A question about draw-polygon and draw-filled-polygon

[若凡陈]

I tried to plot a hexagon using draw-polygon, here a hexagon is defined six vertices

(defparameter hexagon 

                        (list (sdl:point :x (* -10 (cos (/ pi 6))) :y -5)

                              (sdl:point :x (* -10 (cos (/ pi 6))) :y 5)

                              (sdl:point :x 0 :y 10)

                              (sdl:point :x (* 10 (cos (/ pi 6))) :y 5)

                              (sdl:point :x (* 10 (cos (/ pi 6))) :y -5)

                              (sdl:point :x 0 :y -10)))

When I called (sdl:draw-polygon hexagon), I expected to have a close hexagon. However,

one of computer gave me close hexagon and another gave me a open hexagon that

one of edges was missing.

Also, when I called (sdl:draw-filled-polygon hexagon), one gave me what I want and another 

drew nothing at all. So I'm wondering whether my code is not rigorous or it is a bug.

Could someone tell me what's going on? Thanks a lot.

[Elliott Slaughter]

Does the bug go away if you repeat the first point at the end? I suspect that you are not actually closing the polygon.

I find it strange that you would get different results on different systems though. What OS/Lisp/Lispbuilder/SDL versions are we talking about?

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Elliott Slaughter

"Don't worry about what anybody else is going to do. The best way to predict the future is to invent it." - Alan Kay

[若凡陈]

Yeah, I'm not closing the polygon. In fact I have tried closing the hexagon, indeed it would gave a close 

hexagon for both computer, but it won't make any difference for draw-filled-polygon.

Both machine use Kubuntu 14.04 and Clisp, I use Quicklisp for both to install lispbuilder. I also tried SBCL

but the results are the same.

The machine which has problem is using Intel Integrated Graphics Cards, another is using Nvdia GTX650Ti.

在 2015年6月7日星期日 UTC+8上午6:17:17，Elliott Slaughter写道：

[若凡陈]

I'm sorry that the problem may be at other part of my code, some coordinate are not integer when I shift it.

Now it works fine.

Anyway thank you, Mr Slaughter.

By the way, your blackthorn library looks cool :)

在 2015年6月7日星期日 UTC+8上午6:17:17，Elliott Slaughter写道：

Does the bug go away if you repeat the first point at the end? I suspect that you are not actually closing the polygon.

[Elliott Slaughter]

Glad you were able to fix it.