Strip Densitometry

See also Spot densitometry. For a tutorial, view the image file DENSITO2.TIF. Strip densitometry is not as exciting as it sounds. It is similar to spot densitometry, but is easier to perform and interpret because it is not necessary for imal to find the edges of the object.

To obtain a densitometric tracing of a trapezoidal or rectangular region, select ``Strip densitometry'' from the ``image'' menu. The options are:

Coordinates

If `select coordinates' is checked, it is necessary to select the region to scan each time.

Repeat prev. scan

If `repeat previous scan' is checked, the same region as the last time will be re-scanned without the necessity of selecting it. This is useful if you modified the image in some way, such as by filtering it, in order to see the effects on scanning.

Maximum signal

Selects whether a ``black'' or ``white'' pixel is to be regarded as the most intense value.

Scan type

This option determines what type of region to select for scanning. Selecting `Rhomboid, 90${^\circ}$(4 pts)' causes the starting and ending edges of the scan to ``snap'' to either vertical or horizontal, whichever is closest. This is fastest because it does not require any of the anti-aliasing calculations which are needed when scanning diagonally. However, if the image contains objects that are oriented diagonally, some resolution would be lost in the scan because the objects will be scanned at an angle to their true orientation.

The first two points always determine the orientation of scanning. Scanning is always done along a line parallel to these 2 points. The 3rd and 4th points determine the direction and shape of the region to scan. The area being scanned is indicated by a sweeping wave of temporarily inverted pixels.

Selecting Rhomboid (4 points) allows selection of any trapezoidal area with no restrictions on the starting angle. However, the angle of the far side of the trapezoid is always adjusted to make it parallel to the starting side. This avoids confusion that would otherwise be caused by having a small `tail' at the end if the starting and ending sides were not parallel.

Because of the grid nature of the screen, this option is slower because it is necessary to correct for `aliasing' caused by pixels being at unpredictable distances from the starting point. To compensate for the slower speed, the pixels are no longer inverted as they were in earlier versions of imal.

Fixed width only requires two end points instead of 4. Densitometry is performed in a rectangular region between the two points using the specified width in pixels (which can be 1 to 200). If the region is not perfectly vertical or horizontal, anti-aliasing is performed.

This method is the most useful for obtaining results which need to be compared to each other, or when the density of a very narrow region (such as a line) is desired. Of course, a narrower region will tend to be noisier than a wide one.

Fixed width densitometry is slower than 90${^\circ}$rhomboid scanning because more floating point calculations and memory accesses are needed. However, it is more accurate. Using fixed-width densitometry with a width of 1 gives a ``transept line''.