bodmas blog

See a previous post on simulated blood stains for the context of this experiment. we are testing the validity of the formula θ = sin^-1(w/l) where l and w are the length and width of an ellipse fitted to a blood stain.

• Set Preferences -> Units & Rulers from the Photoshop preferences menu
• Open the info swatch by selecting Window -> info from the Photoshop Window menu
• The info swatch follow you mouse and shows the current screen coordinates relative to the top left of the image
• Dragging a line or Arrow Pointer (latter on Photoshop not Elements) will show the width and height of the distance moved from the starting point
• In effect, you get the X and Y coordinates of the two points on the width of the simulated blood stain
• Use Pythagoras to get width: width = √(W² + H²) where the W and H are read from the info swatch

Scanning direct from the simulated bood stains (paper in a plastic wallet to protect the scanner!) at 1200 dpi gives many hundred pixels on the width measurement, thus errors better than a vernier caliper.

This entry was posted on Saturday, November 26th, 2005 at 26/11/05 and is filed under Forensic, Maths. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.