# Zoom problem

Hi everyone,

I want to zoom in on a 2D drawing in OpenGL and so I modify the clipping area when a button is pressed. The zoom algorithm is very simple:

float initialWidth=Math.abs(screenArea.maxX-screenArea.minX);
screenArea.minY += zoomSize;
screenArea.maxY -= zoomSize;
float zoomedWidth=Math.abs(screenArea.maxY-screenArea.minY)*aspectRatio;
screenArea.minX += (initialWidth-zoomedWidth)/2;
screenArea.maxX -= (initialWidth-zoomedWidth)/2;

gl.glOrthof(screenArea.minX, screenArea.maxX,screenArea.minY, screenArea.maxY, 1, -1);

This goes very well up to a certain point, where
screenArea.minX = 3096.8154f;
screenArea.maxX = 3152.815f;
screenArea.minY = 5648.5005f;
screenArea.maxY = 5723.7495f;

From here on, there is no more zooming, but the whole image pans out on the Y axis (the pan is exponential: at the first click it moves a little, at the second click more, etc).

I’m doing this with OpenGL ES for the Android platfrom. Might this be a bug, or I’m doing something wrong?

PS: If I substract from every X coordonate in the scene minX and from every Y coordonate mixY (I translate the whole thing to origin) it works perfectly

Any help would be greatly appreciated.

Sounds like you hit precision issues of your plateform.
What implementation of OpenGL ES do you use ?

Quote from the Android website: “Android currently supports OpenGL ES 1.0, which corresponds to OpenGL 1.3”

That does not say anything about the implementation.
Well I am pretty sure that ES 1.0 can use low precison fixed point arithmetic.

I also tried with fixed point numbers…same result

Another strange behaviour: if I set the vieport smaller than the window, the drawing will pan out of the viewport area and into the rest of the window area.

Any ideas?