Figure 7-3 diagrams setting up the blender node.

Figure 7-3. Setting Up the Blender Node

The blender node is created with the vlGetNode() function. The code fragment in Example 7-1 sets up source, drain, and blender nodes. Notice that the drain nodes are set up before the source nodes.

/* variable definitions */
{
 VLServer vlSvr;
 VLPath path;
 VLNode drn_scr, drn_vid, src_scr, src_vid, blend_node;
}

/* Open a video device */ 
vlSvr = vlOpenVideo(“”);

/* Set up drain nodes on the screen and video */
drn_scr = vlGetNode(vlSvr, VL_DRN, VL_SCREEN, VL_ANY);
drn_vid = vlGetNode(vlSvr, VL_DRN, VL_VIDEO, VL_ANY);

/* Set up source nodes on the screen and video */
src_scr = vlGetNode(vlSvr, VL_SRC, VL_SCREEN, VL_ANY);
src_vid = vlGetNode(vlSvr, VL_SRC, VL_VIDEO, VL_ANY);

/* Set up internal blending node */
blend_node = vlGetNode(vlSvr, VL_INTERNAL, VL_BLENDER,
                       VL_ANY);

Table 7-1 summarizes the generic VL blending controls. For all these controls, access is GST:

Some of the operations are not very useful, for example VL_BLDFCN_ONE, VL_BLDFCN_ONE adds the two images; the hardware clips pixels that are too bright.

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Note: When sending the blender output to video, it is best to blank the chroma.

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You can compose the 12 standard Porter-Duff operations by combining the values of VL_BLEND_A_FCN and VL_BLEND_B_FCN. Figure 7-4 gives some examples of compositing, assuming normalized inputs. Normalized background pixels for a frame are premultiplied by their corresponding alphas before they are blended.

Figure 7-4. Binary Compositing

Table 7-2 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):

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.

For screen inputs, set VL_BLEND_B_NORMALIZE to FALSE if the keyer mode is set to pass-through (VL_KEYERMODE_NONE), because the alpha has been applied once by the graphics in most cases. In other words, set VL_BLEND_B_NORMALIZE to FALSE if it is following another blender.

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

For the OCTANE Digital Video option, pixel source B is normally a flat background, supplied by a flat-background generator on this wire (see Figure 7-2). However, you can use device-dependent controls to set the background as desired:

The software does not prevent you from using values outside of the range (1-254):

The shadow hardware adds back the Y information from the area that was cut by the keyer. If a shadow exists in the cut area, the effect is to dim the pixels in the area of that shadow in the replaced background.

Use these device-dependent controls to set the shadow values:

Shadows that are very dark may be hard to key. These registers affect only the background pixels; foreground areas are passed through by the blender. Tune the values for the best effect.

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 7-6 illustrates the results of luma keying.

Figure 7-6. Luma Keying Application: Titling

The OCTANE Digital Video luma keying controls are summarized in Table 7-3. For each, the type is intVal and access is GST. The default value is persistent: it is initially obtained from the defaults file, but is never reset. Many controls available through the video control panel vcp (for example, the default video input) fall into this category. For this value, changes made by vlSetControl() are persistent across paths, even if the node goes into an unused state.

Figure 7-7 diagrams the relationships between these controls.

Figure 7-7. Relationships Between OCTANE Digital Video Luma Keying Controls

Chroma keying overlays one image on another based on the color value. Figure 7-8 illustrates an example of chroma keying.

Figure 7-8. Chroma Keying Application: TV Weather Map

Table 7-4 summarizes the controls for OCTANE Digital Video chroma keying and gives their ranges. For each, default is persistent, access is GST, and type is intVal.

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Note: VL_MGV_KEYER_FG_OPACITY has no effect on OCTANE Digital Video in chroma key mode.

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Figure 7-9 diagrams the relationships between these controls.

Figure 7-9. Relationships Between OCTANE Digital Video Chroma Keying Controls

The values used with the control VL_MGV_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_MGV_WIPETYPE_FADE;
vlSetControl(vlSvr, vlPath, blend_node, VL_MGV_WIPE_TYPE,
             &val); 

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

Table 7-5 summarizes controls common to all wipe types.

Table 7-6 summarizes the controls specific to wipes or that work differently for wipes. For each, access is GST and the default is persistent, except VL_MGV_WIPE_SYMMETRY and VL_MGV_WIPE_INVERT, for which it is FALSE. Some of these controls work in conjunction with each other.

simpleblend.c 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