# Hugin Fast Preview window

Like the more accurate Hugin Preview window, the fast preview shows something similar to the final stitched output, but with a few important 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 colouring of these images will also be inaccurate when using exposure or white balance correction. For HDR panoramas, the Hugin Preview window is recommended instead.
- Photometric correction only includes white balance and exposure, unless full photometric correction is enabled with the the Photometrics button.
- The remappings are approximate, the output by a tool such as nona is more accurate. If this concerns you more than speed, use the Hugin Preview window instead.
- It's much faster ;-)

The window features a row of tabs at the top, clicking on a tab switches between modes that allow you to interact with the panorama in different ways:

## Preview tab

### Identify

Using this tool you can find where your images are, and match them to their number. You can also edit control points.

When this tool is turned on, move the mouse over the visibility buttons for the images (the numbers at the top of the preview). The image with the number on the button under the mouse lights up red in the preview. Moving the mouse over the panorama highlights all the images under the mouse in different colours. The buttons for those images lights up in matching colours.

When the mouse is on the overlap of two images, click to edit the control points between those images.

### Photometrics

Enables full photometric correction. When turned on, this will cause significant delay when changing photometric parameters. It also takes a while to turn on and off. However, with it enabled you get much better representation of the colours in the output. With it turned off, you get correction only for exposure and white balance. With it turned on, you also get vignetting and colour response correction. The Hugin Preview window does all these things by default, so you may wish to use that instead.

### Show control points

When this tool is turned on, all Control points are drawn as lines with crosses at each end. Green, yellow, orange, and red lines and their crosses indicate 'normal' control points. A red control point is misaligned (the ends of the line are father apart), and a green control point is well aligned (the ends of the line are almost in the same position). Blue indicates horizontal control points, vertical control points or straight line control points.

### Blend mode

The normal blend mode will draw the images as a stack. The difference blend mode will do the same, except the image under the mouse pointer will be subtracted from the rest of the stack. Use this to determine if the alignment went well: where you can see edges in the subtracted image, these edges are misaligned. Be warned that this isn't fully accurate, the other preview has a better difference mode.

### 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.

## Layout tab

The Layout tab shows the entire project as a diagram with colour-coded lines connecting each of the photographs.

Green lines connecting images show the control points have a small error, red lines show a large error. Grey lines show no control points connecting the images.

You can see where the project is OK and where there are problems if it isn't quite right. Just click on any connection and Hugin jumps to the Control Points tab to edit that pair of photos.

## Projection tab

This tab is for adjusting projection of the panorama, some projections have adjustable parameters which will appear when selected.

### 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.

### Field of View

This is the horizontal and vertical angle of view of the output image.

### Projection

Use the drop down list to change the output projection of the panorama, the list is exactly the same as that in the hugin 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.

## Move/Drag tab

Using this tool you can recentre the panorama interactively. With it turned on, try the following:

- Drag the panorama with the left mouse button to rotate the panorama's images. The centre of rotation is the point where you pushed the mouse button down.
- Hold shift when doing the above to constrain movement to yaw or pitch. Note pitch is affected by the centre of rotation.
- Drag the panorama with the right mouse button or hold control and drag with the left to roll the panorama (rotate around the middle)

If the panorama contains unconnected components, they will move individually.

### Drag mode

This determines what parameters that are changed when the images are dragged.

- Normal - When dragged left-right, the yaw parameter is changed and when dragged up-down the pitch parameter is changed. I.e. the camera is tilted in the yaw and pitch angles.
- Mosaic - When dragged left-right, X parameter is changed and when dragged up-down the Y parameter is changed. I.e. the camera is moved in the X and Y dimensions.

### 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 and reoptimise.

### Numeric Transform

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.

## Crop tab

Using this tool you can set the output cropping region interactively. To do this precisely instead, use the Stitcher tab. Initially, the entire panorama is in the output region (i.e. nothing is cropped).

To change the cropping at each edge, move the mouse towards that edge until a white box appears along it, then drag with the left mouse button until the edge is where you want it. The darker areas represent the region that is cropped off. You can move two edges at once by moving the mouse towards the corner shared by the edges until both white boxes appear. If you wish to move the whole region at once, move the mouse into the middle so that all four edges have boxes along them and drag.

### Autocrop

The autocrop button adjusts the crop rectangle so that it is entirely within the image area, i.e. there will be no 'black' borders in the final stitched image. It does this by maximising the area of the rectangle rather than the width or height.

## Displayed images

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

In addition, this display also controls the behaviour of the hugin Optimizer tab and the hugin Stitcher tab - Any photos disabled here are not used in optimisation or stitching.

### 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.

## Preview canvas

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.

## In practice

A panorama with unconnected image groups after optimisation.

Let's try using this preview to help with a panorama where automatic alignment failed. This panorama was taken where a lot of things were blowing around in the wind, and the clouds were changing quickly, so it is not surprising that it aligning it is a struggle. The Assistant tab tells us there are multiple unconnected image groups. We can optimise the panorama and end up with a images correctly positioned amongst the group they are in, but the groups themselves are not aligned. Try this first.

Dragging images with the drag tool.

Using the Drag tool, we can roughly align the groups together:

1. Turn on the tool with the Drag button.
2. Drag each component so the horizon is in the middle, using the left mouse button.
3. Drag with the right mouse button (or hold control and drag with the left) to level the horizon.
   Rotating images with the drag tool.
4. Hold shift and drag with the left mouse button sideways to approximately line up the image with the other groups.

Two images highlighted in the preview window.

When we have the images in approximately the right position, we can begin placing control points to guide the optimiser. The Identify tool lends a hand here. Firstly, turn on the identify tool. Move the mouse on an overlap that was recently created between two image groups. The images in the overlap light up. Move the mouse to a place where there is only two images in that overlap, and click. The two images are opened in the control point editor (there may be a short pause while the images are loaded). Once you have placed control points manually, you can return to the preview to find some more image pairs.

Adjusting the cropping region.

When you are happy that your panorama contains sufficient control points, optimise it again. The panorama will likely have the horizon at the wrong angle, in this case press Straighten on the preview window. You can then frame the panorama using the drag tool (hold shift so you don't make it wonky again!). Use your artistic judgement here. If you want to crop your panorama, click Crop and drag the edges of the cropping rectangle.

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