References to objects such as “From”, “Midpoint between 2 points” and “Extension” allow us to understand how Autocad can indicate points that do not exactly match the geometry of existing objects but can be derived from it, an idea that programmers have used to design another drawing tool called "Point filters" that we can illustrate right away.
Suppose we have a line and two circles on the screen and we want to draw a rectangle whose first vertex coincides on the Y axis with the center of the largest circle and on the X axis with the left endpoint of the line. That implies that the first point of the rectangle could have as a reference points of both objects, but not touch any.
To take advantage of references to objects as an allusion to values for the independent X and Y axis, we use the “Point filters”. With these filters, a geometric attribute of an object - the center of a circle, for example - can be used to determine the value of X or Y from another point.
Let's go back to the rectangle, the line and the circles on the screen. We said that the first corner of the rectangle that the command window asks us for coincides in its X coordinate with the left end of the line, so in the command window we will then write “.X” to indicate that we will use a reference to objects but only to indicate the value of that coordinate. As already stated, the value of the Y coordinate coincides with the center of the major circle. To use this point filter in combination with the reference to the object, press “.Y” in the command window. The opposite corner of the rectangle coincides on its X axis with the other end of the line, but on its Y axis with the center of the smaller circle, so we will use the same point filter procedure.
In many cases, we may only use a point filter and an object reference only for the X coordinate, and for the Y coordinate we give an absolute value, or an absolute value in X and a Y-reference filter. In any case, combined use Filters and object references allows us to take advantage of the location of existing objects even if they do not intersect or fully match their points with other objects.