D. Line spacing measurement Back top | MacLIspix Home

The human eye is very sensitive to straight lines. For measurement, a sensitive algorithm is needed to detect them. A line profile one pixel wide, that is with no averaging, does not show them, whereas a 'fat' line profile does. For the 'fat' profile, pixels are averaged along the width of the rectangle. The line profiles are plotted from zero to the plot maximum, from the original image values (not the scaled image values). In this case, due to the 'washed out' appearance of the original image, the profile plot will be displaced to the top of the graph.

To see a normal single pixel line profile, invoke the MLx -> Show Values -> Line Profiles command.

Move the cursor to the position on the window (which does not need to be in front, but the desired portion of the image must be visible and not covered with another window) and click and hold the mouse button.

Drag the cursor to the other end of the line for the profile, noting the rubber band line that marks the position on the image.

Release the mouse button.

Press the command key. If there is more than one image array in memory at the moment, a dialog will appear for choosing the array(s) or image(s) to plot -- choose the *SRM484* array (note that the file name, window name and array name all correspond). A profile plot will appear in a new window.

To see the averaged profile plot, or 'FAT Line plot', repeat the steps above:

Move the cursor to the starting position of the line. Drag the cursor to the other end of the line. Release the mouse button.

Hold down the mouse button and drag the cursor across the rubber band line. The sides of a rectangle should appear, and be drag-able by the cursor. Note also that the line endpoints are also drag-able, or adjustable. If the end points are moved, then the rectangle may become a parallelogram and the pixel averaging will be along slanted lines that are parallel to the ends of the rectangle. To straighten out the ends, click one side or the other and drag it a little bit.

When the rectangle position is satisfactory, the ends being as parallel as possible to the calibration lines in the image, hold down the command key and proceed as above.

(Optional) If it is desirable to used the scaled image for plotting, which might be the case here because it is the feature positions that are of interest, rather than the numerical pixel values, then the scaled image array can be used in place of the original. The scaled array, which is used normally only for display purposes, must be given a name in its own right, using the MLx -> Image Windows -> Name Array -> Scaled command. MLx -> Pixel Array Ops -> Info command shows what image arrays are accessible, and will then show this new array with the name that you gave it. This new image array may be displayed and treated as the original image array was.

Points in the profile plot can be related back to the image by clicking on the profile plot. Circles will appear on the center line in the image at the appropriate position, and the coordinates and image pixel value at that point will be printed in the Listener window.

Move the plot window or image window so both can be seen.

Click on on one of the peaks in the plot profile window.

Note the small circle at the intersection of the center yellow line and one of the calibration lines in the image window.

The circles are erased in the image window whenever the window is redrawn, such as when one clicks on the lower right corner. When the yellow lines marking the area of the profile plot are no longer desired, they can be erased from the front image window using the MLx -> Graphics -> Clear command.

Next (background subtraction)