Check-in [d0f56ff80a]

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# TIP 507: Include simple svg support with nanosvg
	Author:         René Zaumseil <[email protected]>
	State:          Draft
	Type:           Project
	Vote:           
	Created:        9-May-2018
	Post-History:   
	Keywords:       Tk
	Tcl-Version:    8.7
-----

# Abstract

Tk needs scalable images on high res 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. 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. The out of the box image formats
do not scale and are to tiny on high res mobile devices.
The same goes for the image formats provided by the img extension.

There is already a Tk image extension at https://github.com/auriocus/tksvg
The implementation is using nanosvg. 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 easily included

**svgnano** *-dpi dpiValue -scale scaleValue -unit unitValue -x xValue -y yValue*

*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.

*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.

*xValue* is used to move the created image in x-direction. The default value is 0.

*yValue* is used to move the created image in y-direction. The default value is 0.

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 svgnano** triggers error **"image string format "svgnano" is not supported"**
  
  2. **$img configure -format {svgnano -scale 2} ; $img configure -format {svgnano -x 10}    ;** # the second call takes -scale as the default value (1)

# Supported SVG

The svgnano 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

## Poly Attributes

- points

## Line Attributes

- x1, y1, x2, y2

## Ellipse Attributes

- cx, cy
- rx, ry

## Circle Attributes

# Discussion

- http://code.activestate.com/lists/tcl-core/19871/
- http://code.activestate.com/lists/tcl-core/19994/

## Open questions

- Are all of the above format options necessary? May be remove the *xValue* and *yValue* options.

# Implementation

A patch implementing these changes is available in the fossil repository in the [tip-507 branch]
(https://core.tcl.tk/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"/>

	    # move right and up, all other values are reset
	    foo configure -format {svgnano -x 10 -y -20}
	    # use other unit
	    foo configure -format {svgnano -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.