How can I draw an arrow at the edge of the screen pointing to an object that is off screen?
- by Adam Henderson
I am wishing to do what is described in this topic:
http://www.allegro.cc/forums/print-thread/283220
I have attempted a variety of the methods mentioned here.
First I tried to use the method described by Carrus85:
Just take the ratio of the two triangle hypontenuses (doesn't matter which triagle you use for the other, I suggest point 1 and point 2 as the distance you calculate). This will give you the aspect ratio percentage of the triangle in the corner from the larger triangle. Then you simply multiply deltax by that value to get the x-coordinate offset, and deltay by that value to get the y-coordinate offset.
But I could not find a way to calculate how far the object is away from the edge of the screen.
I then tried using ray casting (which I have never done before) suggested by 23yrold3yrold:
Fire a ray from the center of the screen to the offscreen object. Calculate where on the rectangle the ray intersects. There's your coordinates.
I first calculated the hypotenuse of the triangle formed by the difference in x and y positions of the two points. I used this to create a unit vector along that line. I looped through that vector until either the x coordinate or the y coordinate was off the screen. The two current x and y values then form the x and y of the arrow.
Here is the code for my ray casting method (written in C++ and Allegro 5)
void renderArrows(Object* i)
{
float x1 = i->getX() + (i->getWidth() / 2);
float y1 = i->getY() + (i->getHeight() / 2);
float x2 = screenCentreX;
float y2 = ScreenCentreY;
float dx = x2 - x1;
float dy = y2 - y1;
float hypotSquared = (dx * dx) + (dy * dy);
float hypot = sqrt(hypotSquared);
float unitX = dx / hypot;
float unitY = dy / hypot;
float rayX = x2 - view->getViewportX();
float rayY = y2 - view->getViewportY();
float arrowX = 0;
float arrowY = 0;
bool posFound = false;
while(posFound == false)
{
rayX += unitX;
rayY += unitY;
if(rayX <= 0 ||
rayX >= screenWidth ||
rayY <= 0 ||
rayY >= screenHeight)
{
arrowX = rayX;
arrowY = rayY;
posFound = true;
}
}
al_draw_bitmap(sprite, arrowX - spriteWidth, arrowY - spriteHeight, 0);
}
This was relatively successful. Arrows are displayed in the bottom right section of the screen when objects are located above and left of the screen as if the locations of the where the arrows are drawn have been rotated 180 degrees around the center of the screen.
I assumed this was due to the fact that when I was calculating the hypotenuse of the triangle, it would always be positive regardless of whether or not the difference in x or difference in y is negative.
Thinking about it, ray casting does not seem like a good way of solving the problem (due to the fact that it involves using sqrt() and a large for loop).
Any help finding a suitable solution would be greatly appreciated,
Thanks
Adam
