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# Converting Pixel length into Inches/Centimeters.

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 Converting Pixel length into Inches/Centimeters. by Paul Whishaw on Mar 24, 2004 at 6:32:22 pm

I have used a slider control and an expression to measure the distance of a line drawn by the Write On effect (many thanks to Filip Vandueren). Now I have a length that is measured in pixels. Is there some easy calculation that will tell me how many inches or centimetes that is?

Thanks in advance. Long live the cow.

 Re: Converting Pixel length into Inches/Centimeters.by Rick Gerard on Mar 24, 2004 at 6:39:56 pm

There are no inches in video so you'll have to figure out what scale you want to use. What is the distance supposed to represent?

Rick Gerard

 Good point.by Paul Whishaw on Mar 24, 2004 at 6:52:24 pm

Ah yes, That is a very good point. I suppose everything is relative isn't it?

I have tracked the path of a rat in a circular pool while it looks for a hidden platform. I suppose I need to measure some object in the video and then determine its length in pixels to be able to figure out the distance. Divide the lenght of measured object into the overall length of the path or some evuivalent.

Thanks. Pretty straight forward when I stop to think about it. I'm on the right track, correct?

 Re: Converting Pixel length into Inches/Centimeters.by Steve Roberts on Mar 24, 2004 at 7:00:29 pm

The size of your element will be quite different on a 640x480 monitor setting compared to a 1280x1024 monitor setting, and on a 15" monitor vs. a 21" monitor.

Using math, it should go like this when converting pixels to inches:

L(length in inches)=P(pixels)x H (monitor glass width-inches)/h (monitor display width - pixels)

... L=PxH/h, or...

...for a 400 pixel line on a 21" CRT at 1280x1024 ...
L = 400x16/1280 = 5 inches.

... which can be verified with a ruler on the screen.

Does that help?
Steve

 Re: Converting Pixel length into Inches/Centimeters.by Paul Whishaw on Mar 24, 2004 at 7:36:52 pm

The platform in the pool is 115 cm wide. I used a small graphic layer to travel from one edge to another and then applied the slider/expression to tell me how many pixels wide it was. The platform was 40 pixels wide. So 40 pixels = 115 cm, therefore one pixel = 2.875mm, or .002875m. So I take the length of the line in pixels and times it by .002875 to give me a number in meters.

It seems to be pretty acurate. Can you see any flaws in that method?

 Re: Converting Pixel length into Inches/Centimeters.by Steve Roberts on Mar 24, 2004 at 8:11:54 pm

Close:

40 pixels = 1150 mm

1 pixel = 1150/40 = 28.75 mm, or 2.875 cm, or .02875m.

... it's a "units" thing. :-)

SR

 Re: Converting Pixel length into Inches/Centimeters.by Paul Whishaw on Mar 24, 2004 at 9:32:47 pm

Oops. Sorry. Platform is 115 "mm".

Thanks for the help. This system works great. Very neat.

 Re: Converting Pixel length into Inches/Centimeters.by Filip Vandueren on Mar 25, 2004 at 2:47:47 am

BTW,

more thanks are due to Dan than to me;

Don't we just all love the guy...
Dan for president! 8^)