Gradient.java

/*
  Gradient.java

  (c) 2011-2015 Edward Swartz

  All rights reserved. This program and the accompanying materials
  are made available under the terms of the Eclipse Public License v1.0
  which accompanies this distribution, and is available at
  http://www.eclipse.org/legal/epl-v10.html
 */
package v9t9.gui.client.swt.bars;

import org.eclipse.swt.graphics.Device;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.graphics.ImageData;
import org.eclipse.swt.graphics.PaletteData;
import org.eclipse.swt.graphics.RGB;

/**
 * Generator for linear gradient drawing
 * @author ejs
 *
 */
public class Gradient {

	private ImageData gradientLineData;
	private Image gradientLine;
	private final int[] colors;
	private final float[] weights;
	private final boolean horizontal;
	
	/**
	 * 
	 * @param horizontal true: colors change left-right; false: colors change top-bottom
	 * @param colors
	 * @param weights
	 */
	public Gradient(boolean horizontal, int[] colors, float[] weights) {
		if (colors == null || weights == null || 
				colors.length < 2 || colors.length != weights.length + 1)
			throw new IllegalArgumentException();
		
		this.horizontal = horizontal;
		this.colors = colors;
		this.weights = weights;
	}
	
	public void draw(GC gc, int x, int y, int width, int height) {
		ensure(gc.getDevice(), horizontal ? width : height);
		
		gc.drawImage(gradientLine, 
				0, 0, 
				horizontal ? gradientLineData.width : 1, 
				horizontal ? 1 : gradientLineData.height, 
				x, y, width, height);		
	}

	/**
	 * @param device 
	 * @param sz
	 */
	private void ensure(Device device, int sz) {
		if (gradientLineData == null || gradientLineData.width < sz) {
			PaletteData palette = new PaletteData (0xFF0000, 0xFF00, 0xFF);
			gradientLineData = new ImageData(
					horizontal ? sz : 1, 
					horizontal ? 1 : sz,
					24, palette);
			
			int xd = horizontal ? 1 : 0;
			int yd = horizontal ? 0 : 1;
			
			int x = 0;
			int y = 0;
			
			int idx = -1;
			
			// go through each consecutive pair of colors,
			// each of which contributes sz * weight pixels
			RGB prev = null, limit = null;
			int from = 0, next = 0;
			int range = 0;
			
			for (int i = 0; i < sz; i++) {
				if (idx < 0) {
					limit = device.getSystemColor(colors[++idx]).getRGB();
				}
				if (i >= next) {
					prev = limit;
					range = Math.round(weights[idx] * sz);
					if (range + i > sz)
						range = sz - i;
					limit = device.getSystemColor(colors[++idx]).getRGB();
					
					from = i;
					next = from + range;
				}
				
				RGB cur = new RGB(
						(int) (prev.red + (limit.red - prev.red) * (i - from) / (float) range),
						(int) (prev.green + (limit.green - prev.green) * (i - from)  / (float) range),
						(int) (prev.blue + (limit.blue - prev.blue) * (i - from)  / (float) range)
						);
				
				gradientLineData.setPixel(x, y, palette.getPixel(cur));
				x += xd;
				y += yd;
			}
			
			if (gradientLine != null) {
				gradientLine.dispose();
				gradientLine = null;
			}

			gradientLine = new Image(device, gradientLineData);
		}
	}
}