244966 2022-09-09 03:34:31 -0700 [LBSE] Outermost <svg> elements are not device-pixel aligned 2022-09-16 08:47:05 -0700 1 1 1 Unclassified WebKit SVG WebKit Local Build Unspecified Unspecified RESOLVED FIXED InRadar P2 Normal --- 90738 1 zimmermann zimmermann rbuis sabouhallawa webkit-bug-importer zimmermann oldest_to_newest 1897200 0 zimmermann 2022-09-09 03:34:31 -0700 Whenever a SVG document fragment is placed in an enclosing HTML/CSS context, it receives its position per CSS rules (depending on 'display', 'position', etc.). These are potentially sub-pixel positions, such as x=8, y=79.8456. For HTML/CSS renderers the final position (in local coordinates) is pixel snapped, to enforce device-pixel alignment (no off-device-pixel painting). In WebCore "adjustedPaintOffset" (== paintOffset [external] + location() [intrinsic]) is the quantity that's pixel snapped within the Render*::paint() methods. The SVG render tree does not pixel snap any coordinates/lengths, except for the outermost <svg> renderer. There should be no visual difference between a HTML document drawing a box with a green background, and an equivalent SVG document that does the same using a <rect> element. Therefore the legacy SVG engine always rounded the 'adjustedPaintOffset' to the nearest integer, simulating the effect in the HTML/CSS rendering tree (not correct in all deviceScaleFactor configurations though!). In LBSE, pixel-snapping or rounding the 'adjustedPaintOffset' in RenderSVGRoot won't have the same affect as in the legacy engine: LBSE uses self-painting layers to paint the SVG document -- therefore any transformation needs to be applied on layer-level to affect the descendant renderers (that themselves use self-painting layers!). RenderLayer::paintLayerByApplingTransform() already contains all the code necessary to place outer <svg> elements on device-pixel aligned boundaries. The subpixelOffset is applied as transformation once for the outermost <svg>, and ignored (set to zero) for any other descendant SVG renderer. Pixel snapping is only relevant from RenderView downwards to the (and including) RenderSVGRoot -- the rest of the SVG subtree is not using pixel snapping / device pixel alignment. This will allow us to enable testing the few hundred tests in svg/dynamic-updates, that are currently skipped in LBSE (because they all fail). All of them showed subtle rendering differences caused by the currently _missing_ device pixel alignment for RenderSVGRoot. 1897202 1 zimmermann 2022-09-09 03:56:55 -0700 Pull request: https://github.com/WebKit/WebKit/pull/4158 1898942 2 webkit-bug-importer 2022-09-16 03:35:17 -0700 <rdar://problem/100019924> 1898981 3 ews-feeder 2022-09-16 08:47:03 -0700 Committed 254558@main (de2323d7e2c8): <https://commits.webkit.org/254558@main> Reviewed commits have been landed. Closing PR #4158 and removing active labels.