Pixels to MM2

tuxxaustin

Hi there

I use a vision application to identify different objects, and their size is measured in Pixels.
I have a number of circular finds that I need to convert from pixels to MM squared.
They are 9, 40, 55, 71, 107 and 191 pixels (area).
Now, it also works out that in the future, I am going to have to find objects with area's from 3 pixels to 400 pixels. Is there a way that I can convert these figures from Pixels into MM squared?
I have measured, and a straight line distance of about 45mm is 200 pixels.
I know its a long shot, but consider it a challenge!!
Thanks in advance
Jon

FSL

If your application can return the number of pixels in the image as a whole and you know the actual area of the image, not the area the image takes up on the screen, then you know the area a single pixel represents.

bonzodog2

From your 45mm line, a 45x45 square would be 40000 pixels, so 1mm^2 is 222.222 pixels. 400 pixels would thus be 1.8mm^2. Etc.

Overheal

Be careful with this application, because a pixel is not a standardized unit of measurement. a Pixel on an LCD monitor and a pixel on an iPhone are both different in size and shape. Even between 2 different brands or model of monitor they can differ. This is due to pixel density, measured in ppi, and even that is a bit of a wash for you application. but it would be something.

If you can control what display is used for an application (for instance, this is for a finished product like a touchscreen coffee table or something) then its viable; if its software that will freely run on independent hardware, your feature will not be calibrated correctly and may fail.

Capt'n Midnight

Like overheal said , pixels aren't standard. My phone has roughly the same number of pixels as my TV.

Also pixels aren't necessarily square. Measure both ways.

Some devices report their dpi , but even that won't take into account rectangular pixels

Overheal

http://en.wikipedia.org/wiki/Pixel_geometry