Commodore Amiga - Figure 4-1 Defining Sprite On-Screen Position

figure 4-1. Notice that because of display overscan, position (0,0) (that is, X = 0,
Y
= 0) is not normally in a viewable region of the screen.
(0.0)--.......-- _____ .....,._
I
Visible Screen Area
y
I---+-X----l~~t -::':.~I
Figure 4-1: Defining Sprite On-screen Position
The amount of viewable area is also affected by the size of the playfield display window.
See the "Playfield Hardware" chapter for more information about overscan and display
windows.
Horizontal Position
A sprite's horizontal position (X value) can be at any pixel on the screen from 0 to 447.
To be visible, however, an object must be within the boundaries of the playfield display
window. In addition, the normally usable range of the video screen is from pixel
64 to
pixel 383 (that is, 320 pixels of usable width). A larger area is actually scanned by the
video beam
but is not usually visible on the screen.
If you specify an X value for the sprite of less than 64 or an X value outside the display
window,
part or all of the sprite may not appear on the screen. This is sometimes desir-
able; such a sprite
is said to be "clipped."
To make a sprite appear, unclipped, in its correct on-screen horizontal position, add 64
to the X value. For example, to make the upper leftmost pixel of a sprite appear at a
position 94 pixels from the left edge of the screen, you would perform this calculation:
Sprite Hardware
93