CanvasOpaque - Revision history

Appsforartists: /* Proposed Solutions */

2013-05-01T17:38:54Z

Proposed Solutions

← Older revision		Revision as of 17:38, 1 May 2013
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	== Proposed Solutions ==		== Proposed Solutions ==

	One proposed solution was the <code>opaque</code> attribute on the <code>canvas</code> element. This was implemented by Mozilla and has been shipping in Firefox as the <code>moz-opaque</code> attribute. Semantically, this is exactly what we want. However, it does not match WebGL syntax.		One proposed solution was the <code>opaque</code> attribute on the <code>canvas</code> element. This was implemented by Mozilla and has been shipping in Firefox as the <code>moz-opaque</code> attribute. Semantically, this is exactly what we want. However, it does not match [http://www.khronos.org/registry/webgl/specs/latest/#5.2 WebGL syntax].

	=== Suggested Solution ===		=== Suggested Solution ===

	Allow an optional parameter to the Canvas element's <code>getContext</code> method, which specifies the context creation parameter object. The object contains the boolean <code>alpha</code> property:		Allow an optional parameter to the Canvas element's <code>getContext</code> method, which specifies the context creation parameter object. The object, like its [http://www.khronos.org/registry/webgl/specs/latest/#5.2 WebGL cousin], contains the boolean <code>alpha</code> property:

	<pre>		<pre>

2013-04-22T17:00:08Z

Rationale:

← Older revision		Revision as of 17:00, 22 April 2013
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	'''Q:''' Can you call getContext more than once on the same canvas, with different values for { alpha }?		'''Q:''' Can you call getContext more than once on the same canvas, with different values for { alpha }?

	'''A:''' No. Once it has been called once, the type of the backing store is set. ~~Calling it again with a different argument raises an exception~~.		'''A:''' No. Once it has been called once, the type of the backing store is set. On subsequent calls to getContext(), the context attributes parameter is ignored.

	== Issues: ==		== Issues: ==

2013-04-22T16:56:00Z

Suggested IDL

← Older revision		Revision as of 16:56, 22 April 2013
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	<pre>		<pre>
	~~interface~~ Canvas2DContextAttributes {		dictionary Canvas2DContextAttributes {
	~~attribute~~ boolean alpha;		boolean alpha;
	};		};

2013-04-19T17:30:51Z

Suggested IDL

← Older revision		Revision as of 17:30, 19 April 2013
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	interface Canvas2DRenderingContext {		interface Canvas2DRenderingContext {
	...		...
	Canvas2DContextAttributes getContextAttributes();		Canvas2DContextAttributes? getContextAttributes();
	}		}
	</pre>		</pre>

2013-04-19T14:43:38Z

2013-04-17T22:43:46Z

Rationale:

← Older revision		Revision as of 22:43, 17 April 2013
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	'''Q:''' Why not add a new element attribute instead?		'''Q:''' Why not add a new element attribute instead?

	'''A:''' Implementing the same syntax as WebGL brings consistency to the web platform, obeys the principle of least surprise for developers. It also may allow more code sharing in JavaScript, since with duck typing, the same object can be used for both.		'''A:''' Implementing the same syntax as WebGL brings consistency to the web platform, obeys the principle of least surprise for developers. It also may allow more code sharing in JavaScript, since with duck typing, the same object can be used for both canvas and WebGL.
	<br/>		<br/>

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	'''A:''' No. Once it has been called once, the type of the backing store is set. Calling it again with a different argument raises an exception.		'''A:''' No. Once it has been called once, the type of the backing store is set. Calling it again with a different argument raises an exception.

	== Issues: ==		== Issues: ==

2013-04-17T22:42:28Z

← Older revision		Revision as of 22:42, 17 April 2013
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	:''This proposal is ~~allowing~~ the specification of an opaque backing store for 2D Canvas, to optimize blending and culling.''		:''This proposal is to allow the specification of an opaque backing store for 2D Canvas, to optimize blending and culling.''

	== Use Case Description ==		== Use Case Description ==

2013-04-17T19:48:04Z

Goals

← Older revision		Revision as of 19:48, 17 April 2013
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	# Allow the specification of an opaque backing store for canvas.		# Allow the specification of an opaque backing store for canvas.
	# Modify all canvas operations to respect the opaque backing store (~~and~~ leave it at ~~alpha~~ 1.0).		# Modify all canvas operations to respect the opaque backing store (i.e., to leave all per-pixel alpha at 1.0).
	# Allow the user agent to easily optimize compositing and culling of the canvas.		# Allow the user agent to easily optimize compositing and culling of the canvas.

2013-04-17T16:06:19Z

Goals

← Older revision		Revision as of 16:06, 17 April 2013
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	== Goals ==		== Goals ==

	# Allow the specification of an opaque backing store at ~~getContext(~~) ~~time~~.		# Allow the specification of an opaque backing store for canvas.
			# Modify all canvas operations to respect the opaque backing store (and leave it at alpha 1.0).
	# Allow the user agent to easily optimize compositing and culling of the canvas.		# Allow the user agent to easily optimize compositing and culling of the canvas.

2013-04-17T16:04:27Z

Implementing non-alpha-modifying compositing modes may be difficult in some 2D graphics APIs

← Older revision		Revision as of 16:04, 17 April 2013
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	== Issues: ==		== Issues: ==

	= Implementing non-alpha-modifying compositing modes may be difficult in some 2D graphics APIs =		=== Implementing non-alpha-modifying compositing modes may be difficult in some 2D graphics APIs ===

	The CoreGraphics API provides a mechanism to specify an opaque backing store, via kCGImageAlphaNoneSkipLast. However, it seems that IOSurfaces do not.		The CoreGraphics API provides a mechanism to specify an opaque backing store, via kCGImageAlphaNoneSkipLast. However, it seems that IOSurfaces do not.

← Older revision		Revision as of 14:43, 19 April 2013
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	var ctx = canvas.getContext('2d', { alpha: false });		var ctx = canvas.getContext('2d', { alpha: false });
	</pre>		</pre>

			When alpha is false, the per-pixel alpha values in the canvas backing store must be 1.0 at all times. All canvas operations are modified to preserve this invariant. If the attribute is not specified, the default value of "true" must be used instead.

			When alpha is false, the backing store must be initialized to opaque black (rgba(0, 0, 0, 1.0)), instead of transparent black (rgba(0, 0, 0, 0.0)). In addition, clearRect() clears to opaque black instead of transparent black.

			The behaviour of putImageData() and putImageDataHD() is to premultiply the RGB components by the alpha component as usual, but write 1.0 into destination alpha. In other words, if (r, g, b, a) are the component values in a given pixel passed to putImageData[HD](), then r' = ar, g' = ag, b' = ab are the colour components of the resulting canvas pixel, and (r', g', b', 1.0) is written to the canvas backing store.

			When a canvas has alpha: false, all globalCompositeOperation modes behave as normal and the resulting RGB components are written to the canvas backing store, but the per-pixel alpha component is left unchanged at 1.0.

	== Suggested IDL ==		== Suggested IDL ==