In the previous examples, the panoramic scene was constructed by taking each of the original flat images, cutting them along appropriate lines, warping them according to cylindrical projections, then abutting the resulting warped images. Even if all of these images are perfectly alignes, the seams between adjacent images may still be apparent, especially where there are exposure variations between succesive images.
Instead of abutting warped images, smooth transitions from image to image may be achieved by use of the "Blend" function. This option can be turned on/off by clicking on the "Blend" button in the upper part of the main window. This button remains depressed as long as the "Blend" option is selected. Clicking on this again turns the "Blend" option off and restores the button to then unselected state.
The results of the blending operation are generally superior to the unblended option, but the blended panoramic scenes take longer to update than the panoramas based on unblended (abutted) images. In addition, new problems can arise from the selection of the blended panoramas. Consequently, the initial panoramic scenes are chosen to be unblended. This speeds up the processes of aligning the original images and balancing the exposures, and allows any special problems resulting from image blending to handled after the stitching and exposure balancing conditions have been resolved.
When the blending function is selected, the image in the main window may look like the following example:
If no new problems arise as a result of selecting the blending option, one may continue with the final stage of creating a panoramic image: Saving the results. Otherwise, consult the following material for descriptions of various possible blending problems and techniques for handling these peoblems.
Notice that, like the corresponding unblended panoramic scene image, this image exhibits black "fillets" above and below the seams between adjacent images. These fillets can be removed by reducing the upper and lower field of view angles before saving the final panoramic image.
Return to Blending images (top).Even though the blending process is very simple and reliable, there are particular problems that may arise as a result of blending images instead of simply abutting clipped images. These problems can include:
"Ghosts" occur when something in the overlapping area shared by a pair of adjacent images moves or changes between shots. This is especially common for the overlap between the first and last shots of a complete 360 degree panorama. Several minutes may elapse between the first shot and the overlapping last shot, and it can be difficult to freeze everything in the scene for that length of time.
Obvious things that move can include people, animals, and vehicles. Other things that move or change include clouds, blinking lights, furniture, doors, windows, drawers, trees, bushes, and water. Sometimes ghosting can be considered to be a useful artistic effect, suggestive of motion, but usually it is something to be avoided or eliminated.
Fine detail registration problems arise from details like the blades of grass in a lawn, sand on a beach, the texture of pavement, the wood grain of lumber, etc. Even if nothing moves, the camera was properly mounted, and the images are carefully aligned, fine details like blades of grass never register exactly. This results in a fuzzy band where the images overlap. In many cases, it may look better to use abutted images instead of blending adjacent images.
Area registration problems can be caused by parallax errors resulting from using a hand-held camera, or rotating the camera about an axis that does not pass through the nodal point of the lens. As a result of such parallax errors, a horizontal feature, for example, may be seen as diagonal lines have differing slopes in each of a pair of overlapping adjacent images. For simple abutted images, the adjacent images, as clipped, must be registered only along their shared clipped edges. In this case, registration is limited to a 1-d condition along the vertical registration line. The diagonal lines representing a horizontal feature will cross this vertical registration line at a single point, and parallax errors may appear as a small kink where the diagonal line changes slope where it crosses the vertical registration line.
For blended scenes, adjacent images require 2d registration where features must match over a 2d area instead of a single point. In this case, parallax errors can cause the edge of a horizontal feature to appear as a fuzzy "x".
Return to Blending images (top)One way to solve blending problems is to turn off the blending option. This approach may lead to a panoramic image with half of a person cut off along the seam shared by a pair of adjacent images.
A better means of resolving blending problems may be achieved by double-clicking on any part of the panoramic scene in the main window. This action opens an Image Edit window such as the example shown below:
The Image Edit window shows black regions along the top, bottom, right side, and left side. The edges of these black areas define the limits of the blended areas. These are called the blending boundaries. Everything outside these edges, that is, the portions of the original image covered by the black areas, are ignored by the blending process, as if the original images had been trimmed to the indicated boundaries.
The black areas have a zero width until the blending option is selected for the first time. The thin vertial lines at the right and left indicate where the same image would be clipped to form an abutted image if the blending option were not selected. These lines are called the clipping boundaries. When the blending option is selected for the first time, the blending boundaries are adjusted so that the black areas cover half of the area outside the clipping boundaries.
The amount of space covered by the black areas can be controlled by placing the cursor along any of these edges. When placed on the edges of a black area, the cursor will change to a fat white double arrow. The fat white double-arrow cursor can be used to drag the edges of the black areas to make them larger or smaller.
When the black areas have zero width, the blending boundaries coincide with the original image boundaries. In this case, placing the cursor slightly inside the window boundary should select the fat white cursor. When the cursor is on the image boundary, the cursor changes to a black double arrow that can be used to resize the image window.
Placing the cursor on the clipping lines changes the cursor to a thin white double arrow. The thin white double arrow can be used to drag the clipping lines, thereby altering the image alignment. Be careful not to drag the clipping lines if the adjacent images are already properly aligned.
Dragging the blending boundaries "out" towards the edges of the original image increases the area used to blend adjacent images. This softens the blending, but may exacerbate blending problems. Dragging the blending boundaries "in" towards the image center reduces the area used to blend adjacent images. This results in a harder or sharper transition from one image to the next.
Return to Blending images (top).