In other words :
- When the destination mask use (DMU) parame-
ter is set :
- a pel may be modified when its mask bit is set
- a pel cannot be modified when its mask bit is
reset.
- When the destination mask use (DMU) parame-
ter is cleared :
- a pel may be modified, independently of its
mask bit value.
This provides a very flexible clipping mechanism
not restricted to rectangular windows. (See desti-
nation pointer section for destination mask bit ad-
dressing).
II.3.2 - SOURCE MASK USE (SMU)
A PRINT OBJECT command may be parametered
for source mask use. In this case, the source mask
bit associated with any source pel is read first.
When its mask bit is cleared, a source pel is con-
sidered as transparent. (See source pointer section
for source mask bit addressing).
In other words :
- When the SMU parameter is set, the color of a
destination pel, mapped by a given source pel,
may take this source color value only when this
source bit mask is set. The destination pel keeps
its own color value when the source bit mask is
cleared.
- When the SMU parameter is cleared, a source
pel color may be mapped into destination pel
color independently of the source bit mask value.
The source bit mask acts as a TRANSPAR-
ENCY/OPACITY flag which is enabled by SMU. A
PRINT OBJECT command may be independently
parametered by both SMU and DMU. This provides
a very powerful tiling, print object or move mecha-
nism.
II.4 - Drawing Attributes
0
1
2
3
4
5
6
7
ODD
BANK
EVEN
BANK
68483-08.EPS
Figure 6 : Color Register
The general drawing attributes are the colors, the
drawing mode, and the scaling factor.
II.4.1. COLORS : Registers R1 and R2
Two 8-bit color values, C0 and C1, may be speci-
fied in registers R1 and R2. The low order 4-bit
nibble of a color value is drawn in an even bank.
The high order color nibble is drawn in an odd bank.
When long pels are used, banks 0 and 1 are
generally addressed as the frame buffer. When
short pels are used, any bank may hold a frame
buffer. In this case, the bank parity selects the color
nibble used. (See destination pointer section for
bank addressing).
II.4.2. DRAWING MODE : Register R0
The drawing mode defines the transforms to be
applied to the pels designated by the drawing
commands. There are three drawing modes.
II.4.3. MONOCHROME MODE
Any AREA drawing command, RECTANGLE for
instance, defines through its geometric arguments
an active set of destination pels, that is to say a set
of pels to be modified.
When DMU = 1, this active set is further reduced
by the masking mechanism to only these destina-
tion pels with a bit mask set.
The active destination pels are then modified ac-
cording to two elementary transforms coded in R0.
COLOR TRANSFORM :
The color value C of each active pel is modified
according to one color transform selected out of
four :
- 00 - printed in C0 : C
←
C0
- 01 - printed in C1 : C
←
C1
- 10 - printed in ”transparent” : C
←
C
- 11 - complemented : C
←
C
This yields to a reversible marker mode.
MASK BIT TRANSFORM :
The destination mask bit of each active pel is
modified according to one mask transform selected
out of four :
- 00 - reset bit mask : M
←
0
- 01 - set bit mask : M
←
1
- 10 - no modification : M
←
M
- 11 - complement bit mask : M
←
M
This scheme allows the color bits and the mask bit
of any pel belonging to the active set to be modified
independently. The color transform is performed
first.
II.4.4 - BICHROME MODE
A PRINT CHARACTER command is more complex
because it involves two different active sets :
FOREGROUND and BACK GROUND.
The FOREGROUND is that set of destination pels
printed from set elements in the character cell. The
BACKGROUND is made of all the remaining pels
belonging to the destination window.
When DMU = 1, the FOREGROUND and BACK
GROUND are further reduced by the destination
masking mechanism (see Figure 8).
A bichrome drawing mode is defined by 4 elemen-
tary
and independent transforms
(see Fig-
ure 7) :
- a color transform and a mask transform for the
FOREGROUND PELS
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