texture map stretches funnily when rotating

Tutorials concerning the OpenGL® ES cross-platform API for full-function 2D and 3D graphics on the Google-Android platform.

texture map stretches funnily when rotating

Postby dolgion1 » Fri Apr 03, 2009 5:54 am

Hi there,

I started learning OpenGL ES for Android and i slightly modified the Graphics API example from the SDK
"Textured Triangle".
I turned the shape into a rectangle made of a triangle strip and put the texture upon it.
Now what I don't get are two things:

1. the texture coordinates are not what I think they theoretically should be. As far as I understand it,
the coordinates for the lower left, lower right, upper left and upper right corners should be (0,0), (1,0), (0,1), (1,1) respectively.
A texture coordinate asigns a coordinate on the texture to the corresponding coordinate (vertex) on the polygon, doesn't it?
But here in my example, I ended up with these texture coordinates (by trial and error): (0,0),(-1,0),(0,-1),(-1,-1).
In other words, I have to multiply the coords I originally thought correct by -1. If I don't, the texture is applied upside-down.

2. When I rotate the rectangle along the y axis, the texture somehow 'wobbles' in the centre of the rectangle.

I'll attach the project and I'd be reeaally grateful if somebody more knowledgeable in this matter can enlighten me.
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Postby dohuon » Wed Apr 15, 2009 2:51 am

1. See opengl tutorials

2. I strugged with same problem for a week until I realized it is working perfectly on Android device. :shock:
Not only this problem, I felt emulator has lots of problem on drawing opengl.
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