[JAVA2D] Graphics and Grayscale Images - Maximum luninance is 252 ?

Java Questions Wed, 20 Jul 2005 04:35:46 -0700

Graphics and Grayscale Images - Maximum luninance is 252 ?

Hi everybody,

I am working with 8 bit Grayscale images and am in the process of implementing a rendering interface for an imaging program I work on.

The interface of the "renderer" is quite simple:

import java.awt.image.BufferedImage;

/**
* A renderer is responsible for successfully rendering images to a
* given destination.
*
*/
public interface IImageRenderer {

 /**
 * Render an image.
 *
 * @param sourceImage
 * the image to render
 * 
 * If a <code>null</code> image is passed, no rendering will be
 * performed.
 * 
 * @param horizontalOffset
 * the horizontal offset to apply
 * @param verticalOffset
 * the vertical offset to apply
 */
 public void renderFrame(BufferedImage sourceImage, int horizontalOffset,
 int verticalOffset);
}

I came up with the following implementation:

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;

import javax.swing.SwingUtilities;

import IImageRenderer;

/**
* Render images to a given <code>Graphics2D</code>.
* 
* The areas of the graphics not covered by the image will be filled with a
* custom background color.
* 
*
* @see GraphicsImageRendererTest
*/
public final class GraphicsImageRenderer implements IImageRenderer, Runnable {

 /**
 * Build a renderer for a specified <code>Graphics</code>.
 *
 * @param targetGraphics
 * the graphics where the images are to be rendered
 * @param backgroundColor
 * the background color to use
 * 
 * If <code>null</code>, <code>Color.BLACK</code> will be
 * used.
 * 
 */
 public GraphicsImageRenderer(Graphics2D targetGraphics,
 Color backgroundColor) {
 if (backgroundColor != null) {
 this.currentBackgroundColor = backgroundColor;
 } else {
 this.currentBackgroundColor = Color.BLACK;
 }
 if (targetGraphics != null) {
 this.destinationGraphics = targetGraphics;
 this.destinationGraphics.setBackground(backgroundColor);
 } else {
 this.destinationGraphics = null;
 }
 }

 /*
 * (non-Javadoc)
 *
 * @see IImageRenderer#renderFrame(java.awt.image.BufferedImage, int, int)
 */
 public final void renderFrame(BufferedImage sourceImage,
 int horizontalOffset, int verticalOffset) {
 this.currentSourceImage = sourceImage;
 this.currentHorizontalOffset = horizontalOffset;
 this.currentVerticalOffset = verticalOffset;
 if (!SwingUtilities.isEventDispatchThread()) {
 try {
 SwingUtilities.invokeAndWait(this);
 } catch (Exception exception) {
 // Oops..
 }
 } else {
 run();
 }
 }

 /*
 * (non-Javadoc)
 *
 * @see java.lang.Runnable#run()
 */
 public final void run() {
 if (SwingUtilities.isEventDispatchThread()
 && this.destinationGraphics != null
 && this.currentSourceImage != null) {
 this.destinationGraphics.clearRect(0, 0, Integer.MAX_VALUE,
 Integer.MAX_VALUE);
 this.destinationGraphics.drawImage(this.currentSourceImage,
 this.currentHorizontalOffset, this.currentVerticalOffset,
 this.currentBackgroundColor, null);
 }
 }

 private BufferedImage currentSourceImage = null;

 private int currentHorizontalOffset = 0;

 private int currentVerticalOffset = 0;

 private final Graphics2D destinationGraphics;

 private final Color currentBackgroundColor;
}

My first questions are:

1/ Is my handling of the call to drawImage() appropriate or are the "Swing related" precautions irrelevant (i.e. I should not care about the EDT at this point) ?

2/ How can I retrieve the dimensions of my Graphics2D in order to avoid having to call clearRect() on the largest possible values (no laughing please ;-) ), should I pass them to my constructor (since I will probably have access to the dimensions when retrieving/creating the graphics) ?

Now for the weird part (at least from my point of view).

I have writtent the following JUnit TestCase to see how the background was handled:

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.color.ColorSpace;
import java.awt.image.BufferedImage;

import junit.framework.TestCase;
import GraphicsImageRenderer;

public class GraphicsImageRendererTest extends TestCase {

 public static void main(String[] args) {
 junit.swingui.TestRunner
 .main(new String[] { "GraphicsImageRendererTest" });
 }

 public GraphicsImageRendererTest(String name) {
 super(name);
 }

 public void testBackgroundColor_colorImagesCustomColor() {
 try {
 BufferedImage testDestinationImage = new BufferedImage(10, 10,
 BufferedImage.TYPE_3BYTE_BGR);
 Graphics2D testDestinationGraphics = testDestinationImage
 .createGraphics();

 BufferedImage testSourceImage = new BufferedImage(1, 1,
 BufferedImage.TYPE_3BYTE_BGR);

 Color customColor = new Color(10, 20, 30);
 GraphicsImageRenderer testCustomColorRenderer = new GraphicsImageRenderer(
 testDestinationGraphics, customColor);

 testCustomColorRenderer.renderFrame(testSourceImage, 0, 0);
 assertEquals(customColor.getRed(), testDestinationImage.getData()
 .getSample(9, 9, 0));
 assertEquals(customColor.getGreen(), testDestinationImage.getData()
 .getSample(9, 9, 1));
 assertEquals(customColor.getBlue(), testDestinationImage.getData()
 .getSample(9, 9, 2));
 } catch (Exception exception) {
 fail("Exception: " + exception);
 }
 }

 public void testBackgroundColor_grayScaleImagesMinimumLuminance() {
 try {
 BufferedImage testDestinationImage = new BufferedImage(10, 10,
 BufferedImage.TYPE_BYTE_GRAY);
 Graphics2D testDestinationGraphics = testDestinationImage
 .createGraphics();

 BufferedImage testSourceImage = new BufferedImage(1, 1,
 BufferedImage.TYPE_BYTE_GRAY);

 ColorSpace grayColorSpace = ColorSpace
 .getInstance(ColorSpace.CS_GRAY);

 Color black = new Color(grayColorSpace,
 new float[] { grayColorSpace.getMinValue(0) }, 1f);
 GraphicsImageRenderer testCustomBlackRenderer = new GraphicsImageRenderer(
 testDestinationGraphics, black);

 testCustomBlackRenderer.renderFrame(testSourceImage, 0, 0);
 assertEquals(0, testDestinationImage.getData().getSample(9, 9, 0));
 } catch (Exception exception) {
 fail("Exception: " + exception);
 }
 }

 public void testBackgroundColor_grayScaleImagesMaximumLuminance() {
 try {
 BufferedImage testDestinationImage = new BufferedImage(10, 10,
 BufferedImage.TYPE_BYTE_GRAY);
 Graphics2D testDestinationGraphics = testDestinationImage
 .createGraphics();

 BufferedImage testSourceImage = new BufferedImage(1, 1,
 BufferedImage.TYPE_BYTE_GRAY);

 ColorSpace grayColorSpace = ColorSpace
 .getInstance(ColorSpace.CS_GRAY);

 Color white = new Color(grayColorSpace,
 new float[] { grayColorSpace.getMaxValue(0) }, 1f);
 GraphicsImageRenderer testCustomWhiteRenderer = new GraphicsImageRenderer(
 testDestinationGraphics, white);

 testCustomWhiteRenderer.renderFrame(testSourceImage, 0, 0);
 assertEquals(255, testDestinationImage.getData().getSample(9, 9, 0));
 } catch (Exception exception) {
 fail("Exception: " + exception);
 }
 }

}

The first two tests pass smoothly but for the last one I get a failure on the "white color" test:
junit.framework.AssertionFailedError: expected:<255> but was:<252>

Why are the luminance values only covering the [0..252] range ?

Any extra comment on what I am doing wrong (Java2D, Code conventions, OO, design...) is more that welcome.

Yours,

D.

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[JAVA2D] Graphics and Grayscale Images - Maximum luninance is 252 ?

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