Blaster Master Area 7 Skips

Blaster Master Area 7 Skips
by ZeroNoin

***Work in Progress***

Crawling Drop
There is actually a good use for crawling in this game.
Normally when your character falls 4 blocks distance it does full damage (so it is normally lethal).
However when your character crawls into the fall of 4 block distance it does only 2 bars of damage.
Your position is 4 pixels lower in the RAM when crawling; the main intended purpose of this was probably so bullets would position lower.
When you crawl into a fall, your character pops out of the ledge starting the fall 4 pixels lower.
Instead of reaching a velocity of 62 (0x3E; 3-7/8 pixels per frame), which is set as the minimum velocity for full fall damage, you reach a velocity of 60 (0x3C; 3-3/4 pixels per frame).

Area 7 1st Skip

Description
The skip is done in the room near the start where breakable blocks in the floor prevent progress forward, but allow progress back.
The player exits the tank and does the tunnel reversal glitch 6 times to shift the player's block boundaries right 6 blocks.
The player then moves far enough right, wrapping around the screen, to make the contents of the room to the right start loading.
The player then turns around to fall through the floor while reversing the loading.
The player takes 2 bars of damage falling onto a block that loaded then quickly unloads, then drops to the block below it.
The player is now standing on the boundary of the first platform below shifted right 6 blocks.
Enemies remain on the intended boundaries; the player can either wait for a wall-crawler to pass and move down the left wall, or quickly move hoping that one does.
The player then moves to the left side of the shifted platform boundary and jumps toward the intended platform position.
While in the air, the player pauses and unpauses the game to reset and restore the intended boundaries, and lands on the first platform.
The player then jumps or falls off the left of the platform and falls below to where a left wall-crawler is positioned.
The player takes 2 bars of damage from first colliding with the wall-crawler.
The player then instead of taking deadly fall damage takes damage again from this overlapping enemy; the player can instead Pause while overlapping the enemy to lower fall speed.
Either the wall-crawler moves or the players moves away during the player's next invincibility frames.
The player then drops below and uses the entryway to change the stored return point to that entryway.
The player then purposefully dies to restore the health required to complete the second Area 7 skip in the same room.

The first Area 7 skip is only useful when also using the second skip.
All three lives are required here to skip through Area 7 - one for the first skip, one for the second skip, and one for the boss and death warp.
The timing of the out-of-bounds drop is critical as it will determine how many blocks you fall.
If you fall 1 block, you can jump out of it; this happens to be a breakable block, and I found you can actually move out of breakable (and key) blocks.
If you fall 2 blocks, you will be stuck; although it is also a breakable block, the jump is blocked by the other breakable block above which cannot be moved into.
If you fall 4 blocks, you will die from fall damage unless you crawled off the ledge.
If you fall 5 or more blocks, you will die from fall damage.

Tunnel Reversal Glitch

Fall Speed Absorbing
Being hit by an enemy in side-scrolling subtracts 1 pixel (0x10) per frame from your vertical velocity; this is the knockback.
This can be used to safely hit the ground from a long fall.
A player can take a long fall into being hit by an enemy, pause the game to reset invincibility frames, unpause, get hit again and safely fall slower to the ground.
From terminal velocity in a fall to getting hit by an enemy, pausing, then getting hit again, the player has at most 7 frames to reach the ground before taking more fall damage; the distance fallen during these 7 frames is about 16 pixels (about 1 block).
Player vertical velocity is stored in the RAM at $0407; terminal vertical velocity is 0x40 (4 pixels per frame).
The player accelerates at 1/8 pixel (0x02) per frame during free fall.
A fall less than 0x30 does no damage; a fall with a velocity of 0x30 to 0x3D does 2 bars of damage; a fall with a velocity of 0x3E to 0x40 does 8 bars of damage.

Fall Damage Absorbing
During player invincibility frames, an enemy can override fall damage.
A player can take a long fall, get hit by an enemy to set invincibility frames, then get hit by the ground and an enemy at the same time, and have the enemy override the fall damage.

Pause Scrolling
Blaster Master includes Box Scrolling, specifically your character in side-scrolling is surrounded by a scrolling box, and will only scroll the screen when he pushes into the boundaries of this box (limited also to the boundaries of the current room).
However, when the game is paused and unpaused, it looks to center the character on the screen where possible.
Like in Tunnel Transition Pause Skipping, you can force the scroll using Pause.
If your character jumps then pauses, it will force the scroll higher; if your character ducks then pauses, it will force the scroll slightly lower.

First Drop Strategies
Setting up the drop is particularly difficult.
If the turn-around into the drop is immediate, it has to be frame-perfect.
I originally thought the turn-around into the drop had to be frame-perfect, but apparently if there are some frames without inputs between the turn-around, it can open the window up to about 5 frames.
I noticed crawling off ledges and falling 4 blocks of distance actually only does 2 bar of damage instead of 8.
Crawling into the drop is a highly reliable way to drop, but unfortunately so far often glitches one of the enemies into a bad position much of the time.
The setup for the crawling drop is carefully positioning your crawling character so the shine on the helmet overlaps the left border of the Hover bars.
Once in position, tap right, then quickly hold left, and this will usually do a proper drop.
It's nice to finally find a use for crawling, but I would like to make this more reliable.

Area 7 2nd Skip

Discoveries
The foundation of these skips is the Tunnel Reversal Glitch which probably dates back a long time because it is fairly likely to occur by accident.
There was a lot of analysis especially in the TASVideos Forum noting that it can be used to go out of bounds by warping a distance in the opposite direction and/or shifting the player's boundaries.
BrotherMojo found the now-2nd Area 7 skip about 4 years before the 1st skip was found.
UraniumAnchor (****I think) then found a simpler drop for the second skip using a frame-perfect input following a bounce from a long drop in which your character is already dead.
IluvMario then Skavenger216 years later began integrating this difficult skip into speedrunning attempts of the game.
The second skip then heavily influenced the finding of the first skip.
Dugongue (aka Jigwally) was messing around trying to find random skips during a derust run, especially in this Area 7 room.
He used six tunnel reversals and quick turn-arounds into many promising but unsuccessful falls into the lower room.
He tweeted that he suspected he may have found a new skip; BadBrakes retweeted this; ZeroNoin then noticed this tweet and watched some of the Twitch VoD.
ZeroNoin experimented for a while, eventually centering on six tunnel reversals, and then finding a special drop onto a promising position in the right wall.
He first centered on falling onto the lower platform with a wall-crawler, pausing while overlapping the enemy to prevent fall damage, but then being stranded there with 2 health.
He then noticed wall-crawlers eventually travel down the left wall, and then also noticed it is possible to jump to the top platform.
He then eventually successfully put it all together for a working skip, making and posting a video of the trick.
Skavenger216 then watched the video and worked on some of the specifics and optimization.
ZeroNoin found a crawling setup to make the first drop more reliable.
He then found some Pause Scrolling techniques for helping to deal with the enemy that can glitch during the crawling drop.
ZeroNoin found a slower but easier Area 7 2nd Skip using the crawling technique and the platforms in the room to the right.

References
2005/02/02 - TASVideos Blaster Master Forum Thread - http://tasvideos.org/forum/viewtopic.php?t=1755
2012/12/08 - Original Second Skip Video - https://youtu.be/gY7PxSBV5nE
2012/12/08 - Refined Second Skip Video - https://youtu.be/YnrkU_AifvA
2016/02/04 - Dugongue Tweet - https://twitter.com/Jigwally/status/695449277724688385
2016/02/05 - ZeroNoin Tweet - https://twitter.com/ZeroNoin/status/695603422184108032
2016/02/05 - Original First Skip Video - https://youtu.be/GR3Bajok_FM
2016/02/07 - Original Crawling Drop Video - https://youtu.be/duIDHLJTeDk
2016/02/13 - Some First Skip Techniques Video - https://youtu.be/W7Ov3CmWBCA

Notes
This document may seem bloated and too detailed.
But I feel that noting the details of the process of finding new approaches to games is very much worth noting.
These details can help in outlining and applying logic and unrealized ideas, and in demonstrating to yourself and others how exactly strategies such as these come to exist.

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