Basic of Projection Mapping

pm.pde

// you must prepare image

PImage image;

int[] x=new int[4];
int[] y=new int[4];

void setup()
{
  size(displayWidth,displayHeight,P2D);
  
  image=loadImage("image.png");
}

void draw()
{
  background(0);
  
  fill(0);
  stroke(255);
  strokeWeight(8);
  
  textureMode(NORMAL);
  beginShape();
  texture(image);
  vertex(x[0],y[0],0,0);
  vertex(x[1],y[1],1,0);
  vertex(x[2],y[2],1,1);
  vertex(x[3],y[3],0,1);
  endShape(CLOSE);
}

void keyPressed()
{
  if(key=='0')
  {
    x[0]=mouseX;
    y[0]=mouseY;
  }
  if(key=='1')
  {
    x[1]=mouseX;
    y[1]=mouseY;
  }
  if(key=='2')
  {
    x[2]=mouseX;
    y[2]=mouseY;
  }
  if(key=='3')
  {
    x[3]=mouseX;
    y[3]=mouseY;
  }
}