Java 处理：使用鼠标绕一个点旋转

Question

I am creating a java processing program in which I need to rotate a rectangle about a point when the mouse is held down.

A few restrictions:

1. It can only rotate 180 degrees(in a semicircle)
2. Once it gets to the end of that 180 degrees, it must reverse the direction of its rotation.

Any help would be greatly appreciated. My program is attached.

void setup() {
    size(600, 600);
    smooth();
}

void draw() {
    background(0);
    noStroke();
    fill(169,169,169); 

    translate(width/2, height/2);

    boolean run=true;
    if (mousePressed==true && run==true){
        rotate(radians(frameCount/2));
    }
    if (mousePressed==false){
        run=false;
    }
    if(mousePressed==true  && run==false){
        run=true;
    }
    fill(255);
    rect(-50, -50, 50, 50);
}

Answer 1

The 3rd and 4th parameter of rect is not the bottom right point of the rectangle, it is its width and height.
If a rotation ( rotate() ) is performed, then an object is rotated about its center.

You've to define a rectangle, with a center point of (0, 0). eg:

rect(-50, -50, 50, 50);

rect(-25, -25, 50, 50);

---

If you want to rotate an object around a pivot ( poivot_x , poivot_y ), then you've to:

• Translate the object in that way, that the pivot is at (0, 0): translate(-poivot_x, -poivot_y)

• Then rotate the object: rotate(...)

• Finally translate the pivot back to its original place: translate(poivot_x, poivot_y)

Since matrix operations like rotate() , scale() and translate() , define a matrix and multiply the current matrix by the new matrix, this operations have to be done in reverse order.

eg Rotate the rectangle (0, 0, 50, 50) around the center of the top edge (25, 0):

int poivot_x = 25; // center
int poivot_y = 0;  // top

translate(poivot_x, poivot_y);
rotate(radians(frameCount/2));
translate(-poivot_x, -poivot_y);

rect(0, 0, 50, 50);