Flickering and Convolution

When outputting computer graphics to an interlaced device, there are some important considerations to be made in order to avoid flickering.

As each field is drawn separately, a horizontal line that only appears on one line of one field is drawn for only 1/60th of a second. While the other field is being drawn, the first field is fading to black. Then the line is redrawn again. This causes the horizontal line to "flicker", which can induce a range of reactions in the audience, all negative.

With Radius VideoVision and the AV Macintoshes, the output of computer graphics to an interlaced device can be convolved, in order to minimize flickering. It involves a slight blurring of lines into the adjacent field, so that they are continuously redrawn. VideoVision's convolution is supported at 8- and 24-bit bit depths, while the deflickering option on the AV Macs is only supported at 8-bit. The 16-bit video output supported by the AV Macs is unconvolved.

Radius VideoVision Studio turns convolution off when playing back unzoomed full-screen QuickTime movies in order to avoid blurring and degrading the video image quality. A side effect of this is that Macintosh graphics imported for inclusion with Quicktime movies will flicker, unless convolved manually in the project, or before importing it in the project.

Duplicating Convolution with Photoshop or Premiere

This can be accomplished from within Premiere or with Photoshop. The key is the Convolution filter, which offers a convolution matrix supporting a variety of effects. The convolution matrix used by VideoVision can be duplicated by using the following matrix:

  0 1 0
  0 2 0
  0 1 0
with a multiplier of 6.

Some users prefer using the following matrix to soften the effect:

  0 1 0
  1 4 1
  0 1 0
with a multiplier of 8.

Additionally, stills taken from interlaced material (i.e. frames exported from full-screen QuickTime movies as PICTs) will flicker in the same manner when used in a QuickTime movie. In this case, convolution will not be as effective, and the stills must be deinterlaced instead. This, too, can be accomplished using Photoshop (Filter/Video/Deinterlace) or Premiere (Clip/Field Options/Deinterlace).

Other Convolution Functions in VideoVision and VideoVision Studio

The circuitry used for convolution in VideoVision is also useful for three other functions: Interpolated zoom, averaged reduction, and deinterlacing live video on an RGB monitor.

Interpolation is very important when enlarging, for example, a 320x240 movie to 640x480. Rather than repeating pixels in both dimensions to double the size, the new pixels are averaged in, using the new pixels' neighboring pixels. This minimizes the "jaggies", the stairstepping that otherwise becomes readily apparent when enlarging a movie or bitmapped image to double size.

This averaging also occurs in the other direction, when recording at smaller-than-full-screen window sizes. For example, if a full screen image containing alternate black and white vertical lines is digitized at quarter screen size, the result will be a gray image. Without this averaging, one or the other color would be dropped entirely. In this way, averaging results in a visually more accurate image.

When an interlaced image is displayed on a progressive-scan device like an RGB computer monitor, the image will appear distorted slightly, especially in clips with fast motion, due to the defference between the two fields. When live video is displayed at full-screen on an RGB monitor, VideoVision convolution circuitry will average out one field, by interpolating between the two lines in the other field. This makes the image appear smoother and undistorted. The detail in the eliminated field would be undiscernable.

The only case in which convolution actually alters data is when recording at lower-than-full-screen sizes. The extrapolated zoom, interpolated live video windows, and convolved Mac desktop output affect only the display. These three can also be disabled by turning off convolution in the VideoVision Monitors Options dialog.