Nested for loop storing to an array only giving one column of nested loop into array

Question

So, what I'm trying to do is run through a picture that is 1024x768 (Or, popMap.getWidth x popMap.getHeight), grab the blue color of it, compare it to the highest blue so far, and if it's more, that blue becomes the new 'newBlue'. Basically, find the highest blue value in an image, closest to 255 blue.

Also, I'm attempting to store the blue value of the entire thing into an array popMapArray, which is a 2d array with 3 columns, storing [blueValue][x][y]. Which I will then sort, to get a list from high to low of the bluest values.

My problem, is that with the code below, it's only storing into the array when column=767.

I get 1024 [blue,row,767], and then the rest are all [0,0,0]

Any clue why? Java, by the way.

for (int row = 0;  row < popMap.getWidth(); row++) 
    {
        for (int column = 0; column < popMap.getHeight(); column++)
        {
            System.out.println(column);
            //Find a Pixel
            int c = popMap.getRGB(row, column);
            int red = (c & 0x00ff0000) >> 16;
            //int  green = (c & 0x0000ff00) >> 8;
            //int  blue = c & 0x000000ff;
            // and the Java Color is ...
            Color color = new Color(red); 
            int newBlue = color.getBlue();
            int oldBlue = lastColor.getBlue();
            switch(popArrayRow)
            {
                case 0:
                {
                    arrayVar = newBlue;
                    popArrayRow = 1;
                    break;
                }
                case 1:
                {
                    arrayVar = row;
                    popArrayRow = 2;
                    break;
                }
                case 2:
                {
                    arrayVar = column;
                    popArrayRow = 0;
                    break;
                }
            }
            popArray[row][popArrayColumn] = arrayVar;
            //System.out.println(popArray[row][popArrayColumn]);
            switch(popArrayColumn)
            {
                case 0:
                {
                    popArrayColumn = 1;
                    break;
                }
                case 1:
                {
                    popArrayColumn = 2;
                    break;
                }
                case 2:
                {
                    popArrayColumn = 0;
                    break;
                }
            }


            if(newBlue > oldBlue)
            {
                startX = row;
                startY = column;
                //System.out.print(row);
                //System.out.print(",");
                //System.out.println(column);
                System.out.print("The oldBlue is ");
                System.out.println(oldBlue);
                lastColor = color;
            }



        }
    }

what y? I don't catch your problem

lukastymo
– lukastymo

2011-04-12 21:07:44 +00:00
Commented Apr 12, 2011 at 21:07 — lukastymo
– lukastymo, Commented Apr 12, 2011 at 21:07

jsegal · Accepted Answer · 2011-04-12 21:14:37Z

1

int red = (c & 0x00ff0000) >> 16;
        //int  green = (c & 0x0000ff00) >> 8;
        //int  blue = c & 0x000000ff;
        // and the Java Color is ...
        Color color = new Color(red); 
       int newBlue = color.getBlue();

Do you mean "Color color = new Color(c)"? Your newBlue value will always be 0...

Also, what precisely are you trying to do with the popArray construction? Having your state variables adjust themselves once per pixel is probably not going to accomplish what you want... It sounds like you want a SortedMap<int,Point>, keyed on the blueValue, whose value is the x,y coordinate of the points (stored either as an array or a Point object). Then you'd have your data structure, ordered by blue value, and you can read your points off directly.

Best of luck!

answered Apr 12, 2011 at 21:14

jsegal

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2 Comments

Nick Eichenberg Over a year ago

Well right now it's grayscale so blue or red doesn't really matter, it comes out the same. It's going to stay gray as well, I just arbitrarily chose blue because why not. I'll look into SortedMap, I knew there had to be something that would do what I was trying to do.

Nick Eichenberg Over a year ago

This seems to be the best way to do it, only a few lines to do a majority of what I wanted. Just have to figure out how to access the SortedMap as I've never used one before.

Dave Costa · Accepted Answer · 2011-04-12 21:30:39Z

0

You didn't show the declaration go popArray (as well as some other variables which I'm assuming are ints that are initialized to 0). You describe it as "a 2d array with 3 columns". I'm guessing you've declared it as int[1024][3], so it has one row per row in popMap, then your 3 "columns" which are meant to store the blue value, the original x coordinate, and the original y coordinate.

So first of all it is unclear how you expect to store one entry in this array for each pixel in the original image. But maybe my guess about how you declared it is wrong.

In any case, each time through the inner loop you essentially want to set

popArray[currentPixel] = {blueValue, origX, origY}

but instead you are assigning only one of the three value each time through the loop. So you are doing something like

popArray[0][0] = blueValue //first iteration; blueValue from row 0 col 0
popArray[0][1] = 0 //second iteration; row from row 0 col 1
popArray[0][2] = 2 //third iteration; column from row 0 col 2

So hopefully you can already see that something is wrong, since you are populating "columns" that are supposed to go together with values from different iterations of the loop. Even worse, you then start overwriting those values on the next iteration of the inner loop (which will iterate a total of 768 times before row increments):

popArray[0][0] = blueValue // fourth iteration; blueValue from row 0 col 4; overwrite value assigned on first iteration
etc...

Rather than using 3 array "columns" with different meanings to hold these data elements, it would be wise to make a class that holds the three values and makes clear what's what. popArray would contain this object type. Furthermore, I would make it a List instead of an array since it is more flexible and you can simply call Collections.sort() on it at the end; or @jsegal has a good suggestion of using a data structure that sorts as the items are inserted. Which one is better might depend on what you want to do with them later.

answered Apr 12, 2011 at 21:30

Dave Costa

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2 Comments

Nick Eichenberg Over a year ago

It's actually MAP_HEIGHT*MAP_WIDTH-1 for the first part of the array, as it needs to be every single pixel. I didn't put all the code with variable declarations and the sort because there's a lot more to this code than just this section. With the 'popArray[currentPixel] = {blueValue, origX, origY}', which is actually what I want to do, how do I set three values at once to one row? I'm sure it's completely obvious, but I've blown my mind with this recently.

Nick Eichenberg Over a year ago

FIXED : Now simply : popArray[currentRow][0] = newBlue; popArray[currentRow][1] = row; popArray[currentRow][2] = column; currentRow++; if(currentRow >= (BOX_WIDTH*BOX_HEIGHT-1)) { break; } Setting each of the rows only once, saves them all, and gives me an array full of every single point, with it's color, x, and y. Thank you for pointing out how I failed completely with 2 dimensional arrays, makes so much more sense now!

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