How can I show only a part of an image in Processing? - image

Question

I want to compare two images (same size) fr a presentation with a shifting line. On the left side of this line the one image is should be displayed while on the right side the other picture should stay visible.
This is what I tried (bitmap and ch are the images)
PImage bitmap;
PImage ch;
int framerate = 1000;
void setup() {
size(502, 316);
bitmap = loadImage("bitmap_zentriert.jpg"); // Load an image into the program
ch = loadImage("Karte_schweiz_zentriert.jpg"); // Load an image into the program
frameRate(40); //framerate
}
void draw() {
background(255);
image(ch, 10, 10); // the one image in the back
image(bitmap, 10, 10, bitmap.width, bitmap.height, 10, 10, mouseX, bitmap.height); //show part of the second image in front
rect(mouseX, 10, 1, bitmap.height-1); //make line
}
But the image "bitmap" is the whole image distorted.
How can I do that?

Answer 1

I'd recommend using a PGraphics buffer, which is essentially "Another sketch" that also acts as an Image for drawing purposes, and most definitely not looping at "a thousand frames per second". Only draw something when you have something new to draw, using the redraw function in combination with mouse move events:
PImage img1, img2;
PGraphics imagebuffer;
void setup() {
size(502, 316);
imagebuffer = createGraphics(width, height);
img1 = loadImage("first-image.jpg");
img2 = loadImage("second-image.jpg");
noLoop();
}
void mouseMoved() {
redraw();
}
void draw() {
image(img1, 0, 0);
if (mouseX>0) {
imagebuffer = createGraphics(mouseX, height);
imagebuffer.beginDraw();
imagebuffer.image(img2, 0, 0);
imagebuffer.endDraw();
image(imagebuffer, 0, 0);
}
}
In our setup we load the image and turn off looping because we'll be redrawing based on redraw, and then in response to mouse move events, we generate a new buffer that is only as wide as the current x-coordinate of the mouse, draw our image, which gets cropped "for free" because the buffer is only limited width, and then we draw that buffer as if it were an image on top of the image we already have.

Answer 2

There are many ways to do it, one thing I suggest is to create a 3rd image with the same width and height, then you load the two images pixels and insert in your 3rd image part of image1 pixels and then second part from image2, I wrote this code to test it out, works fine:
PImage img1, img2, img3;
void setup() {
size(500, 355);
img1 = loadImage("a1.png"); // Load an image into the program
img2 = loadImage("a2.png"); // Load an image into the program
img3 = createImage(width, height, RGB); //create your third image with same width and height
img1.loadPixels(); // Load the image pixels so you can access the array pixels[]
img2.loadPixels();
frameRate(40); // frame rate
}
void draw() {
background(255);
// Copy first half from first image
for(int i = 0; i < mouseX; i++)
{
for (int j = 0; j < height ; j++) {
img3.pixels[j*width+i] = img1.pixels[j*width+i];
}
}
// Copy second half from second image
for(int i = mouseX; i < width; i++)
{
for (int j = 0; j < height ; j++) {
img3.pixels[j*width+i] = img2.pixels[j*width+i];
}
}
// Update the third image pixels
img3.updatePixels();
// Simply draw that image
image(img3, 0, 0); // The one image in the back
// Draw the separation line
rect(mouseX, 0, 0, height); // Make line
}
Result :