Working with the BCC Make Alpha Key Filter

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1 Working with the BCC Make Alpha Key Filter Make Alpha Key creates a new alpha channel from one of the existing channels in the image and then applies levels and gamma corrections to the new alpha channel. Make Alpha Key also has a PixelChooser that determines which pixels are used to make the alpha channel. You can create many useful effects by making a Full On (completely opaque) alpha channel and using the PixelChooser to control which parts of the image key out. The Make Alpha Key Þlter includes extensive parameters for you to customize. You may need to scroll to see them all. The illustration at right does not show all the parameters. General Controls Parameter Group At times, you may want to view your clip without the effect that you ve applied. The Bypass Effect checkbox lets you view the source footage without the effect. When the Bypass Effect checkbox is selected, it turns pink. When you applied a BCC effect to an Avid title or matte, enabling the Bypass Effect checkbox does not display the original title unþltered, but rather displays the nested Graphic Fill of the matte key. Bypass Effect is a parameter change; selecting the Bypass Effect checkbox will unrender a rendered effect. To bypass a rendered effect without unrendering, step into the effect in the Avid timeline to see the unaffected video. Selecting the Safe Colors checkbox enables an NTSC/PAL color-safe Þlter that allows only colors that are safe for broadcast. BCC allows RGB values across the full range. When this checkbox is selected, the RGB values are limited to the NTSC/PAL safe range of The Draft Mode checkbox allows you to preview your effect in a Draft Mode to speed previews. This is especially useful for effects with multiple track inputs. Deselect this option before rendering your effect. The Apply to Title-Matte checkbox allows you to apply BCC Þlters to titles created in Avid s Title tool. To apply an effect to a title, drag the effect onto the title (you do not have to Option-drag) and select the Apply to Title-Matte checkbox. The Invert Matte checkbox allows you to invert any matte created by your effect. This is useful when you work with imported images.

2 The Layer Opacity slider sets the opacity of the Þlter layer, which allows you to fade effects. When Apply to Title-Matte checkbox is selected, Layer Opacity affects the graphic Þll layer, which allows you to fade titles. The Field Render menu sets the rendering optimization for BCC. For most Þlters, you can use the default of Speed Optimized. For effects that include edging or DVE moves, you may want to use Quality Optimized. Quality Optimized takes more time to render, but will generally correct any problems with jitter or rough edges on effects. Geometrics Parameter Group The controls in the Geometrics parameter group allow you to add basic DVE moves to any Þlter. The Geometrics parameter group includes the following controls. Selecting the Enable Geometrics checkbox allows you to use the parameters to reposition a track. When this checkbox is deselected, the other parameters have no affect. Position X and Position Y adjust the horizontal and vertical location of the track. Scale X and Scale Y change the size of the image along the X and Y axis, respectively. These parameters scale as percentages of the image s original width and height. Thus, a Scale X value of 200 produces an image twice as wide as the original. Select the Lock Scale checkbox to keep the Scale X and Y values in proportion. Tumble, Spin, and Rotate change the image s perspective along the X, Y, and Z axes respectively. Tumble, Spin, and Rotate can animate over values greater than 360 in order to make the shape complete more than one full revolution. Tumble Spin Rotate

3 The Crop controls crop the edges of the image. Left Crop, Top Crop, Right Crop, and Bottom Crop determine the width, in pixels, of the cropped area on each edge of the image. For the Crop controls refer to the sides of the original image before it is transformed in 3D space. For example, if you tumble an image 180 so that it appears upside-down, cropping the Top affects the top of the original image, which is actually the bottom of the tumbled image. The Blend control soften the edges of the image by reducing their opacity. The Blend parameter determines the width, in pixels, of the transparent band on each edge of the image. These values are unaffected by the Crop values, allowing you to soften the edges of an uncropped image. Drop Shadow Parameter Group The controls in the Drop Shadow parameter group allow you to add an animatable drop shadow to an effect. The Drop Shadow parameter group includes the following controls. The Enable Drop Shadow checkbox turns the shadow on and off. If this checkbox is not selected, the other parameters have no affect. Source Opacity sets the opacity of the source. You can use this parameter to fade in a title or other source, without fading in the shadow. Distance sets the distance (in pixels) between the shadow and the image. Intensity sets the opacity of the drop shadow, and is scaled as a percentage. At a value of 100, the shadow is completely opaque. Lower Intensity values allow the background image to be seen through the shadow. At a value of 0, the shadow is completely invisible. Softness controls the softness of the edges of the shadows. A setting of 0 produces a sharply deþned shadow with hard edges. Increasing Softness produces shadows with softer edges. Angle sets the direction of the drop shadow. A setting of 0 places the shadow to the right of the image; a setting of 90 places it directly below the image. Shadow Color sets the color of the shadow. If the source image is opaque, selecting the Opaque Source checkbox can speed rendering and previews. If your source is partially transparent, deselect this option for best results. Make Alpha Key 1 Parameter Group The Alpha Source Layer menu chooses any layer in your timeline to use as the source layer. If you choose None, the Þltered layer is automatically used as the source.

4 The Alpha From Channel menu speciþes the channel in the Alpha Source Layer used to create the output alpha channel. Luminance, Red, Green, and Blue create mattes that become more opaque as the pixels in the source channel become brighter. You can use such a matte to key out shadows in the source image. You might want to try all channels to see which produces the best matte for your purposes. The channel with the highest contrast is usually best to use. Alpha uses the alpha channel in the source. Use this mode if you are using alpha information from another layer that already has the desired matte. Full On and Full Off create a completely opaque or transparent matte. These settings can be useful when you use the PixelChooser with the Þlter. The Inverse settings (Luma Inverse, Red Inverse, Green Inverse, and Blue Inverse) create keys that become more opaque as the source channel becomes darker. The Folded settings (Luma Folded, Red Folded, Green Folded, and Blue Folded) create a solarized key that is opaque in the middle of the value range and transparent for the lowest and highest values in the source channel. In this illustration, Alpha From Channel is set to Blue. The Þlter keys out portions of the image in which the blue channel value is closest to 0 and keys in the parts of the image in which the blue channel value is closest to 255. Thus the sky region is left opaque and the yellow ßower is keyed out. Setting Alpha From Channel to Blue Inverse has the opposite effect. The Þlter keys out pixels whose blue channel value is highest and retains the pixels whose blue channel value is lowest.

5 If Alpha From Channel is set to Blue Folded, the middle range of the blue channel becomes opaque and the highest and lowest blue values are both keyed out. This image does not contain many pixels with mid-range blue channels, so using the Blue Folded option makes the image almost entirely transparent. The Output menu determines how the output of the Þlter is rendered. In Composite mode, the Þlter uses the new alpha channel, making portions of the image partially or completely transparent when composited over the new background. Composite Inv Alpha inverts the new alpha channel. Show Matte displays the matte instead of the composite, allowing you to view the matte as you make adjustments. Show Invert Matte displays the inverted matte. Input Black and Input White determine which values in the alpha channel are purely transparent and purely opaque, respectively. All pixels more transparent than the Input Black level map to the Output Black transparency level, and all pixels more opaque than the Input White level map to the Output White transparency level. Increasing the Input Black value from the default setting of 0 increases the number of semi-transparent pixels that map to purely transparent (pure black in the matte). Decreasing the Input White value from the default setting of 255 increases the number of semi-transparent pixels that map to purely opaque (pure white in the matte). You can increase Input Black to completely key out areas with low alpha values and decrease Input White to completely key in areas with high values. This is an effective way to create a matte that keys out shadows, keys in highlights, and leaves midtones partly transparent. The following examples demonstrate the effect of Input Black and Input White adjustments on a matte.

6 This is the original image. Because the luminance in the sky and ßower regions is about the same, it is better to make the alpha from a color channel. In this case, the blue channel provides the most contrast. This is the initial matte made from Blue Inverse. Notice that the ßower contains some gray areas that could cause the background image to show through, and the matte, in general, is rather lowcontrast. You can remedy these problems using the Input controls. Decreasing Input White eliminates the light gray areas in the opaque part of the matte by increasing the number of gray values that map to white. Increasing the Input Black value increases the number of gray values that map to black, increasing contrast in the matte.

7 Gamma controls the slope of the curve used to convert from the input color values to output values in pixels brighter than Input Black and darker than Input White. Increasing Gamma from the default of 100 lightens the image and decreases contrast, while decreasing Gamma darkens the image and increases contrast. Post Blur is a blur that is applied to the alpha channel after the key is created. This parameter blurs the entire alpha channel, not just the part created by this Þlter. Use Post Blur to soften the edges of the matte for sources with high-contrast edges. Positive Choke values tighten the matte around the selected regions, while negative values pull the matte away from the edges of the selected regions. The output levels control the range of transparency levels that can appear in the alpha channel. Output Black sets the minimum opacity of the image. Increasing Output Black lightens the black areas of the matte, increasing their opacity. Output White sets the maximum opacity of the channel. Decreasing Output White darkens the white areas of the matte, decreasing their opacity. Alpha matte with Output Black raised Alpha matte with Output White lowered PixelChooser Parameter Group The PixelChooser is included in many Boris Þlters and provides several methods to selectively Þlter an image. You can use the PixelChooser to create a matte between Þltered and unþltered pixels, either by specifying a geometric region or by using the image s luma or color information. See the individual PixelChooser Þlter for more detailed information on the PixelChooser controls. The PixelChooser in this Þlter has one additional control. The Input Alpha menu determines how pixels not chosen by the PixelChooser are affected by the Þlter. Choose Alpha Off to make them opaque, Alpha On to make them transparent, or Layer Alpha to use the source alpha information for these pixels.

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