Check-in [75f5c2c704]

# 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 coordiantes 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

# 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/>