Adds gradient export to SVG library

[tracerstar]

Adding an extension handler to the SVG library to allow linear and radial gradients to export correctly as SVG (currently they export as solid black fills).

The extension handler is a copy of https://gist.github.com/msteiger/4509119 by Martin Steiger with minor changes for Processing.

Several classes and methods in PShapeSVG and PShapeJava2D have been made public or modified to support the export of gradients.

Current SVG export of gradients results in fill:none on the shapes, leaving everything filled black. This adds gradients into the export handler, allowing fill:url(#gradientID) to work as expected.

I originally made this pull request in August 2023 against the old processing4 repo, but I saw that git activity had recently started up again so wanted to make a new PR.

[Stefterv]

Hi @tracerstar, thank you for this PR! Could you provide an example on how this new functionality would be used?

I'd like to test the functionality you wrote, other than that it looks good!

[tracerstar]

Sure thing. A simple sketch like below will demonstrate the export:

import processing.svg.*;

PShape myShape;
boolean canRecord;

void setup() {
	size(1000, 1000);
	background(255);
	canRecord = false;
	myShape = loadShape("test.svg");
}

void draw() {

	if (canRecord) {
		beginRecord(SVG, "frame-####.svg");
	}

	background(255);
	shape(myShape, 0, 0);

	if (canRecord) {
		endRecord();
		canRecord = false;
	}

}

void keyPressed() {
	if (key == ' ') {
		canRecord = true;
	}
}

The SVG file is attached here:

This is primarily to fix gradient exports in the SVG library. If you test the attached file currently, everything will be filled in black. The code I added will allow the exported SVGs to have gradients in the export.

I need to do some additional testing to see if I can add gradient fills to PShape objects dynamically and have them export too, but for SVGs that are imported with gradients and then exported, this code works.

[Stefterv]

Great, tested on macOS arm64 and works as expected!

[micycle1]

BIG

[SableRaf]

@all-contributors please add @tracerstar for code

[allcontributors]

@SableRaf

I've put up a pull request to add @tracerstar! 🎉