Agent Wander
for Atari 8-bit Home Computers

Graphics Implementation

The game uses ANTIC Mode 4 which is a 40 x 24 character mode that can display five colors on screen. Each character has a resolution of 4 x 8 pixels giving the screen a resolution of 160 x 192.

One of the colors is the playfield background color. Two more colors are set and available in any character. The third and fourth colors cannot be used at the same time in the same character. The third color is used for regular characters 0-127 and the fourth color is used for inverse characters 128-255.

More Colors Using Flipping

The game has two character sets. One character set is display on even number frames and the other on odd numbered frames. The result is flipping back and forth between two character definitions with the goal that they blend together through phosphor persistence on CRT monitors. In emulation, there is usually a "blend frames" type of option that will simulate phosphor persistence.

Below are examples of how the platform graphics are formed.

First character set:

Second character set:

Combined appearance using character set flipping:

The result is a platform that appears to have seven colors by blending the four colors available within each character.

The same flipping technique is used with the Player/Missile graphics for the player sprite. There is a single P/M Player used to display the player's sprite and it flips back and forth between two sprites to give the appearance of three colors.

First player sprite:

Second player sprite:

Combined player sprite:

Downside

You may experience different levels of flickering on real Atari hardware on a CRT. Higher quality CRTs will probably experience more flickering due to less phosphor persistence. CRTs with less phosphoer persistence help eliminate "ghosting" that could be seen when a moving object appears on screen, especially if it is a bright object against a dark background. The flipping technique used in this game relies on the ghosting effect to blend the colors.