Chroma Key Hints and Tips

Applies to: Chroma Key Module, Backdrop

Below is a collection of hints and tips which users may find useful when using the Chroma Key Module and Backdrop.
Many of the suggestions are the result of customer feedback and are intended as a guide only.

Study the Chroma Key manual

It is strongly recommended the user becomes totally familiar with the Chroma Key manual available for download from the Support>Licensed downloads section of the website. Quite apart from the explanation of the tools and procedures, the manual describes many keyboard and mouse shortcuts which are invaluable to the busy user.

Evenly exposed background

Ideally the background should be evenly lit and slightly under-exposed by approximately three quarters of a stop. Most users do not light the background. Many users will also carry both a green and a blue background. A white cloth is also useful for subjects wearing both green and blue clothing and can also be used as a reflector.

Image preview size

For the most part images are imported into the software as subsampled previews where the size of the previews can be set in File>Properties. The default size is 512Kb

For the purposes of using the Chroma Key module a larger preview size often gives better results. Suggested sizes include 1024Kb or 2048Kb.

To change the preview size in Version 4 and later

  1. Use tsAdministrator to define a new set of subsamples at the desired size.
  2. In the Timestone Software application switch to the desired subsamples via File>Image previews

To change the preview size in Version 3

  1. Go to File>Properties>Bitmap sizes>Stored size of bitmap used for previewing and enter a new preview size
  2. Import the images - they will be imported at the new size
  3. If images have already been imported they can be refreshed to the new size. Select all the images then select Images>Refresh. This will resample the images to the new size (warning - this can take a long time!)

Choosing the first image to key

The most common workflow when keying images is to select an image, key it, then apply the result to all of the images. Select an image that perhaps requires more than the basic amount of work. People with fair, loose hair often usually provide a good starting image. Key this image to completion including any spill replacement if required. Once the image is as desired, apply the profile to all of the images.

Do not over clean the background

Over cleaning the background is perhaps the single greatest cause of poor results. A single click in the background should be all that is necessary. Click an area at about the eye level midway between the hairline and the edge of the image.

Fall off in background exposure is inevitable even with evenly lit and exposed backgrounds. This is most obvious in the high contrast "matte" view. Avoid the temptation to clean these areas. Such residual noise will scarcely be visible and can actually produce a more natural and pleasing result.

Take care when cleaning the foreground

Do not wander too close to the edges especially at the hairline. Loose, fair hair should be translucent and therefore will not appear as a solid opaque area in the matte view. Zoom in to more easily clean small areas in the foreground.

Toggle often with the right mouse button to the original unkeyed view to monitor your progress. Make sure you are not cleaning areas which should remain green. For example background that is visible between the subject's arm and body, background that is visible between the hair or a green spot on the clothing should not be cleaned as part of the foreground. These areas are difficult to distinguish in the matte view, use one of the other views instead.

Try the Defocus algorithm

It is important to understand the software begins the process of spill replacement the moment the background is clicked in the first step. By default the software uses the "Complement" mode of spill replacement where the colour that is the complementary opposite of the background colour is used to neutralise spill. In the case of green this colour is a magenta tone.

Sometimes a more pleasing result is achieved using the "Defocus" method where a defocused copy of the background is used instead of the background's complement.

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