Processing sketch won't display modified image on screen - image-processing

Question

I am making a sketch which does the following:
Resizes a large image to fit it into a 800 x 600 screen.
displays the image
Applies some effects when keys are pressed
Display the image back on the screen and print a little Done message
Everything works fine expect that it does not display the image back properly.
Here is the black and white effect:
void blackAndWhite() {
loadPixels();
for (int i = 0;i < img.pixels.length;i++) {
int pixel = img.pixels[i];
// println("Working on pixel " + i + " out of " + img.pixels.length);
int red = (int) red(pixel);
int green = (int) green(pixel);
int blue = (int) blue(pixel);
/*
* Luminosity Method.
*/
// red = (int) (red * 0.21);
// green = (int) (green * 0.71);
// blue = (int) (blue * 0.07);
/*
* Average Method
*/
// float avg = (int) (red + green + blue / 3);
// red = green = blue = (int) avg;
/*
* Lightness Method
*/
int mostProminent = max(red, green, blue);
int leastProminent = min(red, green, blue);
int avg = (int) ((mostProminent + leastProminent) / 2);
red = green = blue = avg;
pixel = color(red, green, blue);
img.pixels[i] = pixel;
}
println("Done");
updatePixels();
image(img, WIDTH/2, HEIGHT/2, calculatedWidth, calculatedHeight);
}
However, only the colored image is displayed :(
I know the algorithm works cause I have tried it on other images (not using this sketch).
What is going wrong?

Answer 1

The following code works fine:
PImage img;
void setup () {
img = loadImage ("img.png");
size(img.width, img.height);
}
void draw () {
image(img, 0, 0);
}
void keyReleased () {
blackAndWhite();
}
void blackAndWhite() {
img.loadPixels();
for (int i = 0;i < img.pixels.length;i++) {
int pixel = img.pixels[i];
// println("Working on pixel " + i + " out of " + img.pixels.length);
int red = (int) red(pixel);
int green = (int) green(pixel);
int blue = (int) blue(pixel);
/*
* Luminosity Method.
*/
// red = (int) (red * 0.21);
// green = (int) (green * 0.71);
// blue = (int) (blue * 0.07);
/*
* Average Method
*/
// float avg = (int) (red + green + blue / 3);
// red = green = blue = (int) avg;
/*
* Lightness Method
*/
int mostProminent = max(red, green, blue);
int leastProminent = min(red, green, blue);
int avg = (int) ((mostProminent + leastProminent) / 2);
red = green = blue = avg;
pixel = color(red, green, blue);
img.pixels[i] = pixel;
}
println("Done");
img.updatePixels();
}
My guess is that you either forgot to write img.loadPixels() instead of loadPixels(), or there was something wrong in sketch's draw() method.