HBLs - an extra splash of colour

    Canvas offers the chance to use
many more colours on-screen at once
than most art packages. Up to 512
colours (the whole ST palette) can be
displayed at once, or 4096 on an STE!
However, you wont be able to see all
these colours while you are drawing
your picture - only the usual 16
colour palette (lo-res).
    To access the extra colours is
quite a complex process. There are 3
options on the Colour Specbox with the
letters 'HBL' in them. HBL refers to a
programming technique called Horizontal
Blanking. This is simply a means of
'interrupting' the graphics chip as it
is in the process of drawing the
picture onto the television/monitor
screen. This process happens 50 times
every second to produce the illusion of
a stable picture. Every 4 scan lines
(there are 200 scan lines in lo-res),
the chip is interrupted by Canvas and
given a new palette to display. This
means that more than the usual 16
colours can be displayed. Any number of
HBLs may be set up using the options on
the Colour Specbox so long as they are
each no closer than 4 lines apart
(Canvas wont allow you to put them any
closer).
    To create a HBL, select the palette
you wish the chip to swap over to,
using the Palette Infobar at the top of
the Colour Specbox, and click on the
'Set HBL' option. A line will appear
across the screen. The height of this
line determines at what point the
graphics chip will be interrupted and
the new palette fed in. The Y
co-ordinate is displayed in the top
left of the screen. Click the right
mouse button if you do not wish to set
the HBL or the left button if you do.
    Once you have set some HBLs, select
the palette you want to appear at the
very top of the screen using the
Palette Infobar and click on 'View
HBLs'. See... lots of colours!  Press
any key to return to the Main Menu.

                A Demo

    On the Canvas program disk are some
files which demonstrate how HBls can be
used for stunning effects. First load
in the "SUNSET.CPT" compact picture
which is found in the "_HBLS" folder,
then load the "SUNSET.HBL" file. Now
View HBLs from the Colour Specbox -
pretty eh? You can also try the
"F_348.CPT" and "F_348.HBL" files which
too are found in the "_HBLS" folder.