Dimensions of an item in a picture

[Tony Cox]

Hello Everyone,

I used to have a formula to determine the dimensions of an object in a picture. Like a cabinet or bookcase shelf spacing. Basically you measured the size of the item on a printed picture and then used a known measurement from something in that picture and then used this formula to figure the size of the item you were interested in. I know it sounds a little confusing, but I'm hoping someone gets what I am asking. I can't find this formula and I think it was in my email at my old job. Now it's gone. Any help would be great.

Thanks,

[Nick Laeder]

Hello Everyone,

I used to have a formula to determine the dimensions of an object in a picture. Like a cabinet or bookcase shelf spacing. Basically you measured the size of the item on a printed picture and then used a known measurement from something in that picture and then used this formula to figure the size of the item you were interested in. I know it sounds a little confusing, but I'm hoping someone gets what I am asking. I can't find this formula and I think it was in my email at my old job. Now it's gone. Any help would be great.

Thanks,

Would only work on a picture taken perfectly square to the objects, and only if the two objects were the exact same distance away from the camera, but:

Known object actual size X unknown object scaled size
Known object scaled size


Basically, just determine the scale of the picture. If there's a 1' long ruler in the photo that is 3" long scaled in the photo, the scale is 1/4. Multiply everything by 4 to determine "actual" sizes.

Again, this won't be very precise.

[glenn bradley]

As Nick states, unless the picture was taken specifically for scale comparison I would think a WAG would be as close as not. I have imported images into Visio or SketchUp and measured a dimension I am reasonably sure of. I then apply that guesstimate to another object. I do not take this number as a rule but instead use it to add credence to my guess.

[Tony Cox]

I found what I was looking for. Here it is.

Print the picture and measure the item of the known dimension.

(size in photo)/Actual dimension = ratio

Now measure the item of the non-given dimension.

(non-given dimension)/ratio = size

So I wanted to find the length of a house. I know the railing sections are 6 feet. So I measure the railing in the picture and got 15/16". I convert that to decimal and get .9375.

So, .9375/72 = .013

Then I measure the length of the house in the photo and get 7.5 inches.

So, 7.5/.013 = 576.92 inches. Divide that by 12 and that equals 48.07. So the length of the house is around 48 feet.

It's a pretty good process to get the rough dimensions of something from a photo.

[Lee Schierer]

I found what I was looking for. Here it is.

Print the picture and measure the item of the known dimension.

(size in photo)/Actual dimension = ratio

Now measure the item of the non-given dimension.

(non-given dimension)/ratio = size

So I wanted to find the length of a house. I know the railing sections are 6 feet. So I measure the railing in the picture and got 15/16". I convert that to decimal and get .9375.

So, .9375/72 = .013

Then I measure the length of the house in the photo and get 7.5 inches.

So, 7.5/.013 = 576.92 inches. Divide that by 12 and that equals 48.07. So the length of the house is around 48 feet.

It's a pretty good process to get the rough dimensions of something from a photo.


Tony's got it right. I've used this method several times with great success. When you end up with an odd decimal round it off to the nearest 1/4 or 1/8" dimension. Also, if the photo was taken at an angle, use the appropriate given dimension in that direction for determining actual dimensions in that direction on the photo, it will yield better accuracy. You will be amazed at how close to actual dimension you can get.

[Steve Schoene]

You can refine the accuracy of measurements by using principles of perspective to project the part you want to measure into the same plane as a part where you know the dimension. Jeffery P Greene, American Furniture of the 18th Century, Taunton Press, 1996, has an excellent chapter on measuring items for reproduction.

[Thom Porterfield]

It is handy to have the photo taken "square on" but isn't necessary. The photos produced by most camera lenses are perspectives. As Steve says above, it is not only possible but easy to determine the dimensions of anything in a perspective if you know the dimensions of one object. I would be confident in doing this to achieve an accuracy of the nearest 0.10".

[Dan Hintz]

Thom,

Quick, how tall is this person? ;)

http://1.bp.blogspot.com/_f4SL-0_ALso/Smd-jeHKDtI/AAAAAAAAB-o/KRnieVcS_8I/s400/Eiffel+Tower.jpg

[Thom Porterfield]

Oh, probably average sized...for a human being.

But that tower, man, that's only about one and a half inches tall.

Pretty good model though. . . :D





. . . . . . . . cheater

[Dan Hintz]

Aw, I was hoping measurements to 0.1" accuracy ;)

[Karl Card]

hmmm i think a case of beer or a fifth of wild turkey 101 might help this situation... see all kinds of things then..

actually I have heard of what you are talking about but been to long ago and wasnt anything i needed at the time.

[Dennis Simmons]

I use a program called Paint.net It is free. I find most times that a picture is half scale at 112% zoom out. If you know one dimension then you can use a clear plastic ruler to fine the known dimension down size in the picture, say you know that the base is 4 inches and you measure it at 2 inches. If the camera was not square to the object then you must guess, but you can come close.

[Russell Sansom]

In a software 3d to 2d transformation, the "perspective transformation" is very simple: The X and Y dimensions of lines ( objects ) are scaled by their distance from the eye. So, X(perspective) = X(actual) / Z ( distance from eye ). Doubling the distance cuts the size in half. It seems that it should be more complicated, but it isn't.
This means that if you can judge the distance of two objects, you can compare them by using this transformation.
And if you really want to go crazy, you can also adjust for how far it is rotated. Simply scale the measurement by the cosine of the angle. A road sign that's square to you ( zero rotation ) would be scaled by cos(0.0 degrees ). If it's rotated 45 degrees it will appear to be only "cos (45 degrees)" or .707 times as wide. If it's edge on to you, not counting its thickness, it will be "cos (90 degrees ) or 0.0 times as wide.

[Dan Hintz]

That also assumes, however, there's no lens distortion... a wide angle pic could have serious issues with such a rule of thumb.

Meh, not like it matters, we're not building NASA equipment with these measurements anyway...

[Jon van der Linden]

The short answer is cross multiplication - a formula that is often taught when learning fractions in grade school. This formula is particularly interesting because of it's use in business transactions in Italy, which resulted in the art of the Renaissance. Proportion and it's study is based on this simple formula, if you're interested in learning about its relation to art in history, you can read about it in "Painting and Experience in Fifteenth century Italy" by Michael Baxandall.

In essence you have 3 knowns and an unknown: a1/b a2/X (a1 is known object in picture, a2 is known objects size, b is unknown size in picture, X is unknown size) b x a2 : a1 = X (since the proportions are identical.

This is the same formula you'd use to exchange currencies or to compare prices per volume/weight in the supermarket.