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Editorial: clarify canvas coordinate system #2867

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merged 4 commits into from Sep 28, 2017
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Editorial: clarify canvas coordinate system #2867

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Editorial: clarify canvas coordinate system
annevk Jul 26, 2017
1d7b80a
reword things a bit and drop a confusing sentence
annevk Jul 27, 2017
0f3c28e
reword things a bit more
annevk Sep 21, 2017
dbe8bf7
address nit
annevk Sep 28, 2017
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Expand Up @@ -59276,7 +59276,7 @@ callback <dfn>BlobCallback</dfn> = void (<span>Blob</span>? blob);</pre>
element is available. For example, it is inappropriate to use a <code>canvas</code> element to
render a page heading: if the desired presentation of the heading is graphically intense, it
should be marked up using appropriate elements (typically <code>h1</code>) and then styled using
CSS and supporting technologies such as Web Components.</p>
CSS and supporting technologies such as <span data-x="shadow tree">shadow trees</span>.</p>

<p>When authors use the <code>canvas</code> element, they must also provide content that, when
presented to the user, conveys essentially the same function or purpose as the
Expand Down Expand Up @@ -60066,11 +60066,25 @@ interface <dfn>Path2D</dfn> {

<p>The size of the coordinate space does not necessarily represent the size of the actual bitmaps
that the user agent will use internally or during rendering. On high-definition displays, for
instance, the user agent may internally use bitmaps with two device pixels per unit in the
instance, the user agent may internally use bitmaps with four device pixels per unit in the
coordinate space, so that the rendering remains at high quality throughout. Anti-aliasing can
similarly be implemented using over-sampling with bitmaps of a higher resolution than the final
similarly be implemented using oversampling with bitmaps of a higher resolution than the final
image on the display.</p>

<div class="example">
<p>Using <span data-x="'px'">CSS pixels</span> to describe the size of a rendering context's
<span>output bitmap</span> does not mean that when rendered an equivalent number of <span
data-x="'px'">CSS pixels</span> ends up being used. <span data-x="'px'">CSS pixels</span> are
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Suggestion:

(...) does not mean that when rendered the canvas will cover an equivalent area in <span data-x="'px'">CSS pixels</span>.

reused for ease of integration with CSS features, such as text layout.</p>

<p>In other words, the <code>canvas</code> element below's rendering context has a 200x200
<span>output bitmap</span> (which internally uses <span data-x="'px'">CSS pixels</span> as a
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I think it should be clarified that 200x200 represents both: a) the domain of the canvas's coordinate space in CSS pixels, and b) the size of the canvas's bitmap in actual pixels.

unit for ease of integration with CSS) and is rendered as 100x100 <span data-x="'px'">CSS
pixels</span>:

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It is not clear to me what you are trying to explain in this sentence. By "device pixels" are you referring to the pixels of the canvas's bitmap or the pixels of the physical display device? Whichever it is, this sentence doesn't really provide any actionable information (because of the word "could").

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Referring to the physical device. It was meant as a statement of fact to affirm the details.

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Then perhaps you should state what conditions need to be met in order to get 1 to 1 correspondence with physical device pixels. Something about the layout vs intrinsic size matching the devicePixelRatio.

<pre>&lt;canvas width=200 height=200 style=width:100px;width:100px></pre>
</div>

<hr>

<p>The <dfn>2D context creation algorithm</dfn>, which is passed a <var>target</var> (a
Expand Down