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Sometimes it is necessary to make sure all presented stimuli are of the same absolute size (e.g. a square with width/height of 4cm) on different monitors. In order to accomplish this, a calibration procedure is needed to calculate the pixel to (e.g.) mm  ratio on the monitor a script runs on.

Attached to this message is such a calibration script (Inquisit 4). The zip-file includes one script "AbsoluteSizing.iqx" with the actual calibration procedure and an example script "SizingDemo.iqx" to demonstrate how to use the calibration script to produce a square of a particular size (e.g. 5cm).

Here is an overview of how the calibration procedure works (the same information is also included in the scripts themselves):

(1) <include> script "AbsoluteSizing.iqx"
The code of the calibration script "AbsoluteSizing.iqx" can be be included into any other script (e.g. SizingDemo.iqx) using:

<include>
/file = "AbsoluteSizing.iqx"
</include>

All the elements of "AbsoluteSizing.iqx" are then accessible to the script (e.g. SizingDemo.iqx) and do not have to be re-written.

(2) Calibration Procedure: Underlying idea
The general idea of the calibration procedure itself is to adjust a given line on the monitor screen to be as long as the short side of a US letter-sized piece of paper (8 1/2 inches or ~21.6cm ).  The calibration instructions on the screen read as follows:

<item instructions>
/1 = "Take an 8 1/2 x 11 inch paper (US Letter Size) and
adjust the line on your screen to be 8 1/2 inches (~21.6cm) long.

To increase the line: press '>' on your keyboard
To decrease the line: press '<' on your keyboard

When you are done, press <ENTER>."
</item>

(3) Calculation of 'pixel per mm' ratio
This calibration procedure results in calculating the ratio of pixels per mm on the current monitor:

<expressions>
/ratio_pxpermm = values.linelength_px/215.9
</expressions>

Values.linelength_px is the final length of the comparison line in pixels when it measured 8 1/2 inches or 215.9mm.

NOTE: If the pixel to inches ratio should be calculated instead, the expression would need to be edited to: /ratio_pxperinch = values.linelength_px/8.5

(4) Definition of stimuli of absolute sizes
To present a stimulus of a certain size, the stimulus sizes need to be explicitly coded in pixels.

Take, for example, the square of "SizingDemo.iqx".
Under Editable values, the size of the square is defined as:

<values>
/squaresize = 50
</values>

with values.squaresize setting the desired size of the square to 50mm (5cm). Because the calibration script calculates the pixel/mm ratio it is necessary to set the absolute sizes in mm (and not inches or cm or anything else). If the calibration script calculates the pixel/inch ratio instead, the absolute sizes need to be coded in inches.

The square itself is coded as follows. Note the width and height definitions: they are written explicitly in pixel units (px). The product of (values.squaresize x expressions.ratio_pxpermm) calculates the pixel length that a square of 50mm requires on the current monitor.

<shape square>
/shape = rectangle
/width = 1px * values.squaresize * expressions.ratio_pxpermm
/height = 1px * values.squaresize * expressions.ratio_pxpermm
/color = blue
/position = (50%, 50%)
</shape>

And that's it. More information is included into the demo scripts. If you have any further questions, please, let us know.

cheers,
katja from Millisecond Software