Drawing an arc backwards?

Question

I am porting some of the Slick Graphics class to work with AWT, but I've run into a problem. Slick, being run on OpenGL, draws arcs clockwise. AWT defines them as being drawn counter-clockwise, like in math. I'm wondering how I can draw Slick's arc counter-clockwise, so that it matches AWT.

An image of what I'd expect:

An image of what I get:

Answer 1

This is the function to draw an arc in Slick2D:

public void drawArc(float x1,
                    float y1,
                    float width,
                    float height,
                    float start,
                    float end)

Since it appears that Slick draws these arcs clock-wise (0 - 360 degrees) then you can do some simple arithmetic to get the effect that you wish.

A and B are your original angles that you wish to draw the arc in between like 0 and 90 degrees. drawArc(0,0,1,1,A,B) will render an arc from A to B, but we want an arc from C to D which is the same arc as A to B but counter-clock wise. Since you already know the values of A and B (like 0 and 90) you can get the values for C and D with the following:

C = 360 - B

D = 360 - A

So now we can just use drawArc(0,0,1,1,C,D).

Test it with values:

A = 0

B = 90

C = 360 - 90 = 270

D = 360 - 0 = 360

This will plot an arc from 270 to 360 which is the same as an arc from 0 to 90 counter-clock wise.