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MOPEX User's Guide - IRSA

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8.4.4 Grid Distribution<br />

<strong>MOPEX</strong> User’s <strong>Guide</strong><br />

As a fast alternative of the area overlap method the grid interpolation is implemented. Each input<br />

pixel is filled with Grid Ratio (GRID_RATIO) squared grid points (see Figure 8.20). Each grid<br />

point is assigned the value of the pixel it belongs to. Each grid point is projected onto the output<br />

frame and the flux associated with the grid point is added to the output image pixel into which the<br />

grid point was projected. The value Oj of an output interpolated pixel j is equal to the weighted<br />

average of the input pixels Ii<br />

O j =<br />

n ji<br />

∑ Ii , where<br />

i N j<br />

N j = n ji<br />

Basic Concepts in <strong>MOPEX</strong> 204<br />

Image Interpolation<br />

∑<br />

j<br />

Equation 8.13<br />

where nji is the number of the grid points projected into the output pixel j from the input pixel i.<br />

The parameter Grid Ratio specifies the number of grid points in one direction in an input pixel. In<br />

other words there are Grid Ratio squared grid points per input pixel. If Grid Ratio = 1 and the<br />

input to output pixel size ratio PR = 1, the method is equivalent to the nearest-neighbor<br />

interpolation. In the limit of infinite Grid Ratio the method approaches the area overlap<br />

interpolation. The main purpose of this scheme is to create first-look mosaic images fast, the gain<br />

in speed being up to 10 times as fast as scheme one. The price you pay is the fidelity of the<br />

interpolated images. Do not use these images for science.<br />

Figure 8.20: Input pixel (red) is filled with a grid of points. When the input pixel is<br />

projected onto the output array (blue) each point contributes an equal share of the input<br />

pixel value to the output pixel it is in.

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