hugin provides two pop-up windows showing previews of the final stitched result, the difference between these previews that the Hugin Preview window shows a more accurate image but takes time to redraw, the Hugin Fast Preview window shows updates almost instantaneously and has some useful new features.

The preview window is a smaller version of the output created by saving to a file in the hugin Stitcher tab with some differences:

Reduced resolution input images are used, so some areas can appear blurred that will be sharp in the final output.
Seams are not created, images are simply overlaid with the first image at the bottom of the stack and the last at the top.
Blending by a tool such as enblend isn't shown.
The brightness display of HDR and 16bit images is controlled by settings in the hugin Preferences, these settings are not used when stitching.

The preview window shows the result of panorama creation with the settings chosen by the software. Through the various buttons and slides of the preview window this settings can be changed according to one's taste and needs. This changes will affect the way the final panorama will be created.

When satisfied with the preview result, the window can be closed. Attention must be paid not to close the preview window with the preview window not showing what the user wants.

Buttons

Center

This button horizontally pans the output, changing the yaw of the remapped images so they fit to the centre of the output frame. This is useful if there is a lot of black space on the left or right of the output. This also performs a Fit, equivalent to the next button.

Note that centering a Rectilinear Projection or Fisheye Projection panorama will change the perspective, this may be unwanted.

Fit

This doesn't change any input image parameters, it just readjusts the output Field of View such that all the input images are visible. If the images are all off-centre, then there will be a lot of black space.

Straighten

Straightening the panorama optimises the roll and pitch of the input images without changing their relative positions, levelling the panorama in the process. This normally produces good results, if you need more accurate positioning, try adding vertical control points in the hugin Control Points tab.

Num. Transf.

This opens up a pop-up window that allows you to enter a numerical transform to rotate the input images without changing their relative positions - Effectively this rotates the entire panorama.

Enter roll, pitch and yaw values in degrees, note that you can also realign the panorama by left and right-clicking in the preview image itself, though this is not so precise.

Auto

Setting Auto will regenerate the preview every time something changes in the project. This can be slow with large preview windows, so you may want to disable Auto or reduce the size of the window.

Update

When Auto is unset, the preview doesn't regenerate when something changes in the project. Click Update to force a regeneration manually.

All

By default all input images are shown in the preview, however individual images can be enabled and disabled in the Displayed images section. Use the All button to return to the default and display all the images.

None

Similarly, hide all images with the None button, use this if you want to enable preview images one by one.

displayed images

Every input image in the preview has toggle button where display can be disabled or enabled.

Preview

The image window itself shows a representation of the final stitched output panorama, use the scroll bars to change the horizontal and vertical Field of View.

A Left-Click with the mouse in this image window will shift all the images, without changing their relative positions, such that the clicked point becomes the centre of the panorama.

A Right-Click with the mouse will rotate all the images, without changing their relative positions, such that the clicked point is level with the central horizon line.

Preview options

Projection

Use the drop down list to change the output projection of the panorama, the list is exactly the same as that in the Stitcher tab. Note that for some projections, the scroll-bar sliders to change the Field of View are disabled. If you are having trouble, switch to Equirectangular Projection, adjust the field of view and switch back.

Blend mode

normal blend mode simply shows all selected images as a stack.
difference blend mode highlights differences between overlapping images. If the images are well aligned, there will be few differences and the overlap will appear mostly black.

Output

Changing the Output here tells hugin if you intend to create a low dynamic range (LDR[*]) or high dynamic range (HDR) output panorama - If in doubt select LDR as this is the format of a normal photo, HDR images are only viewable with additional specialist processing.

Selecting HDR will change the preview display to a logarithmic photometric view - This allows you to see something from both dark and bright areas of the scene, though the contrast will be very low.

EV

EV stands for Exposure Value, clicking the Reset button will set it to the average of all the input image exposures or setting it to 0 (zero) will result in no exposure change being applied to the panorama.

EV is a standard photographic scale, each increase or decrease by one unit will change the exposure by the equivalent of one f-stop (ie. halving or doubling the exposure). It is worth adjusting the exposure here in hugin rather than later in an external image editor, since hugin uses the camera response curve calculated in the hugin Exposure tab to perform the correction in a linear colour space.

The average value isn't always wanted. If you see colour artefacts in bright sky areas, set this to the negative of the darkest input image - This has a side-effect of clipping brighter images.