Calibration

Brief description: In this section you will learn how to eliminate distortions using calibration and four-point correction.

Calibration is used to eliminate arbitrary (both linear and non-linear) distortions of monochrome, grey-scale and color raster images.

IMPORTANT: You should calibrate the raster image before applying vectorization (tracing), because if you vectorize a distorted image you will obtain distorted vector data that is much more difficult to correct later.

The original image must contain points with known coordinates. These points are called real points. You can use nodes of a rectangular grid as real points (if you work with mapping materials) and/or other points, whose coordinates are known or can be calculated.

The points that correspond to the real ones on the distorted image are often located with some displacement (i.e. they have different coordinates). These points are called measured points, as their coordinates value can be measured on the raster image.

By applying calibration, the raster image is transformed so that the measured points coincide with, or are extremely close to the real points.

A transformation method is a parametric transformation used for calibration. Each model defines a family of transformations of the same type. There are several transformation methods: affine, bilinear, polynomial, spline, etc.

To work with a set of calibration pairs WiseImage applies an AutoCAD special object, hereafter referred to as the calibration object. Preparing for calibration means creating and editing this object. A list of calibration object properties is available for viewing and modification in the Properties dialog box of AutoCAD.

How to calibrate an image:

Choose Calibration from the rImage menu.

Defining real points

We can define rectangular grid nodes and points with known (estimated) coordinates.

Step 1. Defining grid: A grid is defined by three parameters: start point (coordinates of lower-left node), cell size (by X and Y axis), and grid size (by X and Y axis).

• Press Define Grid on the Calibration toolbar. In the Calibration Grid dialog specify:
  • the number of grid cells along the X and Y axis;
  • the base point;
  • the cell size.

Step 2. Specifying measured points

• Press the Repick button on the Calibration toolbar.
• Specify a point on the image.
• To move to the next point press Next Point . WiseImage places the next point in the center of the screen and highlights it with grips.

You see the real (correct) location of the first point with grips.	Drag the grips to the grid node on the scanned image - this specifies the “measured” point.

Specifying points with known coordinates:

Press the Add point button on the Calibration dialog. You can enter a name for the points in the Label field.

Step 3. Choosing calibration method

Choose the Calibration Method in the Properties dialog with Calibration Entity set to on.

How to choose:

• If you are sure of the correct method to use, choose it from the list. You can estimate the accuracy of this method by pressing the Estimate button.
• If you are not sure of the correct method, select Choose automatically.

Step 4. Running Calibration

Run calibration by pressing the Apply button on the Calibration toolbar.

If you have more than one raster image in your document, select the image(s) to calibrate.

Four-point correction

You can use this operation for quick correction of scanned technical drawings, which contain a frame. This operation assumes that both frame and contents of the image have the same distortion.

How to correct an image by 4 points:

• Choose 4 point correction from the rImage menu. 

Enter the frame size in the Width and Height fields of the 4 point correction dialog. You can also choose a standard frame size from the Frame Size list, or automatically find the closest by selecting Find Closest paper

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