CODEWITHSUNDEEP
javaimagej
sumisu
sumisu

Reputation: 25

How to flip an image horizontally with an ImageJ-Plugin?

I want to develop a Java-Plugin for ImageJ that flips an image horizontally. But my code flips only half of the picture. Maybe, there is something wrong with the construction or the output of the image copy? 

public class flipHorizontal implements PlugInFilter {

public int setup (String arg, ImagePlus imp)
{
    return DOES_ALL;
}

public void run (ImageProcessor ip)
{
    int height=ip.getHeight();
    int width=ip.getWidth();

    ImageProcessor copy = ip;


    for (int x=0; x<width; x++) {

        for (int y=0; y<height; y++) {
            int p=ip.getPixel(width-x-1,y);
            copy.putPixel(x,y,p);
        }
    }
  }
}

Upvotes: 2

Views: 5292

Answers (2)

Mark Longair
Mark Longair

Reputation: 467311

TacticalCoder is correct that you should only be iterating to half way across the image, and you need to save the value from the other side before overwriting it.

There are two additional points that might be worth making, however - one is the the ImageProcessor class already has a method called flipHorizontal, so you can simplify your code to:

public class flipHorizontal implements PlugInFilter {

    public int setup (String arg, ImagePlus imp) {
        return DOES_ALL;
    }

    public void run (ImageProcessor ip) {
        ip.flipHorizontal();
    }
}

The other point that would be worth making is that it seems that you misunderstand what this line means:

ImageProcessor copy = ip;

That's just creating another reference to the same object as ip, so:

copy.putPixel(x,y,p);

... and:

ip.putPixel(x,y,p);

... have exactly the same effect. If you want to create a new ImageProcessor representing the same pixel data, you could do:

ImageProcessor copy = ip.duplicate();

However, that's not necessary in this case.

Upvotes: 0

TacticalCoder
TacticalCoder

Reputation: 6325

Your logic is wrong. What you get is normal: you're not processing half of your image but flipping horizontally once half of your image and twice the other half (if I'm not mistaken).

Anyway, if you want to flip horizontally by manipulating pixels yourself directly, as in your code sample, then instead of going to width, you need to go to half the width (width/2).

You then need to actually invert the two pixels from "left" and "right"

Here's an horizontal flip that works:

    for (int x = 0; x < w / 2; x++) {
        for (int y = 0; y < h; y++) {
            final int l = tmp.getRGB( w - (x + 1), y);
            final int r = tmp.getRGB( x, y);
            tmp.setRGB( x, y, l );
            tmp.setRGB( w - (x + 1), y, r );
        }
    }

There may be "off-by-one" errors in the code above but you should get the idea.

Upvotes: 0

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