Have you ever tried taking a photo of your pattern pieces and tracing them in software, and the dimensions are not quite right? This tool will help you correct the perspective of your photo to assist making accurate traces digitally, for example in Inkscape or Adobe Illustrator. Only corrects 2D perspective, not 3D objects.

Might have bugs, let me know!

For best results photograph your object on a grid, or on a object with 90 degree corners with known dimensions.

• Add 4 points to your image in the following clockwise order: top left, top right, bottom right, bottom left.
• Try and make the rectangle as big as possible for the highest accuracy. You can click and drag the points after the rectangle is drawn.
• Specify the known width and height of what you have selected. The units are dimensionless.
  For example, so if you have placed the object on a grid, count the grid squares. If the object is on a A4 piece of landscape paper, you could place your points around the paper and use 290 x 210 as your aspect width and height.
• Save, then load your image into Inkscape, Illustrator etc and scale to the actual dimensions, and trace!

Double check measurements before applying. The calculation can do some crazy distortions if you make mistakes, or the rectangle you draw is too small to be accurate, which might cause the script to hang for a period of time.

Drop me a message if there are issues or ways to improve. I know there is a algorithm that can estimate the perspective correction just from the drawn rectangle without needing the user to specify the aspect ratio (as used in document scanner apps), but I’ve not been able to implement it yet. The method I use here should be more accurate anyway however as there is no estimation, the user specifies the known width and height.