Thank you very much for these useful tips.
I'm really new to programming and I know I make a lot of mistakes.
I missed imageMode() in the processing reference (I looked for it but probably I was absent-minded).

The sketchFullScreen() is there because I was checking the output on a video projector...
Thanks again.

Thanks again...
I modified the code like this. It looks cleaner now!

1. PImage img[] = new PImage [10];
2. boolean sketchFullScreen() {
3.   return true;
4. }
5. void setup () {
6.  
7.   // setup canvas
8.  
9.   frameRate (25);
10.   size (displayWidth, displayHeight); // comment for debugging
11.   // size (20, 20); // uncomment for debugging
12.   imageMode (CENTER);
13.  
14.  // initialize images
15.  
16.   for (int i = 0; i< 10; i++) {
17.   img [i] = loadImage ((i+1) + ".jpg"); // load images from 1.jpg to 10.jpg
18.   }
19. }
20. void draw () {
21.   if (mousePressed) {
22.     int i = int (random (1, 10));
23.     image (img [i], displayWidth / 2, displayHeight / 2);
24.     println ("drawing image " + (i+1) +".jpg"); // for debugging purpose
25.   }
26.   else {
27.     background (255, 0, 0);
28.   }
29. }

I tried following your advices but there's something still unclear.

loadImage() is a slow process! Just create all images in setup() as an array of PImage, that is PImage[] img = new PImage[ total_num_of_images]. Then you just need to display them using image( img[ index], x, y ) inside draw().

Just to understand the difference with my old code and how computer works:
If I use loadImage() inside setup() would this draw more computer RAM than my previous code because I load all images at the same time?

1. Instead of checking for variable mousePressed, use callback function mousePressed() to deal w/ mouse clicks.

This draws a single image as I click my mouse (or I made something wrong). I want the images to be randomly and continously drawn ad the mose is down. If (mousePressed) works for me but I can't figure out how to get the same calling mousePressed()

Instead of a call to function sketchFullScreen(), you can also use a global boolean variable instead ^_^

I admit I did not understand what you mean here....

I don't think we need to load all the images at once, since they are loaded only on user input, not permanently. Well, only after the change I suggest:

1. PImage img;
2. boolean sketchFullScreen() {
3.   return true;
4. }
5. void setup() {
6.   size(displayWidth, displayHeight);
7.   frameRate(25);
8. }
9. void draw() {
10.   if (mousePressed) {
11.     // display the image to the center of the screen
12.     int x, y;
13.     x = img.width;
14.     y = img.height;
15.     int sx, sy;
16.     sx = displayWidth;
17.     sy = displayHeight;
18.     int centerX = (sx - x) / 2;
19.     int centerY = (sy - y) / 2;
20.     image(img, centerX, centerY);
21.   }
22.   else {
23.     background(255, 0, 0);
24.   }
25. }
26. void mousePressed() {
27.   int i = int(random(1, 10));
28.   String imageName = i + ".jpg";
29.   img = loadImage(imageName);
30. }

(untested)
Note that the image number is drawn only once as long as the mouse is pressed, so the image is stable until the mouse is released.

boolean isFullScreen = true;

Ak... Ok... This is what I was figuring out.

Note that the image number is drawn only once as long as the mouse is pressed, so the image is stable until the mouse is released.

Thank you! I've already tried it. But it was not my goal to have the image drawn only once as long as the mouse is pressed. I want 10 different random images drawn for 1/25th second each so my code seems to be right for that.

I really appreciate your help and I', very pleased to see that Processing has a very wonderful community to help beginners.

Thanks again.