Why set the filmback?

Why not ignore the filmback and use the aspect ratio of the images on disk? You could do this; in fact, the default setting, Same As Image Aspect, does just that. However, without knowing the filmback, Live has no way of deriving the correct angle of view (camera aperture) and focal length, so your rendered images may not match the original footage. The following illustration demonstrates this relationship between filmback, focal length, and angle of view.

Live cannot assume what filmback was used based on the current image size; you must specify it. The reason is that the aspect ratio of images often changes when you scan or transfer them to digital files. For example, film scanners often crop images.

You can check how Live’s predefined filmbacks affect the camera settings by opening the shotCameraShape node in the Attribute Editor (right-click the camera and select shotCamera). The attributes affected are Angle of View, Camera Aperture, and Film Aspect Ratio.

Live sets the camera’s focal length to 35mm by default. If you want to enter a different focal length or have Live calculate it for you, you must set a filmback and change the camera constraints. For more information, see Create a focal length constraint.

To set up filmback and aspect ratios

1. Select a filmback from the Predefined Filmbacks list that best matches how your sequence was shot.

   For film scanned from negative: Select from the 16, 35, or 70 mm filmbacks.

   For film transferred to video: Select a filmback or one of the HDTV settings. Do not select Video Source.

   For a scene shot on video: Select either Video Source or HDTV from Video Source (16:9).

2. Verify the values set by Live; in most cases, leave the defaults.

   Film Aperture

   The standard width and height measurements of the filmback.

   Device Aspect Ratio

   The aspect ratio for the final output device when you render. Live bases it on the Film Aperture settings by dividing the width by height. and multiplying it by the lens squeeze, which is assumed to be one.

   NoteThe device aspect ratio may differ slightly from the industry standard terminology. For example, the aspect for Super 16mm is typically called 1.66, but the actual value is 1.688. Also, the anamorphic aspect ratio is called 2.35 but is actually 2.361.

   Pixel Aspect Ratio

   The aspect ratio of each pixel, which may be square (1.0) or non-square, such as for PAL video. This setting is display-only, but you can change it by turning off Auto-compute. When auto-computed, Live chooses the values from one of the following standards:

   1.0 (square)	1.067 (PAL)	.45 (NTSC, half height)
   .9 (NTSC)	2.0 (anamorphic)	.50335 (PAL, half height)

3. If the Ready To Track button is red, follow the instructions in the next topic, To correct Ready To Track errors

   

4. Click Copy to Render Settings if you eventually want to use Maya to render this shot.

   Live copies the aspect ratios to the resolution settings in the Maya Render Settings. If you make changes in the Setup, click Copy to Render Settings again to update them.

   If you have the Render Settings window open already, the new settings might not appear. Close and open the window to refresh the settings.

When the Ready To Track button is red, it means Maya Live has detected an inconsistency between the device aspect ratio (which is set when you select a filmback) and the actual image aspect ratio. Live detects the inconsistency based on the following relationships:

Device Aspect Ratio (AR) = Image AR x Pixel AR

Device Aspect Ratio (AR) = Filmback AR x Lens Squeeze

The inconsistency typically occurs because your images were cropped during the scan or transfer to digital images.

To correct Ready To Track errors

1. Make sure you have chosen the correct filmback

   Ideally, you can obtain this setting from the cameraman or director. If you can’t, try different settings because some of them have slight differences.

2. Click the Ready To Track button.

   In most cases, you see the following dialog. If you do not, skip to Select Solution 1 (Crop the Width) or 2 (Expand the Height) from the second dialog..

   

3. Do one of the following:
   • Select Solution 1 (Adjust Device Aspect) if you think your images were cropped. First, Live matches the Device Aspect Ratio to the value obtained from measuring your images on disk (Image Aspect Ratio x Pixel Aspect Ratio). Then, Live opens a second dialog with cropping options; see the next step.
   • If you are sure your images were not cropped, select Solution 2 (Adjust the Pixel Aspect Ratio). Live adjusts the pixel aspect ratio value to account for the difference between your images and the predefined filmback. You are now ready to track; skip the next step.
4. Select Solution 1 (Crop the Width) or 2 (Expand the Height) from the second dialog.

   

   This dialog appears if you choose to adjust the Device Aspect from the first dialog (Solution 1), because the same inconsistency exists in reverse order. In the first dialog, the device aspect ratio was based on the film aperture and didn’t match the image measurements. Now it is based on the image measurements and doesn’t match the film aperture. Solution 1 or 2 will adjust the camera aperture values to match the image measurements.

   Solution 3 (Adjust Device Aspect) simply switches back to the first dialog.

   NoteWhen Live adjusts the aperture values, the Predefined Filmback switches to User because these are custom values.