Joe’s Channel Blur builds on a simple blur filter by adding RGB/YUV color channel targeting and aspect-ratio stretching.
Aspect skewing allows the blur to stretch vertically or horizontally. Using extreme aspect adjustment values, Joe’s Channel Blur yields creates an effect similar to a horizontal or vertical motion blur but with some slight off-axis blurring.
Example Images
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| Joe’s Channel Blur |
| Blur: | 50 |
| Aspect: | 1 |
| Target: | R, G |
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| Joe’s Channel Blur |
| Blur: | 50 |
| Aspect: | 0.1 |
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| Blurred Chroma |
| Blur: | 200 |
| Aspect: | 10 |
| Target: | Cb, Cr |
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| Blurred Luma |
| Blur: | 200 |
| Aspect: | 10 |
| Target: | Luma |
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Improving Chroma Keys with Joe’s Channel Blur
Joe’s Channel Blur can help correct a common problem when trying to pull Chroma mattes from DV. As noted on this DV Bluescreen/Greenscreen Solution page, the 4:1:1 color sampling can produce pixelated color artifacts 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 Color Key and Chroma Key effects. Chroma specific channel blurring was not previously possible with Final Cut Pro’s included effects.
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| Joe’s Channel Blur |
| Blur: | 1 |
| Aspect: | 4 |
| Target: | Cb, Cr (Chroma) |
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This frame of a yellow pepper against a solid blue background shows some of the problems with DV Chroma information. Because samples are recorded every four pixels, edges show color blocking and 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.
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| Chroma Keyer |
| Color Range selection only |
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| Joe’s Channel Blur |
| applied before Chroma Keyer |
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Using Joe’s Channel Blur on DV clips before keying helps to smooth blocky Chroma information for better quality masks. Blurring the Chroma also has the effect of broadening the sample area of Chroma Keyer’s eyedropper, making the initial mask selection easier. This effect can also be combined with a Y/C correction using the tools in Joe’s Broadcast Legalizer to better align the chroma with the Luma information.
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| Chroma Keyer |
| color range selection plus: |
| Softening: | 2.5 |
| Thinning: | 75 |
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| Chroma Keyer |
| after Joe’s Channel Blur plus: |
| Softening: | 2.5 |
| Thinning: | 75 |
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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.
Controls
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Blur (0 - 250)
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Sets the radius of the blur, in pixels
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Aspect (0.1 - 10)
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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.
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Target (YUV, RGB)
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Chooses the colorspace to use for the following channel switches.
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Luma, Cb, Cr / Red, Green, Blue, Alpha
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Switches on the blur for the checked channels, the same radius blur will be applied to the channels selected.
A note about controls
Joe’s Channel Blur contains only one Blur Radius slider because multiple controls would have required embedding three copies of the filter into one script. Three sliders would also complicate the interface, requiring a simple adjustment to use the same number of clicks as a more complex adjustment. By using only one blur radius adjustment, the results are faster and the complexity of the process matches the complexity of the process.
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