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Preserve HiDPI image detail when painting ROIs and overlays #302

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9bba9c0
Preserve HiDPI detail in ROI and overlay double buffering
mikelxc Oct 1, 2026
fd942d4
Preserve fractional device pixel phase when presenting buffers
mikelxc Oct 1, 2026
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87 changes: 67 additions & 20 deletions ij/gui/ImageCanvas.java
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,7 @@ public class ImageCanvas extends Canvas implements MouseListener, MouseMotionLis
private Image offScreenImage;
private int offScreenWidth = 0;
private int offScreenHeight = 0;
private boolean offScreenDeviceBuffer;
private boolean mouseExited = true;
private boolean customRoi;
private boolean drawNames;
Expand Down Expand Up @@ -538,10 +539,36 @@ void drawZoomIndicator(Graphics g) {
void paintDoubleBuffered(Graphics g) {
final int srcRectWidthMag = (int)(srcRect.width*magnification+0.5);
final int srcRectHeightMag = (int)(srcRect.height*magnification+0.5);
if (offScreenImage==null || offScreenWidth!=srcRectWidthMag || offScreenHeight!=srcRectHeightMag) {
offScreenImage = createImage(srcRectWidthMag, srcRectHeightMag);
offScreenWidth = srcRectWidthMag;
offScreenHeight = srcRectHeightMag;
AffineTransform transform = ((Graphics2D)g).getTransform();
boolean deviceBuffer = transform.getScaleX()!=1.0 || transform.getScaleY()!=1.0
|| transform.getShearX()!=0.0 || transform.getShearY()!=0.0
|| transform.getTranslateX()!=Math.floor(transform.getTranslateX())
|| transform.getTranslateY()!=Math.floor(transform.getTranslateY());
Rectangle bufferBounds = deviceBuffer
? transform.createTransformedShape(new Rectangle(0, 0, srcRectWidthMag, srcRectHeightMag)).getBounds()
: new Rectangle(0, 0, srcRectWidthMag, srcRectHeightMag);
final int bufferWidth = bufferBounds.width;
final int bufferHeight = bufferBounds.height;
if (bufferWidth<=0 || bufferHeight<=0)
return;
AffineTransform imageTransform = null;
if (deviceBuffer) {
try {
imageTransform = transform.createInverse();
imageTransform.translate(bufferBounds.x, bufferBounds.y);
} catch (NoninvertibleTransformException e) { return; }
}
if (offScreenImage==null || offScreenWidth!=bufferWidth || offScreenHeight!=bufferHeight
|| offScreenDeviceBuffer!=deviceBuffer) {
if (offScreenImage!=null)
offScreenImage.flush();
// Explicit raster dimensions avoid depending on a platform Image's backing scale.
offScreenImage = !deviceBuffer
? createImage(bufferWidth, bufferHeight)
: new BufferedImage(bufferWidth, bufferHeight, BufferedImage.TYPE_INT_ARGB);
offScreenWidth = bufferWidth;
offScreenHeight = bufferHeight;
offScreenDeviceBuffer = deviceBuffer;
}
Roi roi = imp.getRoi();
try {
Expand All @@ -550,22 +577,42 @@ void paintDoubleBuffered(Graphics g) {
imp.updateImage();
}
Graphics offScreenGraphics = offScreenImage.getGraphics();
setInterpolation(offScreenGraphics, Prefs.interpolateScaledImages);
Image img = imp.getImage();
if (img!=null)
offScreenGraphics.drawImage(img, 0, 0, srcRectWidthMag, srcRectHeightMag,
srcRect.x, srcRect.y, srcRect.x+srcRect.width, srcRect.y+srcRect.height, null);
Overlay overlay = imp.getOverlay();
if (overlay!=null)
drawOverlay(overlay, offScreenGraphics);
if (showAllOverlay!=null)
drawOverlay(showAllOverlay, offScreenGraphics);
if (roi!=null)
drawRoi(roi, offScreenGraphics);
if (srcRect.width<imageWidth || srcRect.height<imageHeight)
drawZoomIndicator(offScreenGraphics);
//if (IJ.debugMode) showFrameRate(offScreenGraphics);
g.drawImage(offScreenImage, 0, 0, null);
try {
if (deviceBuffer) {
Graphics2D bufferGraphics = (Graphics2D)offScreenGraphics;
bufferGraphics.setComposite(AlphaComposite.Clear);
bufferGraphics.fillRect(0, 0, bufferWidth, bufferHeight);
bufferGraphics.setComposite(AlphaComposite.SrcOver);
AffineTransform bufferTransform = AffineTransform.getTranslateInstance(-bufferBounds.x, -bufferBounds.y);
bufferTransform.concatenate(transform);
bufferGraphics.setTransform(bufferTransform);
}
setInterpolation(offScreenGraphics, Prefs.interpolateScaledImages);
Image img = imp.getImage();
if (img!=null)
offScreenGraphics.drawImage(img, 0, 0, srcRectWidthMag, srcRectHeightMag,
srcRect.x, srcRect.y, srcRect.x+srcRect.width, srcRect.y+srcRect.height, null);
Overlay overlay = imp.getOverlay();
if (overlay!=null)
drawOverlay(overlay, offScreenGraphics);
if (showAllOverlay!=null)
drawOverlay(showAllOverlay, offScreenGraphics);
if (roi!=null)
drawRoi(roi, offScreenGraphics);
if (srcRect.width<imageWidth || srcRect.height<imageHeight)
drawZoomIndicator(offScreenGraphics);
//if (IJ.debugMode) showFrameRate(offScreenGraphics);
} finally {
offScreenGraphics.dispose();
}
if (deviceBuffer) {
// Present at an integer device-pixel origin without a second resampling.
Graphics2D displayGraphics = (Graphics2D)g.create();
try {
displayGraphics.drawImage(offScreenImage, imageTransform, null);
} finally { displayGraphics.dispose(); }
} else
g.drawImage(offScreenImage, 0, 0, null);
}
catch(OutOfMemoryError e) {IJ.outOfMemory("Paint");}
}
Expand Down
112 changes: 112 additions & 0 deletions tests/ij/gui/ImageCanvasHiDpiTest.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,112 @@
package ij.gui;

import ij.ImagePlus;
import ij.Prefs;
import ij.process.ColorProcessor;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.image.BufferedImage;
import org.junit.Test;
import static org.junit.Assert.*;

/** Deterministic rendering tests; no display peer or microscopy data required. */
public class ImageCanvasHiDpiTest {
private static class Canvas extends ImageCanvas {
Canvas(ImagePlus imp) { super(imp); }
public Image createImage(int width, int height) {
return new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
}
}

private static class SyntheticImage extends ImagePlus {
ImageCanvas canvas;
SyntheticImage(ColorProcessor processor) { super("synthetic checkerboard", processor); }
public ImageCanvas getCanvas() { return canvas; }
}

private SyntheticImage image() { return image(128); }

private SyntheticImage image(int size) {
int[] pixels = new int[size*size];
for (int y=0; y<size; y++)
for (int x=0; x<size; x++)
pixels[y*size+x] = ((x+y)%2==0) ? 0xffffff : 0;
return new SyntheticImage(new ColorProcessor(size,size,pixels));
}

private boolean translated;

private BufferedImage render(Canvas canvas, double sx, double sy) {
BufferedImage result = new BufferedImage(512,512,BufferedImage.TYPE_INT_RGB);
Graphics2D g = result.createGraphics();
if (translated) g.translate(0.25,0.5);
g.scale(sx,sy);
canvas.paint(g);
g.dispose();
return result;
}

private void sameInterior(BufferedImage expected, BufferedImage actual, double sx, double sy, double zoom) {
// Keep comparison away from selection outlines and the zoom indicator.
for (int y=(int)(40*zoom*sy); y<(int)(100*zoom*sy); y++)
for (int x=(int)(40*zoom*sx); x<(int)(100*zoom*sx); x++)
assertEquals("pixel at "+x+","+y,expected.getRGB(x,y),actual.getRGB(x,y));
}

@Test public void roiOverlayAndShowAllPreserveDetail() { checkRendering(false, false); }
@Test public void croppedSourcePreservesDetail() { checkRendering(true, false); }
@Test public void oddCroppedSourcePreservesDetail() { checkRendering(true, false, 129); }
@Test public void interpolatedSourcePreservesDetail() { checkRendering(false, true); }

@Test public void fractionalScaleWithOddLogicalSizePreservesDetail() { checkRendering(false, false, 130); }
@Test public void translatedDeviceTransformPreservesDetail() {
translated = true;
try { checkRendering(false, false, 130); } finally { translated = false; }
}
@Test public void odd129SourcePreservesDetail() { checkRendering(false, false, 129); }
@Test public void oddSourceSizePreservesDetail() { checkRendering(false, false, 131); }

private void checkRendering(boolean cropped, boolean interpolateImages) { checkRendering(cropped, interpolateImages, 128); }
private void checkRendering(boolean cropped, boolean interpolateImages, int size) {
boolean buffered = Prefs.paintDoubleBuffered;
boolean interpolate = Prefs.interpolateScaledImages;
try {
Prefs.paintDoubleBuffered = false;
Prefs.interpolateScaledImages = interpolateImages;
SyntheticImage imp = image(size);
int[] original = ((int[])imp.getProcessor().getPixels()).clone();
Canvas canvas = new Canvas(imp);
imp.canvas = canvas;
if (cropped) {
canvas.setSourceRect(new java.awt.Rectangle(16,16,size-32,size-32));
canvas.hideZoomIndicator(true);
}
for (double zoom : new double[]{0.5,1.0,2.0}) {
canvas.setMagnification(zoom);
for (double[] scale : new double[][]{{1,1},{2,2},{1.25,1.5},{1.5,1.5}}) {
BufferedImage direct = render(canvas,scale[0],scale[1]);
for (Roi roi : new Roi[]{new Roi(2,2,10,10),new OvalRoi(2,2,10,10),new Line(2,2,12,12),new PointRoi(4,4),new TextRoi(2,2,"ROI"),new PolygonRoi(new int[]{2,12,2},new int[]{2,2,12},3,Roi.POLYGON),new PolygonRoi(new int[]{2,12,8,2},new int[]{2,2,12,8},4,Roi.FREEROI)}) {
imp.setRoi(roi);
sameInterior(direct,render(canvas,scale[0],scale[1]),scale[0],scale[1],zoom);
imp.deleteRoi();
sameInterior(direct,render(canvas,scale[0],scale[1]),scale[0],scale[1],zoom);
}
Overlay overlay = new Overlay(new Roi(2,2,10,10));
imp.setOverlay(overlay);
sameInterior(direct,render(canvas,scale[0],scale[1]),scale[0],scale[1],zoom);
imp.setOverlay(null);
canvas.setShowAllList(overlay);
sameInterior(direct,render(canvas,scale[0],scale[1]),scale[0],scale[1],zoom);
canvas.setShowAllList(null);
Prefs.paintDoubleBuffered = true;
sameInterior(direct,render(canvas,scale[0],scale[1]),scale[0],scale[1],zoom);
Prefs.paintDoubleBuffered = false;
}
}
assertArrayEquals(original,(int[])imp.getProcessor().getPixels());
} finally {
Prefs.paintDoubleBuffered = buffered;
Prefs.interpolateScaledImages = interpolate;
}
}
}