POSITIONING SPRITES ON THE SCREEN

The entire display screen is divided into a grid of X and Y coordinates, like a graph. The X COORDINATE is the HORIZONTAL position across the screen and the Y COORDINATE is the VERTICAL position up and down (see Figure 3-4).

To position any sprite on the screen, you must POKE TWO SETTINGS... the X position and the Y position... these tell the computer where to display the UPPER LEFT HAND CORNER of the sprite. Remember that a sprite consists of 504 individual pixels, 24 across by 21 down... so if you POKE a sprite onto the upper left corner of your screen, the sprite will be displayed as a graphic image 24 pixels ACROSS and 21 pixels DOWN starting at the X-Y position you defined. The sprite will be displayed based on the upper left corner of the entire sprite, even if you define the sprite using only a small part of the 24X21-pixel sprite area.

To understand how X-Y positioning works, study the following diagram (Figure 3-5), which shows the X and Y numbers in relation to your display screen. Note that the GREY AREA in the diagram shows your television viewing area... the white area represents positions which are OFF your viewing screen...