Figure 5-2 diagrams setting up the blend node.

The blend node mixes the foreground and background video signals by applying a blend function to the foreground and background pixels.

The blend node then sends the data to the drain node. For example, blending analog video with part of a graphics screen and sending it to video out can be diagrammed as shown in Figure 5-3.

Blending analog video with static frame data and sending it to video out can be diagrammed as shown in Figure 5-4.

The VL uses blending controls to set blending options.

All blending controls—that is, all the controls discussed in this chapter—apply only to blend nodes, except for VL_EV1_ALPHA_NOT_PIXEL, which applies to drain nodes. The order of blending and zooming depends on the node type: for a source, zooming takes place before blending; for a drain, blending takes place before zooming.

Table 5-1 summarizes the VL controls for blending.

The blender has two inputs, path A and path B. Each has an associated alpha that is separately controllable. IMPACT Indigo² Video-specific VL controls set these inputs; they are discussed in “Using VL to Set Up the Blend Node,” later in this chapter. Settings in the panel control blending of these paths.

To use the panel to set the blend functions that control mixing of frames from paths A and B, select “Blend/Wipe Node” in the Pro menu. The following choices appear:

Select “Path Controls”; the Blend/Wipe Node-Path Controls window appears, as shown in Figure 5-5.

Notice that Blend Function A serves as foreground and Blend Function B serves as background.

The choices for foreground and background pixel and alpha values differ, however, as shown in Table 5-2.

Table 5-3 shows the choices for the two blend functions A and B, which correspond exactly.

The value 0.0 sets the display to black (cut the foreground or background value); the value 1.0 sets the display to white (pass the foreground or background value):

For foreground to background wipes, background alpha is set to a constant value of 1.0 so that the background shows through the foreground.

The checkboxes at the top of the Path Controls window set normalization on for each blending source. Normalized background pixels for a frame are premultiplied by their corresponding alphas before they are blended (Porter-Duff model). Figure 5-6 gives some examples of compositing, assuming normalized inputs.

Normally, chroma is multiplied (scaled) by the selected alpha. For example, the value on source A can be multiplied by its own alpha value or that from source B. In a normal blend, f(A), the incoming alpha of source A is applied to the value for A. In the inverse of this blend, f(A)=1-f(A), the region that was considered opaque (turned off), that is, outside the volume defined for keying, is applied to source A.

In another way of blending, the alpha from source B can be applied to the component represented by source A. In the inverse of this blend, f(A)=1-f(B), the region that was turned off for source B is applied to source A.

Luma keying is typically used to overlay a fixed image on video, such as the name and title of an individual being interviewed, a cable channel's logo, or a symbol that denotes an ongoing news story during a newscast. Figure 5-8 diagrams an application.

The Indigo² Video for Indigo² IMPACT luma keying controls are summarized in Table 5-4; each is of type intVal.

Figure 5-9 diagrams the relationships between these controls.

Chroma keying overlays one image on another based on the color value. Figure 5-10 diagrams a common application.

Table 5-5 summarizes the controls for Indigo² Video for Indigo² IMPACT chroma keying and gives their ranges. These controls are all of type intVal.

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Note: VL_EV1_KEYER_FG_OPACITY has no effect on Indigo2 Video for Indigo2 IMPACT in chroma key mode.

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Figure 5-11 diagrams the relationships between these controls.

The values used with the control VL_EV1_WIPE_TYPE determine the type of blending performed:

For example, the following fragment specifies that a fade is to be performed:

VLControlType val;
val.intVal = VL_EV1_WIPETYPE_FADE;
vlSetControl(vlSvr, vlPath, blend_node, VL_EV1_WIPE_TYPE,
             &val); 

Fades, tiles, and wipes go from all-foreground (VL_EV1_WIPE_POSN=0) to all-background (VL_EV1_WIPE_POSN=1000), unless VL_EV1_WIPE_INVERT control is set, in which case they go from all-background (VL_EV1_WIPE_POSN = 0) to all-foreground (VL_EV1_WIPE_POSN = 1000).

Table 5-6 summarizes controls common to all wipe types.

Table 5-7 summarizes the controls specific to wipes or that work differently for wipes. Some of these controls work in conjunction with each other.

In the vcp panel, the Blend/Wipe Node-All window is linked to the controls VL_EV1_WIPE_POSN_PERP, VL_EV1_WIPE_CENT, VL_EV1_WIPE_CENT_PERP, and VL_EV1_WIPE_REPT_PERP.

The role of the keyer is to take a pixel stream and produce an alpha stream. It generates a key for each pixel in each source node:

The keyer determines the alpha value (opacity) of a pixel and sets a value for it ranging from 0 (completely transparent) to 1 (completely opaque). This alpha value can be used downstream for further layering operations. The program simpleblend.c illustrates this procedure; it is included in the software and described at the end of this chapter.

In the video panel vcp, select the “Blend/Wipe Node” choice in the Pro menu. Select “Keying Controls”; The Blend/Wipe Node-Keying window appears, as shown in Figure 5-12.

In this window, Keyer Mode can be ALPHA MODE, LUMA KEY, CHROMA KEY, or TRANSITION. The keyer mode setting determines the values you can set with the sliders in this window.

The sliders in this Blend/Wipe Node-Keying window have the following purposes:

Figure 5-13 shows the relationships between value, range, and detail (transition) for a single channel (for example, A).

Use the sliders to set the values you want, based on what you see on the screen. Once you are satisfied with the settings, use the values shown at the sliders to insert into your code.

simpleblend.c, which blends video with graphics and outputs it to both a graphics window and video out. The program

Like simpleblend.c, simplewipe.c blends video with graphics and outputs it to a graphics window and video out. When the user presses the w key, it executes a wipe.

Specifically, in addition to doing everything that simpleblend.c does, simplewipe.c