Photo Help,

Burton why do you say only if it is printed? There is no need to print the image to find the area.

True, there is no need to actually make the print, but the area referred to in the statement,

I was told that you can get the area of a selection by dividing the number of pixels by the resolution squared. Is this true?

is in no way related to the physical object area that James is attempting to measure. It is, as I said in my previous message, the area of the printed object, whether you print it or not. The A in George's message is also just this area on the print, whether it is printed or not. It has nothing to do with the physical area of the tooth outlined in the Photoshop selection.

To get the physical tooth area, James needs to measure the number of pixels in a centimeter on the centimeter scale included in the photos. This will vary from photo to photo unless there was very precise positioning of the camera in each photograph. It is highly unlikely that the distance from the lens to the tooth was exactly the same from one photo to the next. For a given photo, lets call this measured number of pixels per physical centimeter S. Note that S has nothing to do with the Photoshop image resolution, r. S is a measured scale factor and subject to an inherent measurement error due to rounding the actual physical distance of the centimeter rule image to an integer number of pixels. This measurement error can be reduced by increasing the number of pixels in the total image by upsampling. If more pixels are involved, a roundup error of half a pixel amounts to a smaller physical length error.

Photoshop, in all of its versatility, gives James several ways to make the measurement of S pixels per physical centimeter.

Following George's lead, to use algebra, the tooth area, T, is given by the expression:

T = N/S²

N still represents the measured number of pixels in the selection. Notice that the image resolution, r, is not relevant to the physical tooth area.

-- Burton --

pierre

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Calculate the Area of a Selection.