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OpenGL ES - How to Draw a filled Polygon?

Answer 1

A:

Hi,

Cocos2D is cool library wrapping opengl and has many usefull features ( mainly for games but not limited to ).

For polygon tesselation use : http://flipcode.net/archives/Efficient_Polygon_Triangulation.shtml I've used it before and it worked well.

Cheers, Krzysztof Zabłocki

Krzysztof Zabłocki 2010-10-06 12:42:10

Ok but If possible I would like avoiding Cocos2D just for drawing polygons xD (but by the way, using Cocos2D means adding lot of size to the final archive ? can I create just one View above other UIView without problem ?)

Vinzius 2010-10-06 12:45:15

You can use opengl and normal UIKit without a problem, cocos2d offers much more functionality than drawing polygons so if you only need simple polygons then it's not needed for sure :)

Krzysztof Zabłocki 2010-10-06 13:16:53

yeah I only need polygons, but thanks ;-)

Vinzius 2010-10-06 13:20:45

i added link to working code for tessalation

Krzysztof Zabłocki 2010-10-06 13:21:30

thanks, will take a look too :)

Vinzius 2010-10-06 13:22:33

It seems efficiently for polygon without holes (is saying the header file). So I'm going to study another C code which handle it ^^. If I code C => ObjC I will put the code. Maybe a response later or tomorrow ^^

Vinzius 2010-10-06 13:50:52

I'll begin with this library despite it doesn't support holes and then go to http://www.cs.unc.edu/~dm/CODE/GEM/chapter.html A+ ;-)

Vinzius 2010-10-08 09:48:56

Answer 2

+2 A:

I haven't tried using OpenGL ES, but judging from a quick look in the documentation you should be able to draw a convex polygon using e.g. a "triangle fan":

glVertexPointer(2, ..., arrayOfCoordinates)
...
glDrawElements(GL_TRIANGLE_FAN, ... , arrayOfIndices);

You can think of a "triangle fan" as the spokes of a bicycle-wheel dividing the area of the wheel into "triangles" (the outer edge of a bicycle wheel is of course round, but I hope you get the idea).

UPDATE: I found a small diagram on the web:

S.C. Madsen 2010-10-06 13:02:39

So long as the polygon is convex!

Daniel Paull 2010-10-06 13:08:56

@Daniel: Precisely! (did I not mention that? It would of course be easy to split the polygon into triangles and draw those using GL_TRIANGLES, but it sounded like he wanted to avoid that...

S.C. Madsen 2010-10-06 13:12:29

Yep I'm working on a solution based on a code found here ( http://www.cocos2d-iphone.org/forum/topic/848 ) But it works only if the polygon is convex, like said Daniel. If anything works I tell you.

Vinzius 2010-10-06 13:13:35

I remember seeing algorithms for splitting polygons into convex polygons, you could try Google. Anyway I hope you get your problem solved :-)

S.C. Madsen 2010-10-06 13:16:13

I'm going to search thing about that too. tks. As soon as I get a good solution I put the code here and set the topic as solved ;-) If anyone found something already done, I take it too ! thanks for helping ;-)

Vinzius 2010-10-06 13:17:16

Theres an SO discussion on decomposing to convex polygons here: http://stackoverflow.com/questions/694108/decomposition-to-convex-polygons

S.C. Madsen 2010-10-06 13:25:22

Answer 3

A:

In non-ES OpenGL one would tend to use the tessellator from the GL Utility (glu) library. I found this project which aims to make glu available for the iPhone and it claims to support polygon tessellation - if it works, then it would be a good option.

Daniel Paull 2010-10-06 13:18:55

Thanks, I will take a look to see if it's can be used :-)

Vinzius 2010-10-06 13:21:37

Tried to compile the project but it seems having "some" problems ^^ still thanks

Vinzius 2010-10-07 14:56:39

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OpenGL ES - How to Draw a filled Polygon?

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