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The Pac-Man Dossier

February 23, 2009 Article Start Previous Page 7 of 10 Next
 

Chapter 4: Meet the Ghosts

"First, you've got to learn how to control the monsters. See how the red, pink and blue are grouped together? It's easier to control two monsters than four."-Billy Mitchell, champion Pac-Man player

In the last chapter, we learned how a ghost follows a target tile through the maze. Now we will take a closer look at Blinky, Pinky, Inky, and Clyde to better understand why they behave so differently when in chase mode. They all share the same pathfinding logic for chasing a target tile, so how is it each one behaves differently when following Pac-Man?

The answer is delightfully simple: Pac-Man's tile is not always the target. Every ghost has a distinct method for calculating its target tile in chase mode, resulting in their unique personalities. Some of the ghosts use Pac-Man's actual tile as the target; others only use it as an intermediate step to find another tile.

Sometimes a ghost is targeting a tile that has absolutely nothing to do with Pac-Man at all! Regardless of where a ghost's target tile is at the time, Pac-Man will still be killed if he gets in that ghost's way.

Rumor has it Toru Iwatani and his team spent months doing nothing but tweaking and refining the ghost A.I. routines before releasing Pac-Man to the world. Their efforts show in the final product: Itawani's team created the illusion of complex pathfinding by using very simple logic and very little code.

Blinky: The red ghost's character is aptly described as that of a shadow and is best-known as "Blinky". In Japan, his character is represented by the word oikake, which means "to run down or pursue". Blinky seems to always be the first of the ghosts to track Pac-Man down in the maze. He is by far the most aggressive of the four and will doggedly pursue Pac-Man once behind him.

Of all the ghosts' targeting schemes for chase mode, Blinky's is the most simple and direct, using Pac-Man's current tile as his target. In the pictures above, we can see Blinky's target tile is the same as Pac-Man's currently occupied tile. Targeting Pac-Man directly in this way results in a very determined and tenacious ghost who is tough to shake when he's right behind you.

All ghosts move at the same rate of speed when a level begins, but Blinky will increase his rate of speed twice each round based on the number of dots remaining in the maze. While in this accelerated state, Blinky is commonly called "Cruise Elroy", yet no one seems to know where this custom was originated or what it means.

On the first level, for example, Blinky becomes Elroy when there are 20 dots remaining in the maze, accelerating to be at least as fast as Pac-Man. More importantly, his scatter mode behavior is also modified to target Pac-Man's tile in lieu of his typical fixed target tile for any remaining scatter periods in the level.

This causes Elroy to chase Pac-Man while the other three ghosts continue to scatter as normal. As if that weren't bad enough, when only 10 dots remain, Elroy speeds up again to the point where he is now perceptibly faster than Pac-Man.

If a life is lost any time after Blinky has become Elroy, he will revert back to his normal behavior and speed when play resumes, heading for his home corner during the initial scatter period. But once the last ghost (Clyde) has left the ghost house in the middle of the board, he will turn back into Elroy again.

Keep in mind: he is still in scatter mode the entire time. All that has changed is the target tile-he will still reverse direction when entering and exiting scatter mode as before. As the levels progress, Blinky will turn into Elroy with more dots remaining in the maze than in previous rounds. Refer to Table A.1 in the appendices for dot counts and speeds for both Elroy changes, per level.

Pinky: Nicknamed "Pinky", the pink ghost's character is described as one who is speedy. In Japan, he is characterized as machibuse, meaning "to perform an ambush", perhaps because Pinky always seems to be able to get ahead of you and cut you off when you least expect it.

He always moves at the same speed as Inky and Clyde, however, which suggests speedy is a poor translation of the more appropriate machibuse. Pinky and Blinky often seem to be working in concert to box Pac-Man in, leaving him with nowhere to run.

In chase mode, Pinky behaves as he does because he does not target Pac-Man's tile directly. Instead, he selects an offset four tiles away from Pac-Man in the direction Pac-Man is currently moving (with one exception). The pictures below illustrate the four possible offsets Pinky will use to determine his target tile based on Pac-Man's orientation:

If Pac-Man is moving left, Pinky's target tile will be four game tiles to the left of Pac-Man's current tile. If Pac-Man is moving right, Pinky's tile will be four tiles to the right. If Pac-Man is moving down, Pinky's target is four tiles below.

Finally, if Pac-Man is moving up, Pinky's target tile will be four tiles up and four tiles to the left. This interesting outcome is due to a subtle error in the logic code that calculates Pinky's offset from Pac-Man. This piece of code works properly for the other three cases but, when Pac-Man is moving upwards, triggers an overflow bug that mistakenly includes a left offset equal in distance to the expected up offset (we will see this same issue in Inky's logic later).

Don Hodges' website has an excellent article giving a thorough, code-level analysis of this bug, including the source code and a proposed fix-click here to go there now.

Pinky is the easiest ghost to exert control over thanks to his targeting scheme. By changing direction, you can dictate where Pinky will turn next when he is nearby (see above picture). If you are facing off closely with Pinky, he will turn before he reaches you if he can. This happens due to the fact Pac-Man has come close enough to Pinky for Pinky's target tile to now be behind him.

In the picture above, Pinky chooses to turn up at the intersection because moving left would have taken him further away from his target tile. The longest-lived example of this is the technique known as "head faking". This is where the player shakes the joystick to cause Pac-Man to rapidly change direction back and forth, hopefully causing a ghost to change course in the process.

As it turns out, the shaking is not necessary-one well-timed, quick reversal of direction towards Pinky just before he decides what to do at an upcoming intersection is all that is needed to get him off your tail.

Inky: The light-blue ghost is nicknamed "Inky" and his character is described as one who is bashful. In Japan, he is portrayed as kimagure, meaning "a fickle, moody, or uneven temper". Perhaps not surprisingly, Inky is the least predictable of the ghosts.

Sometimes he chases Pac-Man aggressively like Blinky; other times he jumps ahead of Pac-Man as Pinky would. He might even wander off like Clyde on occasion!

In fact, Inky may be the most dangerous ghost of all due to his erratic behavior. Bashful is not a very good translation of kimagure, and misleads the player to assume Inky will shy away from Pac-Man when he gets close which is not always the case.

Inky uses the most complex targeting scheme of the four ghosts in chase mode. He needs Pac-Man's current tile/orientation and Blinky's current tile to calculate his final target. To envision Inky's target, imagine an intermediate offset two tiles away from Pac-Man's tile in the direction Pac-Man is moving (shown as the dashed, green tile in the picture above), then draw a line from Blinky's tile to that offset. Now double the line length by extending the line out just as far again, and you will have Inky's target tile as shown above.

For the same reasons already discussed in Pinky's case, Inky's offset calculation from Pac-Man is two tiles up and two tiles left when Pac-Man is moving up (shown above). The other three orientations have the expected offset of two tiles in the direction Pac-Man is moving.

Inky's targeting logic will keep him away from Pac-Man when Blinky is far away from Pac-Man, but as Blinky draws closer, so will Inky's target tile. This explains why Inky's behavior seems more variable as Pac-Man moves away from Blinky. Like Pinky, Inky's course can often be altered by Pac-Man changing direction or "head-faking". How much or how little effect this will have on Inky's decisions is directly related to where Blinky is at the time.

Clyde: The orange ghost is nicknamed "Clyde" and is characterized as one who is pokey. In Japan, his character is described as otoboke, meaning "pretending ignorance", and his nickname is "Guzuta", meaning "one who lags behind".

In reality, Clyde moves at the same speed as Inky and Pinky so his character description is a bit misleading. Clyde is the last ghost to leave the pen and tends to separate himself from the other ghosts by shying away from Pac-Man and doing his own thing when he isn't patrolling his corner of the maze.

Although not nearly as dangerous as the other three ghosts, his behavior can seem unpredictable at times and should still be considered a threat.

In chase mode, Clyde's target differs based on his proximity to Pac-Man. When more than eight tiles away, he uses Pac-Man's tile as his target (shown as the yellow target above). If Clyde is closer than eight tiles away, he switches to his scatter mode target instead, and starts heading for his corner until he is far enough away to target Pac-Man again.

In the picture above, Clyde is stuck in an endless loop thanks to his targeting scheme. Outside of the dashed area, Clyde acts exactly as Blinky would, heading straight for Pac-Man, but upon entering the dashed area, Clyde will change his mind and head for his scatter target instead.

Leaving the eight tile perimeter surrounding Pac-Man causes his target to change back to Pac-Man's tile and results in Clyde circling the island indefinitely until Pac-Man moves elsewhere or a mode change occurs.

Clyde's targeting method results in him not being particularly dangerous unless you get in his way as he runs back to his corner or before he can reach an intersection to turn away. Extra care should be taken when Pac-Man is in Clyde's home corner as Clyde is less likely to get out of the way.


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