mouseWheel() / Reference / Processing.org

Name

mouseWheel()

Description

The code within the mouseWheel() event function is run when the mouse wheel is moved. (Some mice don't have wheels and this function is only applicable with mice that have a wheel.) The getCount() function used within mouseWheel() returns positive values when the mouse wheel is rotated down (toward the user), and negative values for the other direction (up or away from the user). On OS X with "natural" scrolling enabled, the values are opposite.

The mouseWheel() function requires a MouseEvent parameter to access the scroll wheel count and other event details. Unlike other mouse functions that can work without the event parameter, mouseWheel() specifically needs the event to get the wheel count via event.getCount(). The event also provides access to the exact mouse coordinates and any modifier keys that were held down during the scroll.

Mouse and keyboard events only work when a program has draw(). Without draw(), the code is only run once and then stops listening for events.

Examples

void setup() {
  size(100, 100);
}

void draw() {} 

void mouseWheel(MouseEvent event) {
  float e = event.getCount();
  println(e);
}

// Enhanced mouseWheel example showing MouseEvent usage
float zoom = 1.0;
int circleX = 50;
int circleY = 50;

void setup() {
  size(200, 200);
}

void draw() {
  background(220);
  
  // Instructions
  fill(0);
  textAlign(CENTER);
  textSize(10);
  text("Use mouse wheel to zoom", width/2, 15);
  text("Hold Shift for horizontal scroll", width/2, 25);
  text("Hold Ctrl for precision mode", width/2, 35);
  
  // Draw circle with zoom applied
  pushMatrix();
  translate(width/2, height/2);
  scale(zoom);
  fill(255, 0, 0);
  ellipse(circleX - width/2, circleY - height/2, 30, 30);
  popMatrix();
  
  // Show zoom level
  fill(0);
  textAlign(LEFT);
  text("Zoom: " + nf(zoom, 1, 2), 10, height-20);
  text("Position: (" + circleX + ", " + circleY + ")", 10, height-10);
}

void mouseWheel(MouseEvent event) {
  float wheelCount = event.getCount();
  int mouseEventX = event.getX();
  int mouseEventY = event.getY();
  
  // Check modifier keys from event
  boolean shiftDown = event.isShiftDown();
  boolean ctrlDown = event.isControlDown();
  boolean altDown = event.isAltDown();
  
  // Different behavior based on modifiers
  if (shiftDown) {
    // Horizontal scrolling
    circleX += wheelCount * 5;
    circleX = constrain(circleX, 0, width);
    println("Horizontal scroll at (" + mouseEventX + ", " + mouseEventY + ")");
  } else if (ctrlDown) {
    // Precision zoom
    zoom += wheelCount * 0.05;
    zoom = constrain(zoom, 0.1, 3.0);
    println("Precision zoom: " + zoom);
  } else if (altDown) {
    // Vertical scrolling
    circleY += wheelCount * 5;
    circleY = constrain(circleY, 0, height);
    println("Vertical scroll");
  } else {
    // Normal zoom
    zoom += wheelCount * 0.1;
    zoom = constrain(zoom, 0.1, 3.0);
    println("Normal zoom: " + zoom);
  }
  
  // Print detailed event information
  println("Wheel count: " + wheelCount + " at (" + mouseEventX + ", " + mouseEventY + ")");
  println("Modifiers: Shift=" + shiftDown + " Ctrl=" + ctrlDown + " Alt=" + altDown);
}

Syntax

mouseWheel(event)

Parameters

event(MouseEvent)the MouseEvent

Return

void

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