7.4.2.2   Extract Edges

After you have prepared the measure task, you continue to choose parameters on the Edges tab such that you can detect the edges between which you want to measure:

• On the tab Edge under Edge Extraction you can specify the parameters that are used to extract edges.

• Edge Selection allows you to group edges to pairs or choose edges with certain features.

• In order to improve the visibility of your ROI and edges, you can change the display parameters.

Now, you can select edges you want to measure and afterwards proceed to the Results tab to view your measuring results.

Open the Line Profile window to inspect edges along an ROI. If necessary, that is, if your edge extraction is not as successful as expected, you can refine the determination of the edges you need for measuring by using the Fuzzy Measure option you find on the Fuzzy tab. Fuzzy measuring allows you to specify certain ranges within which edges are labeled “good”.

Edge Extraction

You can optimize edge extraction by adapting the following parameters on the Edges tab:

• With Min. Edge Amplitude you can specify how strong an edge must be to be selected. Adapt this parameter such that only your desired edges are selected. Note that very small values should only be used with high quality images, otherwise false edges might be caused by noise.

• Smoothing is helpful to reduce noise and therefore the detection of false edges. Please note, however, that smoothing distorts the edge profiles, i.e, edges are detected at a slightly wrong positions and the accuracy decreases. When using the line profile tool, smoothing can either be applied with the Measure Assistant or it can be performed with the smoothing slider within the line profile window. The smoothing values are immediately transfered to the Measure Assistant and the line profile window, respectively.

• With ROI Width you can specify how many pixels perpendicular to the line or arc of measuring are used to detect edges. A larger value helps to reduce noise; however, if the edges themselves are not perpendicular to the line or arc of measure you must choose a smaller value, otherwise the edge will “loose its strength”. The general rule here is to choose the ROI always perpendicular and as wide as possible. This will lead to both a high precision and accuracy. If changing the ROI width leads to an edge length that is inconvenient for the display, for example, because it is very short or very long, you can choose another length for the display instead of the ROI width under Display Parameters.

• With the Interpolation Method you influence the accuracy of the edge extraction, but also the processing time. Processing time and accuracy both increase from nearest_neighbor over bilinear up to bicubic. Activating Highest Accuracy influences the way the interpolation is calculated, it takes a bit more processing time and also slightly improves accuracy.

If one or more edges cannot be found without further processing, or if false edges are detected, the Line Profile which can be opened with the View Line Profile button on the upper right edge of the Edges tab can provide the missing details to solve the problem. In its display, the line profile shows a green line with a gray top for each successfully detected edge that is above a certain level to qualify as an edge. Potential edges that do not qualify as edges are displayed as vertical red lines. It may also be helpful to view the line profile along an ROI while changing parameters for edge detection, to see exactly how your actions influence the line profile and therefore possibly change which edges are detected. This profile can consequently be a basis for the decision which steps should be taken next, that is, what would be necessary to enhance weak edges or suppress false edges.

In order to select which edges or edge pairs from your current results are relevant, proceed to Edge Selection. If you are already satisfied with the detected edges, continue to the Results tab.

Edge Selection

Edge Selection allows you to specify what kind of edges you are looking for. First you decide whether you want to find single edges or edge pairs. Depending on your choice, you have different options for the drop-down menus Transition and Position. These options will be explained for edge pairs in the following paragraph and subsequently for single edges.

You can activate Group Edges to Pairs to find an even number of edges with alternating transitions. The parameter Transition lets you select which edges are chosen to resolve ambiguity: The default setting of all will always accept the first matching edge. The values positive and negative restrict the transition of the very first edge. The *_strongest variants will pick the strongest edge instead of the first edge in a sequence of edges with the same transition. Position lets you decide whether you are looking for all edge pairs, or just want to detect the first or last edge pair.

If you are looking for single edges (instead of edge pairs) you can choose with Transition whether you want edges with all transitions (positive and negative) or just positive or just negative. Using Position, you can then determine whether you are looking for all edges or just want to detect the first or the last edge.

When you are satisfied with your selection, you can view your measuring results on the Results tab.