By Ewout Walraven


The ECS Denise chip (8373-R2a), coupled with the release 2.0
graphics.library, opens up a whole new set of genlocking
possibilities.  Unlike the old Denise, whose only genlocking ability
allowed keying1 on color register zero, the ECS Denise allows keying
on any color register.  Also, the ECS Denise allows keying on any
bitplane of the ViewPort being genlocked.  With the ECS Denise, the
border area surrounding the display can be made transparent (always
passes video) or opaque (overlays using color 0).  All the new
features are set individually for each ViewPort.  These features can
be used in conjunction with each other, making interesting scenarios
possible.  These hardware capabilities and the software support in
release 2.0 offer new challenges for video oriented software.


VideoControl()
--------------

The graphics.library function VideoControl() modifies the operation of
a ViewPort's ColorMap and also reports on the ColorMap's status.

    error = BOOL VideoControl( struct ColorMap *colormap,
                           struct TagItem *videocommandstagarray );

Using VideoControl(), a program can enable, disable, or obtain the
state of a ViewPort's genlocking features.  It returns NULL if no
error occurred.  The function uses a tag based interface.

This article is concerned with the following VideoControl() tags:

    VTAG_BITPLANEKEY_GET
    VTAG_BITPLANEKEY_SET
    VTAG_BITPLANEKEY_CLR
    VTAG_CHROMA_PLANE_GET
    VTAG_CHROMA_PLANE_SET
    VTAG_BORDERBLANK_GET
    VTAG_BORDERBLANK_SET
    VTAG_BORDERBLANK_CLR
    VTAG_BORDERNOTRANS_GET
    VTAG_BORDERNOTRANS_SET
    VTAG_BORDERNOTRANS_CLR
    VTAG_CHROMAKEY_GET
    VTAG_CHROMAKEY_SET
    VTAG_CHROMAKEY_CLR
    VTAG_CHROMAPEN_GET
    VTAG_CHROMAPEN_SET
    VTAG_CHROMAPEN_CLR

See <graphics/videocontrol.h> for a complete list of all the available
tags.

VTAG_BITPLANEKEY_GET is used to find out the status of the bitplane
keying mode.  VTAG_BITPLANEKEY_SET and VTAG_BITPLANEKEY_CLR activate
and deactivate bitplane keying mode.  If bit plane key mode is on,
genlocking will key on the bits set in a specific bitplane from the
ViewPort (the specific bitplane is set with a different tag).  The
data portion of these tags is NULL.

For inquiry commands like VTAG_BITPLANEKEY_GET (tags ending in _GET),
VideoControl() changes the _GET tag ID to the corresponding _SET or
_CLR tag ID, reflecting the current state.  For example, when passed
the following tag array:

    struct TagItem videocommands[] =
    {
        { VTAG_BITPLANEKEY_GET, NULL},
        {VTAG_END_CM, NULL}
    };

VideoControl() changes the VTAG_BITPLANEKEY_GET ID to
VTAG_BITPLANEKEY_SET if bit plane keying is currently on, or to
VTAG_BITPLANEKEY_CLR if bit plane keying is off.  In both of these
cases, VideoControl() only uses the tag's ID, ignoring the tag's data
field.

The VTAG_CHROMA_PLANE_GET tag returns the number of the bitplane keyed
on when bit plane keying mode is on.  VideoControl() changes the tag's
data value to the bitplane number.  VTAG_CHROMA_PLANE_SET sets the
bitplane number to the tag's data value.

VTAG_BORDERBLANK_GET is used to obtain the border blank mode status.
This tag works exactly like VTAG_BITPLANEKEY_GET.  VideoControl()
changes the tag's ID to reflect the current border blanking state.
VTAG_BORDERBLANK_SET and VTAG_BORDERBLANK_CLR activate and deactivate
border blanking.  If border blanking is on, the Amiga will not display
anything in its display border, allowing an external video signal to
show through the border area.  On the Amiga display, the border
appears black.  The data portion of these tags is NULL.

Respectively, the VTAG_BORDERNOTRANS_GET, VTAG_BORDERNOTRANS_SET, and
VTAG_BORDERNOTRANS_CLR tags are used to obtain the status of
BorderNoTransparent mode, and to activate and to deactivate this mode.
If set, the Amiga display's border will overlay external video with
the color in register 0.  Because border blanking mode takes
precedence over BorderNoTransparent mode, setting BorderNoTransparent
has no effect if border blanking is on.  The data portion of these
tags is NULL.

Respectively, the VTAG_CHROMAKEY_GET, VTAG_CHROMAKEY_SET, and
VTAG_CHROMAKEY_CLR tags are used to obtain the status of chroma keying
mode, and to activate and deactivate chroma keying mode.  If set, the
genlock will key on colors from specific color registers (the specific
color registers are set using a different tag).  If chroma keying is
not set, the genlock will key on color register 0.  The data portion
of these tags is NULL.

VTAG_CHROMAPEN_GET obtains the chroma keying status of an individual
color register.  The tag's data field contains the register number.
Like the other _GET tags, VideoControl() changes the tag ID to one
reflecting the current state.  VTAG_CHROMAPEN_SET and
VTAG_CHROMAPEN_CLR activate and deactivate chroma keying for each
individual color register.  chroma keying can be active for more than
one register.  By turning off border blanking and activating chroma
keying mode, but turning off chroma keying for each color register, a
program can overlay every part of an external video source, completely
blocking it out.

After using VideoControl() to set values in the ColorMap, the
ColorMap's ViewPort has to be rebuilt with MakeVPort(), MrgCop(), and
LoadView(), so the changes can take effect.  A program that uses a
screen's ViewPort rather than its own ViewPort should use the
Intuition functions MakeScreen() and RethinkDisplay() to make the
display changes take effect.

The following example, genlockdemo.c, shows how to add genlock support
for an Intuition screen.