Tracer

Tracer is a utility for digitizing data from images of graphs (which could be screenshots, scans, or graphics copied from another application and loaded from the clipboard). The interface includes tools for setting the image scale so that coordinates match those of the graph in the image. In X-scan mode, data are created in an attempt to follow a trace from left to right across the image between endpoints marked by cursors, and the data are extracted at the resolution of the image (one data point per pixel). In equal step mode, a step-length (number of pixels) is selected and the tracing is accomplished by scanning pixels in an arc with a radius of the chosen step-length. When the trace does not form a continuous path between the cursors (owing to poor image resolution, crossing lines, or dashed or dotted line types), the procedure attempts to search for the next line segment. Tracer distinguishes between different trace colors.

In addition to the automated data extraction methods, you can use Tracer to digitize data as individual points picked manually from a graph image, or to draw curves over the image and turn the curves into waves. Tracer has two methods available for curve drawing: the curve option found in the Manual Points section, and the Bezier to Wave method which transforms Bezier drawing objects into waves.

For manual point picking, you might also want to check out the Wavemetrics procedure IgorThief (Look under File > Example experiments > Techniques > Trace Graph), and KZarzana's updated version, IgorThief2, which is available here on IgorExchange. The two IgorThief procedures can correct for image skew, while Tracer is limited to images where the graph axes are parallel to the image edges.

How to use Tracer to extract a trace from an image:

• Select Tracer from the Data menu.
• Load and display an image using the buttons in the Image Selection area of the Tracer panel. The load dialog also allows an image to be loaded from the clipboard.
• Unfold the 'Set Scale' controls to add two pairs of horizontal and vertical cursors to the plot. Drag the cursors to known positions at the ends of the plot axes, then type the axis values in the SetVariable controls. Make sure the cursor positions are accurately reported before moving on.
• Output: Select a name for the wave that will be created with the extracted data.
• Select trace color in the Extract area of the Tracer panel. To sample colors from the image, look for the eye dropper icon in the color-picker dialog.
• The 'fuzziness' slider affects how picky the tracer is about colors. You may have to adjust this setting for tracing to work.
• Place cursors A and B at start and end of a trace and click on 'Extract Trace' to start digitizing. If you hold down the control key (command key on Mac) as you move a cursor the target color will be set to match that of the cursor pixel.
• Play with the gaps, jump threshold, and minimum range settings (or step length, sweep angle and maximum gap settings for equal step mode) to deal with incomplete lines and other problems.
• If you're tracing a series of open symbols, use the minimum range setting to avoid tracing the symbol stroke.
• If needed, you could use interpolate to clean up your traced data.

If you want to help Tracer to take the right path where traces cross one another, you can use the image editing button to touch up the image. Hold down the control/command key and click with the mouse to paint pixels with the selected color. Expand the graph view for more detailed editing. Use command/ctrl and scroll to zoom in and out, hold the shift key to accelerate zoom.

If tracing fails, be sure to try changing the fuzziness, jump threshold/sweep angle, and gap settings.