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No it doesn't, you can not calculate an area unless you incorporate the resolution of your document. 300PPI = 1 Inch 200PPI = 1inch etc. assuming pixels/inch.

Also, S=linear  pixel/cm

When you go to view - image size does the resolution show pixels/inch or pixels/cm

 

Also the selection no matter how its made should be the total pixels selected.

Should be the same for all channels.

[code]

var cnt =0;

histogram = activeDocument.channels["Red"].histogram;

for (i = 0; i <= 255; i++) {

 cnt += histogram[i];

}

alert(cnt);

[/code]

Should be the same as what is shown in the histogram.