geometry
I do not know if this is the place for this question but I am sure I will be told sooner rather than later. Basically I want to know the following.
Is there a way to calculate the height of an object from a certain position.
Example:
A Camera which has taken a picture. The camera was 3 meters off the ground, there is an object 10 meters into the distance. Is it possible to estimate the height of the object.
please see image before to help explain this issue.
The problem with using "Calibration constant, number of pixels per cm at 50cm distance" is that not all of the scene is at 50cm depth or distance from the camera.
I think you need to remove the perspective correction first like in this example... http://opencv-code.com/tutorials/automatic-perspective-correction-for-quadrilateral-objects/
Your green points would be the red and green ones in this example. Now obviously you want your blue points to be the white and blue in this example, which you don't have because they are outside the image. But if you work out where the left and right edges will coincide with the bottom edge of the image (the last row) you can still perform the perspective correction.
Now the correction is removed you can map pixels to cm and estimate where the bottom edge will appear.
If the light source is very far (e.g. the Sun) then you can consider the rays as parallel, so that in your picture you have two similar triangles and $t:h=(t+s):(h+d)$. From that you easily get $d=(h\cdot s)/t$
Best Answer
You can calculate the height of an object using the distance and angle.
distance * cos(angle), where distance is the horizontal distance to the object, and angle is the angle above horizontal of the top of the object (from the viewer). The result will be the height above the viewer.
With a camera, you need more information because you don't necessarily have the angle.
You can estimate the angle by taking a similar photo at the same zoom with known angles, and compare the distance on the two photos between horizontal (or vertical) and the angles. It will be more accurate if you have horizontal (or vertical) in the same place on the two photos.
You might also be able to determine the angle in the photo by the alignment of two objects of different distances from the camera.