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Frame: frame and image are set to the original size, both resize with locked aspect ratio.
Stretch: image always fills to the frame borders, ignoring aspect ratio. (see Fit: Stretch for more details)
Proportional: image scales proportionally within the frame borders.
Manual: image scale, size, position & rotation are set using the input fields below the preview window. Frame can be resized independent of the image.
Proportional, Outside: same as Proportional, but the imagedata is allowed outside (and cropped by) the frame borders.
Smart Fit: Will fit the asset (image) with contextual data in the image frame.
Original Size: The frame will fit around the original image size.
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