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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 the examples in this
chapter, a window with horizontal positions from pixel 64 to pixel 383 is
used (that is, each line is 320 pixels long).  Larger or smaller windows 
can be defined as required, but it is recommended that you read the
"Playfield Hardware chapter" before attempting to do so. A larger area is
actually scanned by the video beam but is not usually visible on the

If you specify an X value for a sprite that takes it outside the display
window, then part or all of the sprite may not appear on the screen. This
is sometimes desirable; such a sprite is said to be " clipped ."

To make a sprite appear in its correct on-screen horizontal position in
the display window, simply add its left offset to the desired X value. In
the example given above, this would involve adding 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

 Desired X position + horizontal-offset of display window ~= 94 + 64 = 158

Thus, 158 becomes the X value, which will be written into the
 data structure .

   Counting Pixels.
   The X position represents the location of the very first (leftmost)
   pixel in the full 16-bit wide sprite. This is always the case, even
   if the leftmost pixels are specified as transparent and do not appear
   on the screen.

If the sprite shown in Figure 4-2 were located at an X value of 158, the
actual image would begin on-screen four pixels later at 162.  The first
four pixels in this sprite are transparent and allow the background to
show through.

                |-- 4 --|
                |pixels |

                |                               |
                |--------- 16 pixels -----------|

                 Figure 4-2: Position of Sprites

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