Transforming Pictures

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Transforming Pictures

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Transformed images of pictures and drawings provide compelling and revealing ways to view and understand geometric and mathematical transformations. Sketchpad provides a number of ways of transforming pictures and creating objects based on pictures.

Translate, Rotate, or Scale an Original Picture

If an original picture is independent or attached to a single point, you can transform the original in various ways.


If the picture is attached to two points or to three points, you must move the points to which it is attached in order to transform the original. You can still create transformed images of the original using either built-in or custom transformations.

Use the Translate Arrow tool to translate a selected picture to a different location in your sketch.

Use the Translate Arrow tool to scale a selected picture by dragging a resize handle vertically, horizontally, or both. The picture is resized relative to the opposite handle.

You can stretch or shrink the picture independently in the horizontal and vertical directions, changing its aspect ratio (ratio of horizontal to vertical size). Hold the Shift key while resizing to preserve the aspect ratio.

Use the Rotate Arrow tool to rotate a selected picture by dragging a resize handle. The picture is rotated around the opposite handle.

Hold the Shift key while rotating to constrain the rotation to multiples of 15°.

Use the Dilate Arrow tool to dilate a selected picture by dragging a resize handle. The picture is dilated about the opposite handle.

The picture’s aspect ratio (ratio of horizontal to vertical size) remains constant.

Transform a Picture Using the Translate, Rotate, Dilate, and Reflect Commands

Use Sketchpad’s built-in transformations to create a transformed image of a picture.

Select the picture and choose Translate, Rotate, Reflect, or Dilate from the Transform menu. Depending on the transformation you want, you may need to mark various objects or values to serve as the center, distance, angle, ratio, or mirror.

These four transformations, and any transformation that combines them, are called similarity transformations because the transformed image is similar to the original: angles and ratios of distances are preserved.

Transform a Picture Using a Custom Transformation

Use Sketchpad’s custom transformations to create an arbitrary transformed image of a picture.

Use two points, one of which depends on the other, to define a custom transformation.

Select the picture you want to transform.

Choose the custom transformation you want from the bottom of the Transform menu.

If the two points that define the custom transformation are related using only Sketchpad’s built-in transformations, the result is a normal transformed picture, just as if you had applied the various transformations directly to the original picture.

Such a normal transformed picture can be displayed more quickly and with less distortion than a sampled picture.

If the two points that define the custom transformation are related in some other way, the result is a sampled transformed picture.

Sampled Transformed Pictures

A sampled transformed picture is a custom transformed image of a picture in which the custom transformation doesn’t rely strictly on Sketchpad’s built-in transformations of translation, rotation, reflection, and dilation.

Transformed images of pictures constructed using the built-in transformations retain the essential shapes of their pre-images, though their size and orientation may change.

A rectangular area in the pre-image always corresponds to a similar rectangular area in the image.

There is no guarantee of similarity for a transformed image of a picture constructed in other ways. The resulting image must be displayed using a sampling process: The transformed picture is displayed by sampling rectangles within the pre-image and then transforming each sample to create the transformed image. The smoothness and level of detail of the transformed path depend on the number of samples used. To change the number of samples and the allowable distortion, select the transformed picture and choose Edit | Properties | Plot.