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If I have a photograph of something that I know to be 20' long and I know the pixel size of the photo frame, can I determine the dimensions of items in that photograph by scaling (using the pixel count)?

dusty wrote:If I have a photograph of something that I know to be 20' long and I know the pixel size of the photo frame, can I determine the dimensions of items in that photograph by scaling (using the pixel count)?

Maybe!

Camera angle(and lense) may introduce 'irregularities'.

I think a reasonable approximation can be gleaned that way.

For starters, how accurate is the 20'?

Take this in the spirit of "everything looks like a nail, to a hammer" . At one point about 6 years ago, I was messing with importing photographs into Google Sketchup, and then lining up the vanishing points, estimating object placements, etc. It was possible to then have a Sketchup model that could be viewed from any new aspect point, and have dimensions taken from it. I only did this once, so I really can't remember how to do it any more....

nuhobby wrote:Take this in the spirit of "everything looks like a nail, to a hammer" . At one point about 6 years ago, I was messing with importing photographs into Google Sketchup, and then lining up the vanishing points, estimating object placements, etc. It was possible to then have a Sketchup model that could be viewed from any new aspect point, and have dimensions taken from it. I only did this once, so I really can't remember how to do it any more....

Dusty, I think you will need something like Sketchup to help you. Its use of perspective view mimics the camera's captured image pretty well. If the axis orientations of sketch up are alined closely with the pictured object there should be a way to set the scale in Sketchup so that the known dimension is 20' in Sketchup. Other dimensions should then be able to be captured. There may be other variables due to the kind of lens, but that is where I would start.

You can most likely "do the math" by looking at the file (detail) properties of the .jpg file.
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I use it fairly frequently. Have been able to come up with really close dimensions after taking a picture of a piece of furniture somewhere with something of known dimensions in the picture....my cap for example.

dusty wrote:If I have a photograph of something that I know to be 20' long and I know the pixel size of the photo frame, can I determine the dimensions of items in that photograph by scaling (using the pixel count)?

Usually works in TCad, should work in Sketchup I would think.
If your picture is pretty much straight-on (like for a 20' piece the camera should be 57' or more away).
What works best for me is to find something within the whole pic that is very clearly defined and whose size I can know independently, and scale to that (use the Calculator function). Often the outer edges of the Whole (esp. when 20' wide) are hard to zero in on.