Relatively simple perspective question

[Marvin Mattelson]

The simple answer here is that you can't. As planes recede into the distance they get forshortened. So on a cube positioned such as yours The length of the vertical (A) will always exceed the length of horizontal (B or C) going into perspective. If you objectively look at the "cube" you illustrated, it looks more rectangular. Sorry.

[Ken Smith]

This does look rectangular now that you mention it.

Just to make sure I'm explaining myself clearly, I understand that the B&C lines will always be shorter than the A line due to foreshortening. But what I'm wondering is: is there a method to precisely define the length of the B&C line in relation to the A line (while taking the foreshortening into account), since they're all supposed to be 4 inches long? All the perspective stuff I've read (except for plan perspective) seem to define the length of foreshortened lines only in relation to themselves (as in finding half distances).

I got into this question when I started to draw a rectangular background object (in perspective) that I knew was 46 inches tall and 6.5 ft. long. The 46 inches tall was no problem, but I couldn't figure how to draw the 6.5 ft. length accurately, in perspective, relative to the 46 inch height.

Is this still a "can't be done" situation?

[Allan Rahbek]

Hi Ken,

This is easy. You draw your cube in scale 1:10 on a paper. Then you mark your point of view and draw the lines, and measure.
Allan