a good template.  A new 
"command code constant" would have to be added to represent "This is a 
clip rectangle request".

NGCanvas would then need to digest the new buffer commands and I believe 
that the existing Prism call g.setClipRect() would work to enable the 
scissor clip (fast rectangular clipping).

The question is what is the proper API?  If we have it take doubles, 
would that imply to developers that there would be soft clipping of the 
edges similar to if you used a rectangular path and clip()?  Right now 
Node.setClipNode(Rectangle) will do the fast scissor clip 
(g.setClipRect()) if the coordinates fall on integer axis-aligned 
coordinates, but it will do soft-edged clipping if there is 
rotation/skewing, or the coordinates are not integers.  That would 
probably be the best API to mimic since HTML5 doesn't have a similar 
"cliprect" method...


On 5/27/14 2:57 PM, Tom Schindl wrote:
> Is there anything I could do to help getting rectangular clipping into
> JavaFX - I tried to find my way through the sources but I'm not sure I
> have enough knowledge to provide a patch in this area.
> BTW it looks like I'm not alone with the clipping performance problem
> see
> Tom
> On 27.05.14 23:47, Jim Graham wrote:
>> Canvas is, essentially, a "draw pixels" mechanism.  We have to bundle
>> the requests into a command stream due to threading issues, but when the
>> requests get to the render thread then they get turned into pixels so
>> the command stream is a temporary intermediary.  Some of the hw J2D
>> pipelines also have a temporary command stream due to platform threading
>> issues as well.  It all depends on which pipeline you use and on which
>> platform in the case of J2D.  FX simply normalized the threading on all
>> pipelines/platforms so that we have a separate UI and render thread in
>> all cases, but that concept is not foreign to J2D either.
>> I'm fairly certain that the lack of simple rectangular clipping is
>> probably the biggest cause of your performance problems.  We do AA on
>> everything in FX, though, whereas rendering to a BufferedImage by
>> default will be non-AA unless you requested AA using the graphics hints.
>>   But on the up-side, we hw accelerate just about every operation in FX
>> so it should be on par with performance there, modulo the lack of
>> rectangular clipping...
>>              ...jim
>> On 5/23/14 5:46 PM, Tom Schindl wrote:
>>> Hi,
>>> As an experiment I've now written a SWT-GC implementation using a
>>> BufferedImage & Graphics2D and transfering the pixels over to JavaFX and
>>> the performance is as it is with native SWT.
>>> I always thought Canvas works similar to Image and one only draws pixels
>>> - looks like that is not the case, having a dep in my application
>>> java.awt is not what I'm aiming at but without acceptable performance in
>>> conjunction with clipping it looks like i have to go this route :-(
>>> Tom
>>> On 23.05.14 23:57, Tom Schindl wrote:
>>>> In the current usecase it is a rect all time but that's just in this
>>>> special use case.
>>>> I guess that rect clipping is the most common one so having an
>>>> optimization for rects and a slow path for none rects might help.
>>>> Tom
>>>> Von meinem iPhone gesendet
>>>>> Am 23.05.2014 um 23:35 schrieb Jim Graham <james.graham at>:
>>>>> Are you clipping to an arbitrary path in all cases or just a
>>>>> rectangle?  Unfortunately we only offer the arbitrary
>>>>> clip-to-current-path method that isn't optimized for basic
>>>>> rectangular clipping and it implements soft clipping.
>>>>> There is an outstanding tweak that we added faster clipping support
>>>>> for WebNode and we need to start using it for
>>>>> Node.setClipNode(non-rectangle) and Canvas, but we haven't
>>>>> implemented that yet.
>>>>> (  It basically is a
>>>>> direct "render this texture through that other texture as a clip"
>>>>> operation instead of the current code that runs it through some
>>>>> Blend effect filters.  It would definitely improve your run times,
>>>>> but I'm not sure how much.
>>>>> Even more savings could be had for rectangular clips if we provided
>>>>> some way to communicate them to the GC...
>>>>>              ...jim
>>>>>> On 5/23/14 11:47 AM, Tom Schindl wrote:
>>>>>> Hi,
>>>>>> Maybe as some of you might know I've been working since sometime on
>>>>>> SWT
>>>>>> on JavaFX and to implement direct drawing operations we use
>>>>>> JavaFX-Canvas.
>>>>>> I've today tried to run a heavy direct drawing grid implementation and
>>>>>> it performed very bad because it makes heavy use of clipping.
>>>>>> For a grid I've counted ~1500 clipping operations the library works
>>>>>> something like this:
>>>>>> boolean activeClip;
>>>>>> Canvas canvas = new Canvas();
>>>>>> public void setClipping(PathIterator pathIterator) {
>>>>>>     GraphicsContext gc = canvas.getGraphicsContext2D();
>>>>>>     if(activeClip) {
>>>>>>       gc.restore();
>>>>>>       activeClip= false;
>>>>>>     }
>>>>>>     if( pathIterator == null ) {
>>>>>>       return;
>>>>>>     }
>>>>>>     activeClip = true;
>>>>>>     float coords[] = new float[6];
>>>>>>          gc.beginPath();
>>>>>>          float x = 0;
>>>>>>          float y = 0;
>>>>>>          gc.moveTo(0, 0);
>>>>>>          while( ! pathIterator.isDone() ) {
>>>>>>              switch (pathIterator.currentSegment(coords)) {
>>>>>>              case PathIterator.SEG_CLOSE:
>>>>>>                  gc.lineTo(x, y);
>>>>>>                  break;
>>>>>>              case PathIterator.SEG_CUBICTO:
>>>>>>                  gc.bezierCurveTo(coords[0], coords[1], coords[2],
>>>>>> coords[3],
>>>>>> coords[4], coords[5]);
>>>>>>                  break;
>>>>>>              case PathIterator.SEG_LINETO:
>>>>>>                  gc.lineTo(coords[0], coords[1]);
>>>>>>                  break;
>>>>>>              case PathIterator.SEG_MOVETO:
>>>>>>                  gc.moveTo(coords[0], coords[1]);
>>>>>>                  x = coords[0];
>>>>>>                  y = coords[1];
>>>>>>                  break;
>>>>>>              case PathIterator.SEG_QUADTO:
>>>>>>                  gc.quadraticCurveTo(coords[0], coords[1],
>>>>>> coords[2], coords[3]);
>>>>>>                  break;
>>>>>>              default:
>>>>>>                  break;
>>>>>>              }
>>>>>>    ;
>>>>>>          }
>>>>>>          gc.clip();
>>>>>>          gc.closePath();
>>>>>> }
>>>>>> Am I doing something ultimately wrong, totally wrong? Has anyone an
>>>>>> idea
>>>>>> how I would work around the problem?
>>>>>> Tom

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