(Show frame with Topics and Control Panel)

AVX Plug-Ins

 User Guide

"Tips" appear in green. Tips will save you time but are not essential information.

"Cautions" appear in red. Be sure to read and understand this information.

3Prong's FlickerFiX AVX Plug-in helps to remove flicker and other changes in brightness, contrast, and color levels from video or film footage. These problems might be caused by:

• changing lighting conditions during shooting such as a subject moving from indoor to outdoor lighting
• camera auto-iris affected by changes in the scene
• analog signal path disturbances such as misaligned heads in a VTR
• film processing or exposure variations

FlickerFiX works by measuring the average levels within user-defined sample zones, and taking a snapshot of these reference values when the user presses the "Other Options" button. Then, when rendering each flickering field, the levels in enabled zones are compared with the reference values, and the differences are used to apply gain and offset compensation. This brings each field back to the correct levels, matching that of the reference.

FlickerFiX is very versatile, with modes for removing flicker in luma, saturation, Cb/Cr, or RGB channels independently. As well, you may choose to remove gain flicker, offset flicker, or both.

Quick Start

FlickerFiX has many features, but turning them all on will probably not produce the best results! Less is better. The default settings (Composite mode, the Light Zone only enabled) will work for most common flicker problems:

• Apply FlickerFiX to a segment in the timeline.
• Drag the Light Zone box from the upper left corner to a light area of the image that does not contain a lot of movement or detail.
• If necessary, add keyframes and adjust the Light Zone to track this area.
• Press the Other Options button (appears next to the FlickerFiX effect name at the top left of the Effect.) This sets the reference values. Click OK.
• Render.

If this is not effective, consider the likely source of the problem, and examine the footage to determine the best settings. Some experimentation may be required.

Types of Flicker

There are two processing modes in FlickerFiX: Composite and Component. There are three sampling zones available, Light, Chroma, and Dark, to measure the flicker in each of these ways and apply the appropriate corrections. As well, there are controls for gain, black and saturation levels to provide some overall correction of the footage or compensate for sampling errors.

Luminance and Composite Flicker

Examine the footage carefully using a waveform monitor and vectorscope. The Avid software waveform monitor only displays a single line of video at a time, so you will find a hardware waveform monitor much more useful. Set the waveform monitor to filter out the chroma, so that you can observe changes in luma alone.

Does the upper part of the waveform (brighter areas of the image) vary in level more than the lower part (darker areas)? This is gain flicker, the most common type, where the entire signal is magnified or reduced. Use the Light Zone to measure and fix this type of flicker.

Do the lowest parts of the waveform (approaching black) vary quite a bit? This is offset flicker, where the entire signal is riding up and down on an offset, and is not common. It may be present with or without gain flicker. Enable the Dark Zone to measure and fix this.

Color Flicker

If you are seeing color changes in your image independent of brightness or contrast, those may be fixed or reduced by FlickerFiX. FlickerFiX provides four choices to deal with color flicker.

You can use the vectorscope for assistance in telling color gain flicker from color offset flicker and saturation flicker. However, it is not possible to tell from a vector display, or even a component waveform monitor, whether the color flicker is RGB or CrCb. If you consider the origin and path of the signal, this may provide a clue. Film and telecine flicker is more likely to be RGB, whereas CrCb flicker is more likely from a component analog path such as a Betacam VTR. Saturation flicker that is independent of luminance is more likely caused by a composite device such as a Umatic or VHS VCR.

Generally, try Component mode first (RGB), before enabling the Chroma Zone. This is a simpler approach and more commonly successful. If that doesn't work, enable the Dark Zone if it is not already in use. You may try the enabling the Chroma Zone.

Referring to the vectorscope, normally there will be a blob near the center that represents the darker areas of the picture as well as the monochromatic lighter areas. Does this blob move erratically? That may be offset flicker in the color channels. The Dark Zone must be enabled to measure and fix this, and Component mode selected. (Color offset flicker may be present with or without color gain flicker, but the Chroma Zone will determine whether processing occurs as RGB or CrCb.)

Observe the larger vectors that come out of the center of the vector display. Do they flicker independently from the central blob? If they just move in and out of the center of the display, this saturation flicker is likely just due to gain flicker present. However, if you still notice saturation flicker after trying the default mode, you should enable the Chroma Zone and remain in Composite mode.

However, if color vectors flicker more in a horizontal than vertical direction than the central blob, you are seeing color component flicker. Component mode with the Chroma Zone disabled is used for RGB flicker. Component mode with the Chroma Zone enabled is used for CrCb flicker.

Locating Zones

Turn on the group enable button for each zone, depending on the type of flicker you are trying to fix. The chosen areas must not clip (touch the maximum limit of any of the system's RGB channels) at any point in the segment. To detect clipping, look for some flattening in peaks of the waveform.

The Light Zone is used to measure and fix gain flicker in either Composite or RGB. Place this over a lighter area of the image, avoiding any areas that are so light that they are clipped.

The Chroma Zone is used to measure and fix color gain flicker in Saturation or CrCb. Place this over a brightly colored (saturated) area of the image, preferably one containing multiple diverse colors. Avoid any areas where chroma clipping might be occurring.

The Dark Zone is used to measure offset flicker in Composite, RGB, or CrCb. Place this over a darker and unsaturated area of the image, avoiding any areas that are so dark that they are clipped.

Proper location of the enabled zones is crucial to successful removal of flicker. Choose areas where contrasting detail does not enter or exit the zone during the effect, and there is a minimum of motion. The larger the zone, the more tolerance (forgiveness) of detail, motion, and clipping within the zone there will be.

The chosen areas must remain unobscured throughout the effect. If no such area is visible at both the start and end of the segment (such as in the case of a sweeping pan), you must subdivide the segment further using the Avid's add-edit key. In this case, you may use several instances of the FlickerFiX effect on the adjacent segments, each with different reference areas, and apply dissolves between them.

Similarly, if the source footage has been previously edited, isolate each pre-recorded cut using add-edits and apply multiple FlickerFiX effects. You cannot use FlickerFiX over pre-recorded dissolves. (If used on a pre-recorded fade, FlickerFiX will remove most of the fade, except for the last few frames, which can be used to recover additional footage!)

Operation

From the Effect Palette, apply the FlickerFiX effect to the problem segment.

For identification, the Light Zone starts in the top left of the frame and the Dark Zone at the bottom left. The Chroma Zone starts at the middle of the right hand side.

Using the onscreen rectangle, move the desired zones by dragging the corner points or the entire rectangle. If you close any of the zones or move them outside of the frame, no correction of that type can take place. If you close one accidentally, you may open it again by adjusting one of the closed sides with a slider control in the Effect Editor.

If required, add keyframes and adjust at each one, moving the sample zone to follow the motion of the subject. The size of the zone may vary without a problem, but the average levels in the zone must remain constant.

Locate a field with the proper levels which you would like to use as a reference. Then press the Other Options button (appears next to the FlickerFiX effect name at the top left of the Effect Editor.)

A message appears indicating that the levels of the current image will be used as the reference values for this effect.

(The Unlock button takes you to the dialog box for ordering a license and unlocking the plug-in.)

If an error message pops up saying that one or more of the reference RGB channels is clipping, you should choose a slightly darker area for the light Zone, or a slightly lighter value for the Dark Zone.

If you later change either the Light or Dark Zone, you must reset the reference values or the results will be poor. If you never set the reference values, rendering will produce NO results.

If desired, adjust the gain and black level controls. If these don't seem to work, there are no reference values set. They will function when rendering, so return them to normal till after the reference values are set.

Render the effect.

Limitations

You can repair many flicker and level problems with FlickerFiX. However, FlickerFiX cannot remove some types of flicker:

• where areas of an image are flickering in differing ways (for example, foreground flickering in a different way than the background). This is sometimes seen in old film footage, video or computer monitors in the shot, or lighting flicker caused by power line frequency strobing with the frame rate.

• flicker that is not linear by nature (for example, darker parts of an image flickering in a different way than the lighter parts). This is sometimes seen in old film footage.

• interlace twitter, the flickering effect caused by detail present on one field but not on the other. This is often seen in computer-generated images or character generator output.

However, your purchase of a FlickerFiX license also includes 3Prong's TwitterFiX, which may be helpful for these types of problems. TwitterFiX uses field blending to average out any differences. See the TwitterFiX documentation for more information.

Also see AVX Limitations in the Common Issues section.

Troubleshooting

(Back to top, with Topics and Control Panel frame)