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How can I Zoom without scaling texture?

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Hello, I have a region that is texture mapped, and when I zoom in, I want the texture to remain fixed, as if the region is a window onto the texture, while zooming in/out is simply making the region bigger or smaller, respsectively. I thought this was as easy as scaling the U,Vs as a function of the camera height (I'm using shader language). This keeps the texture a constant size, but it scrolls as I zoom. By default, when you increase the U,V factor (say from 1.0,1.0 toward 0.0,0.0), the texture zooms toward the point 0,0 (the upper left corner). This has a scrolling effect. The texture stays the same size, but it slides diagononally as I zoom. I can zoom to any point on the main diagonal using the equation: y=-.5x+1.5 where x is the U,V scale factor and y is the new U and V. I can zoom to any point on the main diagonal if I substitute the .5 and 1.5 with say, .2 and 1.2 (to zoom to .2,.2). That is, U,V=fx*x+(1.0+fy) where fx,fy = the point you want to zoom to. However I'm trying to zoom to any point on the texture, and when I try say,( .25,.75) the texture starts out off center. Thanks in advance for any ideas, Neil

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