# TIP 507: Include simple SVG support with nanosvg
	Author:         René Zaumseil <r.zaumseil@freenet.de>
	State:          Final
	Type:           Project
	Vote:           Done
	Created:        9-May-2018
	Post-History:   
	Keywords:       Tk
	Tcl-Version:    8.7
	Tk-Branch:      tip-507
	Votes-For:      DKF, JN, FV, SL, AK
	Votes-Against:  none
	Votes-Present:  none
-----

# Abstract

Tk needs scalable images on high resolution mobile devices. This TIP proposes to let Tk be able to read an [SVG][] image (plus information about orientation and pixel scale) and make it into a photo image. It is therefore a (lossy and single direction) conversion operation from an [SVG][] format to a pixel format.

# Rationale

Tk is running on desktop and mobile devices. However, the out-of-the-box photo
image formats do not scale and are too tiny on high resolution mobile devices.
The same goes for the image formats provided by the [Img][] extension.

The response to this challenge in general (on the web, in applications, etc.)
has been to adopt the [SVG][] format, as that is scalable and does not
typically depend on being rendered at a particular resolution.
Moreover, there is already a Tk image extension to do the conversion,
[tksvg][]. The implementation is using the [nanosvg][] library. It has no other
external dependencies and is only 2 header files.

[nanosvg][] was choosen because it:

- has a suitable license
- is written in 2 plain C header files and can be easily included
- is really lightweight
- is tested with tksvg
- can render images
- has no other external dependencies

# Specification

The already existing [tksvg][] extension will be adapted and included in Tk.
A new photo image format will be created. The photo image format name is **svg**.
The new format will support a small list of options.
Note that scope still exists in the future for full integration of SVG as its
own full image type (which would allow more options for rendering) instead of
having the side-trip via the photo image's pixel buffer, but this is out of
scope of this TIP.
A description of the features and limitations of the current implementation is in section [Supported SVG](#Supported-SVG). It will be also described in the photo(n) man page.

The **svg** image format has the following format suboptions:

 > **svg** **-dpi** _dpiValue_ **-scale** _scaleValue_ **-unit** _unitValue_

*dpiValue* is used in conversion between given coordinates and screen resolution. The value must be greater then 0.0. The default value is `96`.

*scaleValue* is used to scale the resulting image. The value must be greater then 0.0. The default value is `1.0`.

*unitValue* is the unit of all coordinates in the svg data. Available units are `px` (default, coordinates in pixel), `pt` (1/72 inch), `pc` (12 pt), `mm`, `cm` and `in`.

The given format options are only used at creation time of the image and are not preserved in the image. This means that:

  1. `$img data -format svg` triggers the error _"image string format
     "svg" is not supported"_; Tk cannot convert a photo image into an
     [SVG][].

  2. In this:

        $img configure -format {svg -scale 2}
        $img configure -format {svg -dpi 96}

    the second call takes `-scale` as the default value (1).

There is no intention to provide a reverse mechanism for saving a photo image
as an SVG image.

<a name="Supported-SVG"></a>
# Supported SVG

The **svg** format supports a wide range of [SVG][] features, however some features (e.g. 'text') are missing and silently ignored when reading the [SVG][] data.

## Elements

- `g`
- `path`
- `rect`
- `circle`
- `ellipse`
- `line`
- `polyline`
- `polygon`
- `linearGradient`
- `radialGradient`
- `stop`
- `defs`
- `svg`
- `style`

## Attributes

- `width`, `height`
- `viewBox`
- `preserveAspectRatio` with `none`, `xMin`, `xMid`, `xMax`, `yMin`, `yMid`, `yMax`, `slice`

## Gradient Attributes

- `gradientUnits` with `objectBoundingBox`
- `gradientTransform`
- `cx`, `cy`, `r`
- `fx`, `fy`
- `x1`, `y1`, `x2`, `y2`
- `spreadMethod` with `pad`, `reflect` or `repeat`
- `xlink:href`

## Poly Attributes

- `points`

## Line Attributes

- `x1`, `y1`, `x2`, `y2`

## Ellipse Attributes

- `cx`, `cy`
- `rx`, `ry`

## Circle Attributes

- `cx`, `cy`
- `r`

## Rectangle Attributes

- `x`, `y`
- `width`, `height`
- `rx`, `ry`

## Path Attributes

- `d` with `m`, `M`, `l`, `L`, `h`, `H`, `v`, `V`, `c`, `C`, `s`, `S`, `q`, `Q`, `t`, `T`, `a`, `A`, `z`, `Z`

## Style Attributes

- `display` with `none`, `visibility`, `hidden`, `visible`
- `fill` with `nonzero`, `evenodd`
- `opacity`
- `fill-opacity`
- `stroke`
- `stroke-width`
- `stroke-dasharray`
- `stroke-dashoffset`
- `stroke-opacity`
- `stroke-linecap` with `butt`, `round`, `square`
- `stroke-linejoin` with `miter`, `round`, `bevel`
- `stroke-miterlimit`
- `fill-rule`
- `font-size`
- `transform` with `matrix`, `translate`, `scale`, `rotate`, `skewX`, `skewY`
- `stop-color`
- `stop-opacity`
- `offset`
- `id`
- `class`

# Discussion

In <http://code.activestate.com/lists/tcl-core/19994/> François Vogel mentioned his concerns.

- A.1 Alternatives are described here at the end.
- A.2 The interface is small (-format ...). Other, better implementations could provide it also.
- A.3 is now described in section "Supported SVG" above.
- A.4 The following, not necessary needed option will be removed:
  - **-x** *xValue* is used to move the created image in x-direction. The default value is 0.
  - **-y** *yValue* is used to move the created image in y-direction. The default value is 0.
- B.3 **svg**
- B.4 has the fix surfaced?
- C is about better documentation

Thread <http://code.activestate.com/lists/tcl-core/19871/> is about compilers.

Thread <http://code.activestate.com/lists/tcl-core/19985/> was the last discussion.

> <http://code.activestate.com/lists/tcl-core/20110/> and <http://code.activestate.com/lists/tcl-core/20111/> talk about a better solution, not using the **photo** comand and instead making a p.e. **svg** command with full functionality (especially writing). This is imho out of scope of this tip. It should be addressed in a separate tip. If done so the new function can then be used to support the functionality in this tip.

There was some discussion on the further usage and maintenance of this image type. Here are the current state of the implementation in this area:

- This tip will provide an **readonly** display of svg images with a **limited** set of funtionality described above.
- The created image will have no information about the [SVG][] data and format options.
- The included nanosvg header files will be remain diffable to the original ones. So changes in the original project can be ported to our implementation.
- The format options will be limited to a minimum. So the implementation can later be substituted with a full [SVG][] compatible one.
- If the implamenation will be replaced it will occur in a different tk version. So there is no need to create a scpecial variable describing the used [SVG][] implementation.
- To further reduce the interface it is possible to remove the **-dpi** and **-unit** options. There is currently no demand for this.

# Implementation

A patch implementing these changes is available in the fossil repository in the [tip-507 branch](https://core.tcl-lang.org/tk/timeline?r=tip-507).

The new format is described in the photo(n) man page.

# Example of use

    # the image data
    set data {<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
    <path fill="none" stroke="#000000" d="M0 0 h16 v16 h-16 z"/>
    <path fill="none" stroke="#000000" d="M8 4 v 8 M4 8 h 8"/>
    <circle fill="yellow" stroke="red" cx="10" cy="80" r="10" />
    <ellipse fill="none" stroke="blue" stroke-width="3" cx="60" cy="60" rx="10" ry="20" />
    <line x1="10" y1="90" x2="50" y2="99"/>
    <rect fill="none" stroke="green"  x="20" y="20" width="60" height="50" rx="3" ry="3"/>
    <polyline fill="red" stroke="purple" points="80,10 90,20 85,40"/>
    <polygon fill ="yellow" points="80,80 70,85 90,90"/>
    </svg>}
    # create image
    image create photo foo -data $data
    # change size
    foo configure -format {svg -scale 2}
    # change dpi value
    foo configure -format {svg -dpi 96}
    # use other unit
    foo configure -format {svg -unit mm}

# Alternatives

- Use of another library with full [SVG][] support instead of the current proposal for partial support: could be done as a further step.

- Direct manipulation of SVG vector graphics (as opposed to the present proposal of converting SVG to pixel format in a photo) would be desirable. However this can always be done later, for instance in the frame of the canvas widget. It is deemed an already large improvement to have Tk be able to read SVG images and convert them to pixel-based photos.

# Copyright

This document has been placed in the public domain.

[nanosvg]: <https://github.com/memononen/nanosvg>
[tksvg]: <https://github.com/auriocus/tksvg>
[SVG]: <http://www.w3.org/TR/SVG11/>
[Img]: <https://sourceforge.net/projects/tkimg/>
