Joe's Filters: Joe's Channel Blur

The Joe's Channel Blur plugin is a part of Joe's Filters, shareware utility and image processing effects for Final Cut Pro.

What it does

Joe's Channel Blur adds specific channel targeting options and aspect-ratio stretching to a basic blur filter. The filter can target any combination of either RGB or YUV channelsets regardless of original colorspace.

Aspect skewing allows the blur to stretch vertically or horizontally. Using extreme aspect adjustment values, Joe's Channel Blur yields results very similar to a horizontal or vertical motion blur, with some slight off-axis blurring. The method used by this effect is much, much faster than Final Cut Pro's Motion Blur routines.

Example Images

Original Image	Joe's Channel Blur Blur: 50 Aspect: 1 Target: Red
Joe's Channel Blur Blur: 5 Aspect: 10 Target: YUV all	Joe's Motion Blur Distance 36 Quality: 7 Target: YUV all
Original Image
Blurred Chroma Blur: 30 Aspect: 10 Target: Cb, Cr (Chroma)	Blurred Luma Blur: 30 Aspect: 10 Target: Luma

Improving Chroma Keys with Joe's Channel Blur

The limitations of DV Chroma cause problems for color-based keying effects. Joe's Channel Blur can help create smoother mattes by pre-blurring only the chroma. By softening the source image's color information color and chroma based mattes will be smoother and more organic.

Joe's Channel Blur can be used to correct a common problem when trying to Chroma matte DV's 4:1:1 color sampling. As noted on this DV Bluescreen/Greenscreen Solution page, the low color sampling can produce noticeable artifacting along color edges. Blurring the chroma channels with a horizontally biased aspect ratio helps to correct this problem. The ability to blur Chroma makes this effect a good companion to Final Cut Pro 3.0's new Chroma Key effect. YUV specific channel blurring was not previously possible with Final Cut Pro's included effects.

The bright color of the mango in the examples below help to show how Joe's Channel Blur can improve the results of Final Cut Pro's new Chroma Keyer plugin by pre-softening the source image's Chroma:

Original Image	Joe's Channel Blur Blur: 1 Aspect: 4 Target: Cb, Cr (Chroma)
This frame of a mango against a solid blue background shows some of the problems with DV Chroma information. Samples are recorded every four pixels, resulting in blocky edges which make keying difficult. The detail of the pre-filtered edge shows how interlacing helps to increase color information vertically. Applying Joe's Channel Blur with a horizontal bias helps smooth the blocky chroma without sacrificing vertical detail.
Chroma Keyer Color Range selection only	Joe's Channel Blur applied before Chroma Keyer
Using Joe's Channel Blur before masking results in more organic masks. The cleaner starting mask means less to clean up and better detail. Blurring the Chroma also has the effect of broadening the sample area of Chroma Keyer's eyedropper, making the initial mask selection easier.
Chroma Keyer color range selection plus: Softening: 2.5 Thinning: 75	Chroma Keyer after Joe's Channel Blur plus: Softening: 2.5 Thinning: 75
Since Chroma Keyer performs it's softening and thinning on the chroma-keyed mask, blocky artifacts are still visible. The mask generated from the Chroma-blurred image is smoother and maintains more detail with a truer edge.

Since this filter also works in Final Cut Pro 2, those who haven't yet upgraded can get smoother masks by pre-blurring image chroma before using Color Key or other color-based keying filters.

Controls

• Blur (0 - 250)

  Sets the radius of the blur, in pixels

• Aspect (0.1 - 10)

  Defines the effect aspect ratio. Values less than one skew vertically, values greater than one skew horizontally. This amount is multiplied by the sequence aspect ratio, so a value of 1 is round no matter what aspect ratio is being used in the sequence.

• Target

  Chooses the colorspace to use for the following channel switches.

• Luma / Red, Cb / Green, Cr / Blue

  Switches on the blur for the checked channels, the same radius blur will be applied to the channels selected.

This plugin only contains one Blur Radius slider because multiple controls would just embed three copies of the filter into one script. Three sliders would also complicate the interface, requiring a simple adjustment to use as many clicks as a complex adjustment. By using only one radius adjustment, the results are faster and the workflow grows in complexity to match the process.