Warp

This transformation lets you rotate, crop, perspective correct, stretch, and warp an image. It works by stretching the four corners of a warp region in the input image out to the four corners of the result image, distorting the interior of the image accordingly.

Positioning the Warp Region

The warp region is displayed as an overlay on the input image; you position the warp region by dragging its corners, sides, or interior. The interior of the warp region is filled with a reference grid which is particularly useful for perspective correction. Lines in the input image that parallel horizontal or vertical grid lines in the input image will become perfectly horizontal or vertical in the output image. The number of grid divisions can be increased or decreased (see below).

The idea is to line up the grid with the parts of the image you want to make vertical and horizontal. The input image will then be warped accordingly.

If you hold down the shift key while dragging one of the corners of the warp grid, it will only move horizontally or vertically.

Proportions Slider

This slider lets you adjust the proportions of the output image. This proportions are initially set to the proportions of the entire input image. Altering this setting lets you make the output image taller and narrower or shorter and wider. Changes you make are reflected in the width and height readouts (see below).

When you apply perspective correction to an image, it is often necessary to adjust its proportions to obtain a natural appearance. If you click Auto Proportions, Picture Window adjusts the proportions slider automatically so as to preserve the proportions of the center part of the input image. This usually works well, but you may can also adjust the Proportions slider if necessary.

Scale Factor Slider

This slider lets you adjust the ratio of the area of the output image to the area of the warp region. It is initially set to 1:1. Altering this setting lets you increase or decrease the dimensions of the output image. Changes you make are reflected in the width and height readouts (see below).

Width, Height, and Resolution

These settings display and let you enter the dimensions and resolution of the output image.

Grid Divisions

This control lets you make the grid overlay on the input image finer or coarser.

Settings Menu

Rigid -- causes the warp region to scale and rotate about its center as you move any of its four corners. Use this option to rotate or scale part of the input image while preserving its proportions. When this setting is in effect, dragging a side of the warp region is disabled.

Rotate Only -- causes the warp region to rotate about its center as you move any of its four corners. Use this option to rotate part of the input image while preserving its proportions. When this setting is in effect, dragging a side of the warp region is disabled.

Free -- lets you adjust the four corners of the warp region independently. Use this option to skew, perspective correct, or warp part of the input image.

Perspective -- warping is slower than bilinear warping but more accurate, especially when making large perspective corrections. A true perspective warp keeps straight lines in the input image as straight lines in the output image, regardless of their orientation. A bilinear warp only preserves straight lines that are parallel to the grid lines.

Tips

Rotating an Image

To rotate and image without stretching or distorting it, select Rigid or Rotate Only from the options menu. This will restrict the warp transformation to just scaling, rotating, and cropping. A useful technique for correcting for camera tilt is to align the grid with the horizon. Adjusting the number of grid divisions can make this easier in some cases.

Perspective Correction

To correct the perspective of an image, position the four corners of the grid so that the grid lines are parallel to the horizontal and verticals of the building or other object you want to correct. Adjusting the number of grid divisions can make this easier in some cases. Click the Auto Proportions button when you are done to set the proportions. You may need to further adjust the proportions using the Proportions slider to make the result look more natural.

Blur before Reducing an Image

When you use the warp transformation to decrease the file size of an image, it may introduce artifacts such as moiré patterns, especially if the input image contains any kind of repeating pattern. To avoid this problem, use the blur transformation on the input image before warping it.

Sharpen after Enlarging an Image

When you use the warp transformation to increase the file size of an image, it may reduce its sharpness slightly. To compensate for this effect, use the Sharpen transformation on the output image.

Leave extra room around your subject when you take the photograph

When you warp an image, you always lose part of it. So, if you anticipate needing to warp an image of a subject -- especially if you will warp it a lot, make sure you leave enough room around the subject to allow for the cropping that will necessarily occur.