Comparison by reference points applied to 2D drawings in PDF, image or CAD
If you have already used image comparison software by overlay, you will quickly realize the limitations of this software.
To compensate for this limitation, some software allows you to make a comparison by selecting 3 reference points on the first file, then the 3 corresponding points on the second file. The comparison calculates a transformation matrix with these reference points. This transformation matrix applies to both files. It allows better detection of similarities and differences, but, in general, this method of comparison by reference points has 3 defects than the “COMPARE” software doesn't.
The comparison applies to the entire document
To our knowledge, no software allows you to select a part of the image and apply this comparison by reference points only on the selected parts. Software, when it has this functionality, applies it to the ENTIRE image.
It is possible that the title block has not undergone any modification, but that the elements or certain elements present in this title block need a comparison by reference point to best detect similarities and differences.
This functionality is possible in the “COMPARE” tool, which is a strong point of the tool. You select the elements on which you want the transformation matrix to apply.
The need to determine 3 points
This necessity comes from the fact that this functionality has often been developed to straighten and calibrate an image which, for example, has been scanned.
When comparing PDF files, it is very rare that the files to be compared come from a scan. They come from the CAD software which made it possible to create the digital model. On the other hand, it is very possible that the elements between the two versions of the digital model production were not placed in the same place, with the same orientation or the same scale. In this case, being able to apply a comparison by reference point on these selected elements is one of the strong points of the “COMPARE” tool.
But when there is only one TRANSLATION between the 2 files, it is not necessary to specify the 3 points, 1 point on each of the files is sufficient (this is 90% of cases).
Likewise, if we have elements in TRANSLATION which have also undergone a ROTATION or a CHANGE OF SCALE in one axis or in the 2 axes, simply specify 2 points on the 2 files.
The 3 points are only necessary when we have TRANSLATION + ROTATION + AFFINITY (which is extremely rare, less than 1% of cases).
Tolerance
When software has this possibility of comparison by reference point, it is not possible to choose the tolerance. The choice of this value is an important point because the result obtained by applying the transformation calculated from the reference points does not correspond exactly to the graphic data in the file and the tolerance thus makes it possible to better take into account the precision of these calculations.