In situations where the Reference Scale cannot be used because the object of interest is captured with some perspective distortion, the Perspective Reference can be used. It allows measurements on flat objects and works with any camera capture angle, but needs a reference rectangle to be specified.
This method is useful if you want to measure many things that are on the same, flat object.
The first example (Figure 1) shows a kitchen wall during construction. We want to make a plan of the location of all the sockets. In this case, we can take the whole wall as the reference rectangle. We have to measure the real width and height and enter them into the reference rectangle (green color), but all remaining measurements (yellow) are computed automatically based on the given wall size.
Note that the wall is perspectively distorted. The wall height in the image is smaller on the left side than on the right side of the image. However, ImageMeter does compensate for this effect. Note the grid that ImageMeter draws whenever the reference is selected. It provides a visualization of the plane on which ImageMeter carries out the measurements and can be useful to adjust the reference rectangle accurately.
In Figure 2, we use a frame that has been cut out of a sheet of A4-sized paper as reference rectangle. As the size of an A4 paper is known as (29.7cm x 21cm), this can be a handy reference. Based on this reference, ImageMeter calculates the side lengths of the complex stone shape and its area, which might be useful to estimate the number of stones needed to pave a given area.
You can use any kind of object as references as long as it is rectangular. For measuring houses, windows are a good option. You can also pin a sheet of paper on the wall and use this. However, you should choose the reference as large as possible to increase accuracy. The reference even does not have to exist physically. Just four points that form an imagined rectangle will work.
Note that you can only measure things that are on the same plane as your reference rectangle. For example, you cannot take the door of your house as reference and then try to compute the length of the walkway to your door.
In Figure 3, the yellow rectangle in the center is taken as the reference. All remaining measures have been computed based on that reference. The measures that were computed correctly have been colored green, while the red measures are wrong (these have been manually colored, ImageMeter has no way to detect this automatically). Also note the superimposed grid that visualized the reference plane (you may want to view the image at full resolution).
Note that the two 75cm measurements both show the same distance even though their size is very different in the image. This is because of the perspective distortion that ImageMeter takes care of. The same for the two 50cm measures. Any measurement that is carried out on the top of the object will be correct, taking this distortion into account.
The two measurements in red color are wrong as they are not in the reference plane, but ImageMeter bases its calculation on the assumption that they are within that plane.