How to convert java swing panel to quality image

Question

my panel looks like:

when I convert to image

public BufferedImage createImage(JPanel panel) {

int w = (int) PageSize.A4.getWidth();//panel.getWidth();
int h = (int) PageSize.A4.getHeight();//panel.getHeight();

BufferedImage originalImage = new BufferedImage(panel.getHeight(), panel.getWidth(), BufferedImage.TYPE_BYTE_INDEXED);
Graphics2D gg = originalImage.createGraphics();
gg.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
        RenderingHints.VALUE_INTERPOLATION_BILINEAR);
gg.setRenderingHint(RenderingHints.KEY_RENDERING,
        RenderingHints.VALUE_RENDER_QUALITY);
gg.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
        RenderingHints.VALUE_ANTIALIAS_ON);
panel.paint(gg);

BufferedImage resizedImage = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
Graphics2D g = resizedImage.createGraphics();
g.drawImage(originalImage, 0, 0, w, h, null);
g.dispose();
g.setComposite(AlphaComposite.Src);

g.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
        RenderingHints.VALUE_INTERPOLATION_BILINEAR);
g.setRenderingHint(RenderingHints.KEY_RENDERING,
        RenderingHints.VALUE_RENDER_QUALITY);
g.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
        RenderingHints.VALUE_ANTIALIAS_ON);

return resizedImage;
}

it looks terrible - larger and worse quality

how I can change quaity of this image to the quality of panel ?

EDIT:

library which you need to compile this code:

import com.itextpdf.text.PageSize;
import java.awt.AlphaComposite;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import javax.swing.JPanel;

Answer 1

To literally answer your question:

how I can change quality of this image to the quality of panel ?

Simple, do not alter the size of your image. Drop the whole 'stretching your image to A4 size' as this is the cause of the quality loss.

public BufferedImage createImage(JPanel panel) {
  BufferedImage originalImage = new BufferedImage(
        panel.getHeight(), panel.getWidth(), 
        BufferedImage.TYPE_BYTE_INDEXED);
  Graphics2D gg = originalImage.createGraphics();
  gg.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
        RenderingHints.VALUE_INTERPOLATION_BILINEAR);
  gg.setRenderingHint(RenderingHints.KEY_RENDERING,
        RenderingHints.VALUE_RENDER_QUALITY);
  gg.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
        RenderingHints.VALUE_ANTIALIAS_ON);
  panel.paint(gg);
  gg.dispose();
  return originalImage;
}

Not sure however if this is what you have been looking for. If you really want the image to be on A4 size, I suggest trying to get your panel sharply rendered on A4 size before converting it to an image. Stretching a small image to a larger version will always result in quality loss.

Answer 2

you can use the Graphics2D transform so the panel's paint immediately goes to the scaled image

BufferedImage resizedImage = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
Graphics2D g = resizedImage.createGraphics();
g.transform(AffineTransform.getScaleInstance((float)panel.getWidth()/w,
          (float)panel.getHeight()/h));//this might need to be inverted I'm not sure...
panel.paint(g);
g.dispose();

btw the setRenderingHint won't do anything useful after you disposed the Graphics