[l0ckz0r]

Hey there, I am coming back into Java having not programed with it since I left uni about a year ago.
I currently am trying to write a program that creates an RGB histogram from a bufferedimage. However I haven't been able to find a lot of useful guidance on the web.

Does anyone know of any tutorials that are helpful? Many of the websites that explain it are very brief or incomplete .

Maybe someone could help me with the logic of it.

[macosxnerd101]

Essentially, the BufferedImage is just there so you can getGraphics() from it. You use the Graphics object to draw your histogram. Remember that the origin in Java is the top-left corner. Check out the Graphics documentation.

From there, you can create an ImageIcon from the BufferedImage, and pass that to a JLabel to display.

[l0ckz0r]

I've got my buffered image and up to creating the histogram. I have 2 things I need a bit of help with.

1. I'm using some code from a tutorial as my base however there were a few things that I didn't understand:

int height = image.getHeight();
        int width = image.getWidth();
        
        Raster raster = image.getRaster();
        for(int i=0; i < width ; i++)
        {
            for(int j=0; j < height ; j++)
            {
                //if(gray)
                //{
                //    bins[0][ raster.getSample(i,j, 0) ] ++;
                //}
                //else
                //{
                    bins[0][ raster.getSample(i,j, 0) ] ++;
                    bins[1][ raster.getSample(i,j, 1) ] ++;
                    bins[2][ raster.getSample(i,j, 2) ] ++;
                //}
            }
        }

I've commented out the if(gray) -- the tutorial was a summary and there was no mention of gray, why would I need that there? Shouldn't I just be able to fill the bins without it?
Also when I run this code I get a nullpointer exception, I've initialised the bins variable as int[][] bins = null; Am I doing this wrong?

2. Once I've filled by bins correctly, how do I plot the histogram, all the tutorials i've seen use JAI. I would prefer to use use a JPanel, is that possible?
How does the plot work, where can I find some info on that?

This post has been edited by l0ckz0r: 11 September 2011 - 03:24 AM

[l0ckz0r]

Just to explain more simply

1. I need to know, for an RGB histogram, do I need to have an if(gray) statement, what is it's purpose?

2. How do I draw the histogram in Java 2d once I have it in the bins[][] I can't find anything useful online

[macosxnerd101]

Why would you need an if statement to test if the Color is grey? Have you checked out the Graphics class yet? The fillRect() method seems appropriate.

Quote

Once I've filled by bins correctly, how do I plot the histogram, all the tutorials i've seen use JAI. I would prefer to use use a JPanel, is that possible?
How does the plot work, where can I find some info on that?

It looks like the JAI has a Histogram class. I've never worked with the JAI API before. If you wanted to do it with Swing, you can draw to a BufferedImage and create an ImageIcon from that to be displayed on a JLabel. Or you can extend JPanel, override the paint() (or paintComponent()) method and use the Graphics param for the method you override to draw the Image to the JPanel.

[l0ckz0r]

Sorry what I need help is, I don't know how the drawing process of a histogram actually works

So I have my bins

[0][44000][0]...
[23][233][434]...
[232][123][23]...

I can't draw a line thats 44000 pixels long, so whats the process I need to go through?

[macosxnerd101]

The keyword here is scale. If you have a bin with 4400, you need to determine what one pixel represents. Is one pixel = 20 in your bin? The API is very self-explanatory. I think you need to think it through logically now.

[l0ckz0r]

Thanks for all your help - I have got it working. One final question now:

The first image here is my histogram - as you can see it's quite spiky and there's a few gaps


However the photoshop histogram for the same image is much more rounded out and there are no gaps - it's the same image:


Is it okay that my graphs are spiky and have gaps or do I need to use some algorithms to change it? Let me know if you need to see some of my code.

[pbl]

Try to make the JFrame wider to see if the problem persists
Seems that, due to some rounding, you skip a pixel X sometimes

[l0ckz0r]

The Panels themselves are a little wider than the drawing, and they stay the same size no matter how big the Jframe is.

I actually checked the input and for say the colour red, there is no value of 3 - hence the gap. Is this normal for images? Or have I messed something up?

[pbl]

Obviously rounding error
If an X is absent, use the preceeding Y or the following Y or even better the mean between the two

[l0ckz0r]

When I actually get the value from the raster getsample() function - there is no value of 3 for the red channel, hence the gap. So RedBin[3] never gets incremented. Is that still a rounding error? Do I still need to do the mean trick?

[cmpshr]

if (RedBin[3] == 0) RedBin[3] = (RedBin[2] + RedBin[4]) / 2;
Is that the idea ?

[l0ckz0r]

That's the idea, but if there is no pixel with that value should I really be drawing it in the graph? I don't know how these things work.

[pbl]

If there is really no values chances that there will be no value at the left or the right or both so (0 + 0) / 2 = 0 or (5 + 0) / 2 = 2
You won't be far