NullPointerExeption when using img.get()

Processing Coding Questions
1

Hey, I am trying to get the values of a pixel in an image as well as the values of the pixels around it, however the get() function is returning a null pointer exception.
Most likely something wrong in my code:

void feedForward(String[] images){
  img = loadImage(images[int(random(images.length))], "png");  //Gets a random image from the images[] array
  img.resize(img.width, img.height); //Makes sure that the image is the original size
  image(img, 0, 0);
  for(int y = 0; y <= img.height; y++){
    for(int x = 0; x <= img.width; x++){
        desired[0] = img.get(x,y);
        desired[1] = img.get(x+1, y);
        desired[2] = img.get(x,y+1);
        desired[3] = img.get(x+1, y+1); 
        desired[4] = img.get(x-1, y);
        desired[5] = img.get(x, y-1);
        desired[6] = img.get(x-1, y-1);
        desired[7] = img.get(x-1, y+1);
        desired[8] = img.get(x+1, y-1);
    }
  }
//The rest of the funtction is irrelevant

Any help or pointers would be appreciated, I have been pulling my hair out on this one for the past week, thanks.

2

Try this next, three changes per line:

for(int y = 1; y < img.height-1; y++){
    for(int x = 1; x < img.width-1; x++){

The important thing here is how to debug this code. A simple mental substitution of x to zero shows that you get a negative index which get() will not know how to handle and simply return null. You either manage the null or re-design the algorithm so restricts your calculation within a valid are of your image.

Btw, this is redundant: img.resize(img.width, img.height); aka. you don’t need this.

Kf

3

Hey thanks for the help. I made the changes and debugged with that in mind however I am still getting the null pointer. I haven’t quite had time to fully delve into the issue and I will get to that soon, so I may of missed something.
Also thanks for the pointer on the img.resize() I wasn’t sure if this was required or not.

4

You could start the for loops at 1 and let them go to image width and height -1

You could write your own function as a wrapper that checks whether it’s <0 before executing get()

Also in which line does the error occur exactly?

5

Those seem like good ideas, I will give them a go.
This error is very annoying because it only says:
“NullPointerExeption”
I would like for it to say more, but sadly that’s all it tells

6

Interesting.

But does it mark exact line? In yellow or red?

7

Yes I forgot to mention that, the first desired[0] = img.get(x,y); highlights red, if I comment out this line the next is highlighted

8

**Highlights Yellow
Sorry the error is red, the line highlights yellow

9

Thank you for all the help, after some mucking around I found that the solution was stupidly simple. As it always tends to be.

The reason for the error was that I had declared the desired[] array as such:

float[] desired;

This caused the array to not have any data and therefore being Null the way I solved this was by filling the array with the value 0. The end result looked like this:

float[] desired = {0, 0, 0, 0, 0, 0, 0, 0, 0}; 

void feedForward(String[] images){
  img = loadImage(images[int(random(images.length))], "png");  //Gets a random image from the images[] array
  image(img, 0, 0);
  for(int y = 1; y <= img.height -1; y++){
    for(int x = 1; x <= img.width -1; x++){
        desired[0] = img.get(x,y);
        desired[1] = img.get(x+1, y);
        desired[2] = img.get(x,y+1);
        desired[3] = img.get(x+1, y+1); 
        desired[4] = img.get(x-1, y);
        desired[5] = img.get(x, y-1);
        desired[6] = img.get(x-1, y-1);
        desired[7] = img.get(x-1, y+1);
        desired[8] = img.get(x+1, y-1);
    }
  }
//The rest of the funtction is irrelevant

Thank you for the help

10

Ah, okay. You forgot to initial the array

We could find something like this out by using println before that line