Add support for device-pixel-content-box-size #3554 #4476
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@gregwhitworth - mind taking a first pass review on this? Please bear with me as this is my first spec PR. Thanks! |
astearns
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Oct 31, 2019
astearns
reviewed
Oct 31, 2019
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chrishtr
reviewed
Nov 1, 2019
chrishtr
reviewed
Nov 1, 2019
chrishtr
reviewed
Nov 1, 2019
Reference browser-specific behavior, rather than implementation specific. Mention that the device pixels are not affected by transforms. Clarify which box contentRect refers to.
chrishtr
reviewed
Nov 1, 2019
| before applying any CSS transforms on the element or its ancestors. | ||
| This size must contain integer values. | ||
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| <p class="note"> |
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How about adding a non-normative note specifying a typical way that UAs compute this box?
e.g. "How a UA computes the device pixel box for an element is implementation-dependent. One way could be to multiply the box size and position by the device pixel ratio, then round the resulting (floating-point) size and position of the box to integer values in some way that maximizes quality of rendered output".
chrishtr
reviewed
Nov 1, 2019
resize-observer-1/Overview.bs
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| <p class="note"> | ||
| The {{device-pixel-content-box}} can be approximated by multiplying <a>devicePixelRatio</a> by the {{content-box}} size. | ||
| However, due to browser-specific subpixel snapping behavior, | ||
| authors cannot determine the correct way to round the scaled {{content-box}} size. |
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s/the scaled/this scaled/, to make clearer that you're referring to the output of multiplying by devicePixelRatio.
chrishtr
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Nov 4, 2019
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Minutes: #3554 (comment)
Authors need a way to get the content box size in integral physical pixels. This allows for the correct sizing of the backing store of a canvas to match the snapped CSS size and prevents moire patterns that arise when the sizes are mismatched.
Currently authors could use devicePixelRatio and rounding in order to approximate this, but due to implementation differences in pixel snapping, can still get the wrong answer. Additionally, this can be affected by position changes which authors cannot currently hook into in a reasonable manner.