Measuring bloodstains with Photoshop

PhotoShop or similar image editor provides a way of measuring a scanned image accurately, but you need Pythagoras...

Using Photoshop to measure simulated bloodstains

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.

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bodmas.org, 26 November 2005