1674681 - Setting devPixelsPerPx to 1.75 causes rounding error and blurry canvas

Status: NEW

(Core :: Graphics, defect, P3)

Description

[Tatsuyuki Ishi]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.111 Safari/537.36 Edg/86.0.622.56

Steps to reproduce:

Set layout.css.devPixelsPerPx to 1.75 in about:config. Or simply use Firefox on a computer with 175% scaling factor.

Actual results:

window.devicePixelRatio becomes 1.7647058823529411 which is a 0.8% "drift". This results in canvas applications becoming blurry, since they can no longer render to physical pixels 1:1. (Can be tested with PDF.js: https://mozilla.github.io/pdf.js/web/viewer.html)

Expected results:

The DPR should be accurate for precisely presentable floating point values. (e.g. 1.75, 1.125 is accurately representable, but 1.1, 1.3 is not.)

Comment 1

[Masatoshi Kimura [:emk]]

AppUnitsPerCSSPixel() = 60.[1]

mAppUnitsPerDevPixelAtUnitFullZoom = round(AppUnitsPerCSSPixel / devPixelsPerCSSPixel(1.75)) = 34.[2]

nsDeviceContext::mAppUnitsPerDevPixel = round(mAppUnitsPerDevPixelAtUnitFullZoom) / mFullZoom) = mAppUnitsPerDevPixelAtUnitFullZoom = 34.[3] (Assuming full zoom is not applied.)

nsDeviceContext::AppUnitsPerDevPixel() = nsDeviceContext::mAppUnitsPerDevPixel = 34.[4]

mCurAppUnitsPerDevPixel = mDeviceContext->AppUnitsPerDevPixel() = 34.[5]

nsPresContext::AppUnitsPerDevPixel() = mCurAppUnitsPerDevPixel = 34.[6]

devicePixelRatio = AppUnitsPerCSSPixel() / presContext->AppUnitsPerDevPixel() = 60 / 34 = 1.764705882352941.[7]

[2] brings the rounding error.

Comment 2

[Dzmitry Malyshau [:kvark]]

Great investigation, Masatoshi! Talking to folks from gfx team, it sounds like changing mAppUnitsPerDevPixelAtUnitFullZoom to be floating point is the way to go, but it may be associated with quite a bit of implementation complexity.

Comment 3

[Kelsey Gilbert [:jgilbert]]

I'm not sure about this, unfortunately.
This was more or less explored here: https://github.com/KhronosGroup/WebGL/issues/2460#issuecomment-624463142

It comes down to asking what DPR is for.
In particular, at "175%" 1920x1080 becomes one of two things:

• 1097.142 x 617.142 @ dpr: 1.750
• 1088.000 x 612.000 @ dpr 1.7647058823529411 (current behavior)