How to crop the picture in the amplification process?

            
There are pictures, resolution is 320x480, the screen resolution is also 320x480, just display the whole image.
When asked for larger image doubled, the picture becomes 640x960, assuming that the display is a central location, then displays the top left corner of the chart is the original image (160,240)
When the graphic enlarged three times, the picture becomes 960x1440, assumptions or centered, then the upper left corner of the display is the original figure of how much?

Prepared by a general formula how to calculate the upper-left corner of different magnification and a different display, the coordinates corresponding to the original number is?

Reply:
Screen coordinates to the desired location to catch down slightly. . .