Tutorial 6: Notre Dame in Perspective

Objectives 

1) make a vertical panorama,

2) create perspective images,

3) understand imaging.

Open PanoStitcher

 Load

As the alternative to the standard loading utility of "Image Browser",  drag-and-drop from Windows Explorer is used here.  Hence close  "Load Images" window.

1) Start Windows Explorer from the computer Start | Programs | Windows Explorer".  Open the folder "tutorial 4/NotreDame" to show the images: 

2)  Select all photos by clicking on the first and then the last image file name while pressing the [Shift] key.

3) Drag the two selected images onto PhotoBench. 

4) Click on PhotoBench "Rotate right" button. Click on any image while pressing [Ctrl] or [Shift] key.  All images will rotate 90 degrees clockwise.

5) Arrange the images by dragging them to their correct locations on PhotoBench. 

Fig 1. PhotoBench containing the raw photo thumbnails

Fig 2. PhotoBench containing the properly rotated photo thumbnails

Settings...

1) Click on the Stitch Toolbar "Settings..." button to bring up the "Stitch settings" dialog box.

2) It is known that at the focal length (28mm) of the camera used in this example there is substantial distortion towards the periphery of an image, which needs to be corrected for stitching. Click on the "Enable" check box to activate the "Select..." button.  Click on the button to pop-up the "Select Camera Info" dialog box.  Select the Camera Info file "Olympus C-3000 (28mm).txt" and Click OK. 

3) Click OK (with the default stitching "Auto Mode" and "Auto-balance intensity" should already be selected).

Stitch

1) Click on the Stitch Toolbar [Stitch] button.  PanoStitcher will stitch the image pair to make the panorama preview. 

2) Right-click on the panorama and choose "Unbend panorama"/"Rotate panorama".  Drag vertically on the panorama so that the long edge of the Notre Dame (closest to the camera) is leveled and straight.

Fig. 3  Panorama preview:  a) raw;   b) after unbend/rotate

Blend

1) Click Blend button to make panorama, which is auto-intensity balanced and auto-cropped with the default Resolution Ratio of  0.75. 

Save

1) Click on File | Save Project  to save the stitching project to the project file NotreDame2.psp in the "NotreDame" folder.  Later you can continue working by loading this .psp file to PanoStitcher.

2) Right-click in the panorama window and select "Create 90 degree image" to make the vertical panorama.

3) With the vertical panorama window as the active window, right-click in the vertical panorama window.  Select "Save at 100% resolution" to save the vertical panorama to the name "NotreDame_v.jpg".

Fig. 4  Vertical panorama saved at 50% (screen) resolution

Create Perspective Images

1) Right-click on the panorama and select "Select perspective region".  Then press and drag on the panorama to draw a box region.  Adjust each side to contain the structure (Fig. 5).

2) Right-click on the panorama and select "Create  perspective view" to create a perspective image in a new window

3) Right-click on the perspective image and select "Rotate left 90" to create the final perspective image of Fig. 6. 

Fig. 5  Select perspective region:  OC at center

Fig. 6  Created  perspective view (with OC at center).  This is equivalent to the picture taken with a camera with a high-end wide angle lens -- large angular coverage and distortion free.  Now with PanoStitcher you can get the same result and much more with a regular camera.

Let us now compare the perspective image of Fig.6 and the panorama image of Fig. 4.  Note that the panorama has the building's straight edges curved, i.e., the scene is warped, while the perspective image avoids the warping like a picture taken with a distortion-free camera.  A wide panorama is an invert looking world map -- a world map is a sphere surface which has to be warped to be displayed on a planar paper! A panorama can display the whole surrounding while a perspective image can only cover a view span of less than 180 degrees.  If the building is tall such that you have angular span of >150 degrees, non-warping view actually looks worse due to the divergence towards +-90 degrees. 

By design, when taking a picture, the optical center (OC) of a camera's lens corresponds to the picture's center, where the divergence effect "originates".  With PanoStitcher you can get away with this limit.  Actually the circle in the perspective image box is the OC, which can be dragged off the default box center (of Fig. 5).

Now let us create perspective images by :

4) Drag the OC circle off the center the perspective image box, to the bottom of the building (see Fig 7)

5) Right-click on the panorama and select "Create  perspective view" to create a perspective image in a new window

7) Right-click on the perspective image and select "Rotate left 90" to create the perspective image of Fig. 9. 

Repeat 4)-7) but with the OC to the top of the building (see Fig. 8) to create the perspective image of Fig. 10.

You can also drag the OC to other locations to see what happens.

Fig. 7  Select perspective region:  OC at building's bottom

Fig. 8  Select perspective region:  OC at building's top

Fig. 9  Created  perspective view (with OC at bottom)                   Fig. 10  Created  perspective view (with OC at top)

Optics & Imaging

From the perspective images of Fig. 6/9/10 (which corresponds to the pictures from a distortion-free camera with wide-angle lens), it can be observed that the further away an object is from the OC the more stretched out it looks in the images.  You can do an experiment with your own camera:  1) Take a picture with an object in the center of the view; 2) Take another picture with the same object at a corner of the view; 3) Compare the shape of the object in the two pictures.

Also note that such stretch-out effect is less in the panorama image of Fig. 4.   Hence you might prefer the look of a vertical panorama than its corresponding wide-angle perspective views, despite the warping.