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The Pac-Man Dossier


February 23, 2009 Article Start Previous Page 4 of 10 Next
 

Cornering

Pac-Man is able to navigate the turns in the maze faster than his enemies. He does not have to wait until he reaches the middle of a turn to change direction as the ghosts do (see picture below). Instead, he may start turning several pixels before he reaches the center of a turn and for several pixels after passing it.

Turns taken one or more pixels before reaching the center are "pre-turns"; turns taken one or more pixels after are "post-turns". Players learn to consistently move the joystick in the direction Pac-Man should go well before arriving at the center of a turn, ensuring each pre-turn is started as many pixels away from center as possible.

This technique is known as cornering and is one of the first skills a new Pac-Man player should master. For every successful pre-turn maneuver, Pac-Man puts a little more distance between himself and any ghosts following close behind.

Such a small gain in distance may not seem terribly significant at first, but cornering through a quick series of turns will shake off even the most determined pursuer. It is a vital tool for survival in the higher levels of the game.

Whenever Pac-Man makes a pre-turn or post-turn, his orientation changes, and he starts to move one pixel in his new direction for every pixel traveled in his old direction, effectively doubling his speed as he moves at a 45 degree angle. Once he reaches the centerline of the new direction's path, he starts moving purely in that direction and his speed returns to normal.

The greatest distance advantage is thereby gained by making the earliest pre-turn possible. The illustration below shows the layout of pre-turn pixels (shown in green), center point pixels (shown in yellow), and post-turn pixels (shown in red) for each of the four possible directions a turn can be approached. Each example shows Pac-Man entering the same four-way intersection from a different direction.

When entering from the left, there are three pre-turn pixels before the center of the turn, and four post-turn pixels. Conversely, entering the same intersection from the right yields four pre-turn pixels and three post-turn ones. Entering from the top as opposed to the bottom exhibits the same property.

For any turn that is made later than the earliest possible pre-turn, Pac-Man will be one frame behind where he would be for every pixel of "lateness" in the turn. Basically, it pays to move the joystick well before reaching a turn to maximize your speed.


(click image for full size)

Turning at the earliest pre-turns possible is also required to successfully execute most any pattern. Patterns are meant to be played with perfect cornering because it removes the human element of uncertainty as to when Pac-Man will turn. Without cornering, it would be nigh-impossible to reproduce the exact timing of every turn as made by the pattern's author, thereby increasing the possibility of unpredictable ghost behavior due to Pac-Man not being in the exact same tile at the exact same time anymore.

Typically, the most popular patterns have been those that tend to "hold together" well when small input timing flaws occur (turning three pixels away from center instead of four when approaching a turn from the right is a timing flaw, for example). Other patterns-especially those that bring Pac-Man very close to the ghosts late in the sequence-tend to "fall apart" unless every turn is perfectly cornered.

During a long Pac-Man session, even the best players will make occasional timing mistakes during a fast series of turns and have to deal with the possible consequences. As such, one should aim for perfect cornering at all times but remain alert for unexpected ghost behavior from subtle input timing flaws creeping into the pattern.

Home Sweet Home

Commonly referred to as the ghost house or monster pen, this cordoned-off area in the center of the maze is the domain of the four ghosts and off-limits to Pac-Man.

Whenever a level is completed or a life is lost, the ghosts are returned to their starting positions in and around the ghost house before play continues-Blinky is always located just above and outside, while the other three are placed inside: Inky on the left, Pinky in the middle, and Clyde on the right.

The pink door on top is used by the ghosts to enter or exit the house. Once a ghost leaves, however, it cannot reenter unless it is first captured by Pac-Man-then the disembodied eyes can return home to be revived. Since Blinky is already on the outside after a level is completed or a life is lost, the only time he can get inside the ghost house is after Pac-Man captures him, and he immediately turns around to leave once revived.

That's about all there is to know about Blinky's behavior in terms of the ghost house, but determining when the other three ghosts leave home is an involved process based on several variables and conditions. The rest of this section will deal with them exclusively. Accordingly, any mention of "the ghosts" below refers to Pinky, Inky, and Clyde, but not Blinky.

The first control used to evaluate when the ghosts leave home is a personal counter each ghost retains for tracking the number of dots Pac-Man eats. Each ghost's "dot counter" is reset to zero when a level begins and can only be active when inside the ghost house, but only one ghost's counter can be active at any given time regardless of how many ghosts are inside.

The order of preference for choosing which ghost's counter to activate is: Pinky, then Inky, and then Clyde. For every dot Pac-Man eats, the preferred ghost in the house (if any) gets its dot counter increased by one. Each ghost also has a "dot limit" associated with his counter, per level.

If the preferred ghost reaches or exceeds his dot limit, it immediately exits the house and its dot counter is deactivated (but not reset). The most-preferred ghost still waiting inside the house (if any) activates its timer at this point and begins counting dots.

Pinky's dot limit is always set to zero, causing him to leave home immediately when every level begins. For the first level, Inky has a limit of 30 dots, and Clyde has a limit of 60. This results in Pinky exiting immediately which, in turn, activates Inky's dot counter. His counter must then reach or exceed 30 dots before he can leave the house.

Once Inky starts to leave, Clyde's counter (which is still at zero) is activated and starts counting dots. When his counter reaches or exceeds 60, he may exit. On the second level, Inky's dot limit is changed from 30 to zero, while Clyde's is changed from 60 to 50. Inky will exit the house as soon as the level begins from now on.

Starting at level three, all the ghosts have a dot limit of zero for the remainder of the game and will leave the ghost house immediately at the start of every level.

Whenever a life is lost, the system disables (but does not reset) the ghosts' individual dot counters and uses a global dot counter instead. This counter is enabled and reset to zero after a life is lost, counting the number of dots eaten from that point forward.

The three ghosts inside the house must wait for this special counter to tell them when to leave. Pinky is released when the counter value is equal to 7 and Inky is released when it equals 17. The only way to deactivate the counter is for Clyde to be inside the house when the counter equals 32; otherwise, it will keep counting dots even after the ghost house is empty.

If Clyde is present at the appropriate time, the global counter is reset to zero and deactivated, and the ghosts' personal dot limits are re-enabled and used as before for determining when to leave the house (including Clyde who is still in the house at this time).

If dot counters were the only control, Pac-Man could simply stop eating dots early on and keep the ghosts trapped inside the house forever. Consequently, a separate timer control was implemented to handle this case by tracking the amount of time elapsed since Pac-Man has last eaten a dot. This timer is always running but gets reset to zero each time a dot is eaten.

Anytime Pac-Man avoids eating dots long enough for the timer to reach its limit, the most-preferred ghost waiting in the ghost house (if any) is forced to leave immediately, and the timer is reset to zero. The same order of preference described above is used by this control as well. The game begins with an initial timer limit of four seconds, but lowers to it to three seconds starting with level five.

The more astute reader may have already noticed there is subtle flaw in this system resulting in a way to keep Pinky, Inky, and Clyde inside the ghost house for a very long time after eating them. The trick involves having to sacrifice a life in order to reset and enable the global dot counter, and then making sure Clyde exits the house before that counter is equal to 32.

This is accomplished by avoiding eating dots and waiting for the timer limit to force Clyde out. Once Clyde is moving for the exit, start eating dots again until at least 32 dots have been consumed since the life was lost. Now head for an energizer and gobble up some ghosts. Blinky will leave the house immediately as usual, but the other three ghosts will remain "stuck" inside as long as Pac-Man continues eating dots with sufficient frequency as not to trigger the control timer.

Why does this happen? The key lies in how the global dot counter works-it cannot be deactivated if Clyde is outside the house when the counter has a value of 32. By letting the timer force Clyde out before 32 dots are eaten, the global dot counter will keep counting dots instead of deactivating when it reaches 32. Now when the ghosts are eaten by Pac-Man and return home, they will still be using the global dot counter to determine when to leave.

As previously described, however, this counter's logic only checks for three values: 7, 17, and 32, and once those numbers are exceeded, the counter has no way to release the ghosts associated with them. The only control left to release the ghosts is the timer which can be easily avoided by eating a dot every so often to reset it. Click on the YouTube video below to see a demonstration of this curious behavior:

The last thing to mention about the ghost house is how to determine whether a ghost will move right or left after exiting the home. Ghosts typically move to the left once they get outside, but if the system changes modes one or more times when a ghost is inside, that ghost will move to the right instead of the left upon leaving the house.

Areas To Exploit

The illustration above highlights four special "zones" in the maze where ghost behavior is limited by certain conditions which can be exploited to the player's advantage. The two red zones represent the areas where ghosts are forbidden to make upward turns. Once a ghost enters either of these two zones, it may only travel from right-to-left or left-to-right until exiting the area.

Thus, only Pac-Man has access to these four, upward-facing tunnel entrances. It will serve the player well to remember the ghosts can still access these tunnels from the other end! The red zone restrictions are enforced during both scatter and chase modes, but in frightened mode the red zones are ignored temporarily, allowing the ghosts to turn upwards if they so choose.

The pink zones are in the two halves of the connecting side-tunnel. As mentioned previously, any ghost that enters the tunnel will suffer an immediate speed penalty until leaving the zone. This slow-down rule is always enforced and applies to ghosts only-Pac-Man is immune.


Article Start Previous Page 4 of 10 Next

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