Tuesday, October 23, 2007

Let me count the ways ...

Oh how I love the measurement tool ... let me count the ways.

Many in the forensic realm have heard of Photogrammetry, but have shied away from trying it for fear of that dreaded word ... math. Never heard of Photogrammetry? In a general sense, its a method used to determine the geometric properties (ie. height, width, distance, etc) of an object in an image. It is far more complicated than the simplistic definition that I've offered, but I think that you get the point. I bring this up as a way to introduce the new Measurement feature in Photoshop CS3 Extended.

So lets say that you need to measure an area or an object in an image. No problem, right? You should be able to get that using the info palette, right?. Well, almost ... What happens when you want to track and log the measurement data for forensic purposes?. Still want to use the info pallet? No? No worries, Photoshop CS3 Extended is here to help.

You can measure any area defined with one of the Selection tools, the Ruler tool, or the Count tool. The measurement feature in Photoshop CS3 Extended allows you to compute and track data points, such as height, width, area, and perimeter. It conveniently tracks the measurement data in the Measurement Log palette, which you can customize to display the information you want. 

Important Tip: before you start to measure, set the measurement scale to specify what you want X number of pixels to represent in units, such as inches or millimeters (damn that metric system). An especially cool feature lets you place scale markers on an image to display the measurement scale.

Want to give it a go? Let's try it together.

Click Analysis>Set Measurement Scale>Custom.

Enter the pixel length and logical length, and then specify the logical units. You will have to have some info from the scene. For example, if a desk in the scene measures 42" from floor to its top, and the ruler measures it as 237 pixels in the image, then your pixel length would be 237, your logical length would be 42, and your units would be inches.

(If you have a lot of similar images to process, you can save the measurement scale as a preset. Click Save Preset, type a name, and then click OK.)

When all of this is finished, click OK.

Placing a Scale Marker in your image

A scale marker can be a helpful visual reminder of the scale used in the measurements.

Analysis>Place Scale Marker.

Enter a number for the length of the scale marker in pixels.

To show the logical length and units for the scale marker, select the Display Textcheck box.

Click the Bottom or Top option to specify where you want the text caption. It's your choice here. Use your best judgement based on what's relevant in the image.

Click the Black or White option to set the scale marker and caption color. If your image is dark, you'll want to click white. If your image is light, you'll want to click black.

Click OK.


Open a document.

Click Analysis>Select Data Points>Custom.

Select the check boxes next to the data points you want to measure and track for the different tools. You may only want to check width, length, or height.

Click OK.

Select the Ruler tool in the toolbox, and then click and drag the tool to measure what you want.

Click Window>Measurement Log to open the Measurement Log palette.

Click Analysis>Record Measurements to record the count to the Measurement Log.

When you are finished, click on Select All from the palette's fly-out menu. Then click on Export Selected. This will save your results in a text file that can be imported to your word processor or spread sheet application.

Give it a try. It's accuracy depends on accurate measurements of known objects at the scene. Also be aware of the camera's angle and height relative to the scene. There may be a keystone effect happening in your image that may effect the accuracy of your measurements.

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