SVG Working Group Meeting Report — Sydney

The SVG Working Group had a four day face-to-face meeting in Sydney this month. The first day was a joint meeting with the CSS Working Group. I would like to thank the Inkscape board for allocating the funds for this trip as well as all the Inkscape donors that made it possible. This was an expensive trip as I was traveling from Paris and Sydney is an expensive city… but I think it was well worth it as the SVG WG (and CSS WG, where appropriate) approved all of my proposals and worked through all of the issues I raised. Unfortunately, due to the high cost of this trip, I have exhausted the budgeted funding from Inkscape for SVG WG travel this year and will probably miss the two other planned meetings, one in Sweden in June and one in Japan in October. We target the Sweden meeting for moving the SVG 2 specification from Working Draft to Candidate Recommendation so it would be especially good to be there. If anyone has ideas for alternative funding, please let me know. Highlights:

• CSS will support color palettes for OpenType SVG fonts.
• Publishing Shapes Level 2 (needed for SVG 2 auto-wrapped text in a shape) as a Working Draft approved.
• New clipped miter line-join approved.
• Using ‘z’ to auto-close curved paths approved.
• Text on a shape approved (extension of text on a path).
• Option to auto-smooth mesh gradients approved.

A summary of selected topics, grouped by day, follows:

Joint CSS and SVG Meeting

Minutes

• SVG sizing in HTML.

  We spent some time discussing how SVG should be sized in HTML. For corner cases, the browsers disagree on how large an SVG should be displayed. There is going to be a lot work required to get this nailed down.

• CSS Filter Effects:

  We spent a lot of time going through and resolving the remaining issues in the CSS Filter Effects specification. (This is basically SVG 1.1 filters repackaged for use by HTML with some extra syntax sugar coating.) We then agreed to publish the specification as a Candidate Recommendation.

• CSS Blending:

  We discussed publishing the CSS Blending specification as a Recommendation, the final step in creating a specification. I raised a point that most of the tests assumed HTML content. It was requested that more SVG specific test be created. (Part of the requirement for Recommendation status is that there be a test suite and that two independently developed renderers pass each test in the suite.)

• SVG in OpenType, Color Palettes:

  The new OpenType specification allows for multi-colored SVG glyphs. It would be nice to set those colors through CSS. We discussed several methods for doing so and decided on one method. It will be added to the CSS Fonts Level 4 specification.

• Text Rendering:

  The ‘text-rendering‘ property gives renderers a hint on what speed/precision trade-offs should be made. It was pointed out that the layout of text flowed into a box will change as one zooms in and out on a page in Firefox due to font-hinting, font-size rounding, etc. The Google docs people would like to prevent this. It was decided that the ‘geometricPrecision’ value should require that font-metrics and text-measurement be independent of device resolution and zoom level. (Note: this property is defined in SVG but both Firefox and Chrome support it on HTML content.)

• Text Properties:

  Text in SVG 2 relies heavily on CSS specifications that are in various states of readiness. I asked the CSS/SVG groups what is the policy for referencing these specs. In particular, SVG 2 needs to reference the CSS Shapes Level 2 specification in order to implement text wrapping inside of SVG shapes. The CSS WG agreed to publish CSS Shapes Level 2 as a Working Draft so we can reference it. We also discussed various technical issues in defining how text wraps around excluded areas and in flowing text into more than one shape.

SVG Day 1

Minutes

• CamelCase Names

  The SVG WG decided some time ago to avoid new CamelCase names like ‘LinearGradient’ which cause problems with integration in HTML (HTML is case insensitive and CamelCase SVG names must be added by hand to HTML parsers). We went through the list of new CamelCase names in SVG 2 and decided which ones could be changed, weighing arguments for consistency against the desire to not introduce new CamelCase names. It was decided that <meshGradient> should be changed to <mesh>. This was mostly motivated by the ability to use a mesh as a standalone entity (and not only as a paint server). Other changes include: <hatchPath> to <hatchpath>, <solidColor> to <solidcolor>, …

• Requiring <foreignObject> HTML to be rendered.

  There was a proposal to require any HTML content in a <foreignObject> element to be rendered. I pointed out that not all SVG renderers are HTML renderers (Inkscape as an example). It was decided to have separate conformance classes, one requiring HTML content to be rendered and one not.

• Requiring Style Sheets Support:

  It was decided to require style sheet support. We discussed what kind of style sheets to require. We decided to require basic style sheet support at the CSS 1 or CSS 2.1 level (that part of the discussion was not minuted).

• Open Issues:

  We spent considerable time going through the specification chapter by chapter looking at open issues that would block publishing the specification as a Candidate Recommendation. This was a long multi-day process.

SVG Day 2

Minutes

Note: Day 2 and Day 3 minutes are merged.

• Superpaths:

  Superpaths is the name for the ability to reuse path segment data. This is useful, for example, to define the boundary between two shapes just once, reusing the path segment for both shapes. SVG renderers might be able to exploit this information to provide better anti-aliasing between two shapes knowing they share a common border. The SVG WG endorses this proposal but it probably won’t be ready in time for SVG 2. Instead, it will be developed in a separate Path enhancement module.

• Line-Join: Miter Clipped:

  It was proposed on the SVG mailing list that there be a new behavior for the miter ‘line-join’ value in regards to the ‘miter-limit’ property. At the moment, if a miter produces a line cap that extends farther than the ‘miter-limit’ value then the miter type is changed to bevel. This causes abrupt jumps when the angle between the joined lines changes such that the miter length crosses over the ‘miter-limit’ value (see demo). A better solution is to clip the line join at the ‘miter-limit’. This is done by some rendering libraries including the one used on Windows. We decided to create a new value for ‘line-join’ with this behavior.

• Auto-Path Closing:

  The ‘z’ path command closes paths by drawing a line segment to the first point in the path. This is fine if the path is made up of straight lines but becomes problematic if the path is made up of curves. For example, it can cause rendering problems for markers as there will be an extra line segment between the start and end of the path. If the last point is exactly on top of the first point, one can remove this closing line segment but this isn’t always possible, especially if one is using the relative path commands with rounding errors. A more detailed discussion can be found here. We decided to allow a ‘z’ command to fill in missing point data using the first point in the path. For example in: d=”m 100,125 c 0,-75 100,-75 100,0 c 0,75 -100,75 z” the missing point of the second Bezier curve is filled in by the first point in the path.

• Text on a Shape:

  An Inkscape developer has been working on putting text on a shape by converting shapes to paths while storing the original shape in the <defs> section. It would be much easier if SVG just allowed text on a shape. I proposed that we include this in SVG 2. This is actually quite easy to specify as we have already defined how shapes are converted to paths (needed by markers on shapes and putting dash patterns on shapes). A couple minor points needed to be decided: Do we allow negative path offsets? (Yes) How do we decide which side of a path the text should be put? (A new attribute) The SVG WG approved adding text on a shape to SVG 2.

• Marker knockouts, mid-markers, etc:

  A number of new marker features still need some work. To facilitate finishing SVG 2 we decided to move them to a separate specification. There is some hesitation to do so as there is fear that once removed from the main SVG specification they will be forgotten about. This will be a trial of how well separating parts of SVG 2 into separates specifications works. The marker knockout feature, very useful for arrowheads is one feature moved into the new specification. On day 3 we approved publishing the new Markers Level 1 specification as a First Public Working Draft.

• Text properties:

  With our new reliance on CSS for text layout, just what CSS properties should SVG 2 support? We don’t want to necessarily list them all in the SVG 2 specification as the list could change as CSS adds new properties. We decided that we should support all paragraph level properties (‘text-indent’, ‘text-justification’, etc.). We’ll ask the CSS working group to create a definition for CSS paragraph properties that we can then reference.

• Text ‘dx’, ‘dy’, and ‘rotate’ attributes:

  SVG 1.1 has the properties ‘dx’, ‘dy’, and ‘rotate’ attributes that allow individual glyphs to be shifted and rotated. While not difficult to support on auto-wrapped text (they would be applied after CSS text layout), we decided that they weren’t really needed. They can still be used on SVG 1.1 style text (which is still part of SVG 2).

SVG Day 3

Minutes

Note: Day 3 minutes are at end of Day 2 minutes.

• Stroking Enhancements:

  As part of trying to push SVG 2 quickly, we decided to move some of the stroking enhancements that still need work into a separate specification. This includes better dashing algorithms (such as controlling dash position at intersections) and variable width strokes. We agreed to the publication of SVG Strokes as a First Public Working Draft.

• Smoothing in Mesh Gradients:

  Coons-Patch mesh gradients have one problem: the color profile at the boundary between patches is not always smooth. This leads to visible artifacts which are enhanced by Mach Banding. I’ve discussed this in more detail here. I proposed to the SVG WG that we include the option of auto-smoothing meshes using monotonic-bicubic interpolation. (There is an experimental implementation in Inkscape trunk which I demonstrated to the group.) The SVG WG accepted my proposal.

• Motion Path:

  SVG has the ability to animate a graphical object along a path. This ability is desired for HTML. The SVG and CSS working groups have produced a new specification, Motion Path Module Level 1, for this purpose. We agreed to publish the specification as a First Public Working Draft.