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Re: [whatwg] Grouping in canvas 2d

From: Ashley Gullen <ashley@scirra.com>
Date: Fri, 14 Jun 2013 18:13:31 +0100
Message-ID: <CAABs73ijNJiH8Ah1PqB1aAZbkE+yqn41esvHuW_T0G7s7vPrEg@mail.gmail.com>
To: Rik Cabanier <cabanier@gmail.com>
Cc: "public-canvas-api@w3.org" <public-canvas-api@w3.org>
IMO this looks like something that belongs to a framework rather than the
web platform. I don't see why you'd ever need "lots of temporary canvas
objects", it should only ever require one, which probably has a low enough
overhead to not need to worry about too much.


On 14 June 2013 04:57, Rik Cabanier <cabanier@gmail.com> wrote:

> Last year, I requested if grouping could be added to canvas.
> The API would look like this:
> void beginLayer();
> void beginLayer(unsigned long x, unsigned long y, unsigned long w, unsigned
> long h);
> void endLayer();
> When you call beginLayer, you inherit everything from the graphics state
> except shadows, opacity and the current compositing mode. Those will reset
> to their initial value.
> At endLayer time, the graphics state will be restored and you will draw the
> content of the group.
> so for instance, if you want multiply blend with opacity of .5
> ctx.globalAlpha = .5;
> ctx.globalCompositeOperation = "multiply";
> ctx.beginLayer();
> ... // drawing
> ctx.endLayer();
> This would cause everything between begin/endLayer to be drawn normally.
> This result is then blended and composited at endLayer time.
> Last year, people argued that you could achieve the same effect by using a
> temporary canvas so this API is not really needed.
> I would like to see if people are more open to this API now.
> Reasons to have it:
> - it is easily achievable with existing graphics libraries.
> - it is a very common idiom. You can cut down on the amount of JS needed.
> - if you want to avoid antialiasing issue, you need to carefully set the
> CTM on the temporary canvas and remove the CTM from the current canvas
> - creating a temporary canvas has a higher overhead than simply starting a
> layer.
> - creating lots of temporary canvas objects creates memory overhead
> - is polyfillable for older canvas implementations
> Rik
Received on Friday, 14 June 2013 17:13:58 UTC

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