Advanced Topics; Hold-And-Modify Mode - Commodore Amiga Hardware Reference Manual

Advanced Topics
This section describes features that are used less often or are optional.
INTERACTIONS AMONG PLAYFIELDS AND OTHER OBJECTS
Playfields share the display with sprites. Chapter 7, "System Control Hardware," shows
how play fields can be given different video display priorities relative to the sprites and
how playfields can collide with (overlap) the sprites or each other.
HOLD-AND-MODIFY MODE
This is a special mode that allows you to produce up to 4,096 colors on the screen at the
same time. Normally, as each value formed by the combination of bit-planes is selected,
the data contained in the selected color register is loaded into the color output circuit for
the pixel being written on the screen. Therefore, each pixel is colored by the contents of
the selected color register.
In hold-and-modify mode, however, the value in the color output circuitry is held, and
one of the three components of the color (red, green, or blue) is modified by bits coming
from certain preselected bit-planes. After modification, the pixel is written to the screen.
The hold-and-modify mode allows very fine gradients of color or shading to be produced
on the screen. For example, you might draw a set of 16 vases, each a different color,
using all 16 colors in the color palette. Then, for each vase, you use hold-and-modify to
very finely shade or highlight or add a completely different color to each of the vases.
Note that a particular hold-and-modify pixel can only change one of the three color
values at a time. Thus, the effect has a limited control.
In hold and modify mode, you use all six bit-planes. Planes 5 and 6 are used to modify
the way bits from planes 1 - 4 are treated, as follows:
o
If the 6-5 bit combination from planes 6 and 5 for any given pixel is 00, normal
color selection procedure is followed. Thus, the bit combinations from planes
4 - 1, in that order of significance, are used to choose one of 16 color registers
(registers 0- 15).
If only five bit-planes are used, the data from the sixth plane is automatically
supplied with the value as 0.
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