442133 - Canvas text sizes depend on browser text zoom

Status: VERIFIED FIXED

(Core :: Graphics: Canvas2D, defect)

Description

[Philip Taylor]

To reproduce:
* Open http://philip.html5.org/tests/canvas/misc/changing-font-size.html
* Zoom the page in/out

Expected results:
The text drawn into the canvas should be a consistent size in canvas coordinate space units - "16px" should always correspond to 16 canvas units.

Actual results:
The canvas text size is affected by the browser's zoom.

This is a problem since it makes careful layouts much less reliable: people will assume a 16px font is about the same size as a rect(0, 0, w, 16), but if a user zooms in (or increases their text-only zoom) to 200% then that assumption will be very wrong.

Comment 1

[Takeshi Kurosawa :takenspc]

Attachment: Patch

use |AppUnitsPerCSSPixel()| instead of |AppUnitsPerDevPixel()| when creating |gfxFontStyle|.

Comment 2

[Vladimir Vukicevic [:vlad] [:vladv] (needinfo me, slow to respond)]

Comment on attachment 327323 [details] [diff] [review]
Patch

Looks right to me, Eric can you confirm?

Comment 3

[Eric Butler [:ebutler]]

Comment on attachment 327323 [details] [diff] [review]
Patch

Worked when I tested it, and measureText and drawText seem to behave consistently under different zooms.

Comment 4

[Eric Butler [:ebutler]]

Comment on attachment 327323 [details] [diff] [review]
Patch

I just noticed that while the text draws correctly, measureText returns erroneous results when text-only zoom is used. measureText should return the same value regardless of zoom. This is because dev pixels are used in the drawText and measureText instead of CSS pixels

Comment 5

[Eric Butler [:ebutler]]

Attachment: ver 1.0

Includes the correction from the previous patch for full-page zoom, as well as a new correct for text-only zoom. Canvas text will now be completely unaffected by text zoom.

Comment 6

[Takeshi Kurosawa :takenspc]

Thanks ebutler for your patch!
# I'm being busy temporary :-(.

Comment 7

[Rob Arnold [:robarnold]]

Pushed changeset 94b3ceae4041

http://hg.mozilla.org/index.cgi/mozilla-central/rev/94b3ceae4041

Comment 8

[Tony Chung [:tchung]]

Verified fix on Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b2pre) Gecko/20081023 Minefield/3.1b2pre.

Comment 9

[Brett Zamir]

Attachment: measureText fails with non-font characters

I was working on a hack to figure out whether a character were supported in a given font (by converting to canvas to see whether boxed hex code patterns were shown or not), and I ran across the fact that measureText returns a different value depending on zoom if the character is not supported in the font.