Hi time for another update:
- level select screen
- GameOver And level cleared dialog screens
4 Likes
This is great! I had never played the original, but I've beaten all the levels twice now on my playdate and it is great fun. The easier levels are very meditative.
I have three questions.
-
Have you considered making the strength of gravity a changeable parameter? Maybe as an achievement you could need to explore a level while the gravity was a strong as Jupiter or null gravity.
-
I read somewhere the original had twenty levels. Any plans to update the game to include the other 10?
-
In level 3, off to the right, there's a platform with a bonus crate. If you take the crate it makes the same sound as the +1 life box, but it doesn't increment your lives when you take it, nor does it increase your carrying capacity. Is this a bug or does the box do something I'm missing?
Seriously, though, this is a great addition to my playdate and has given me several hours of enjoyment already. The game has been very stable for me. It crashes sometimes when I re-load the same level I'm currently on, but otherwise I've had no crashes.
1 Like
Any chance to get a compiled .pdx for the .5 update to try out the menu selection screen?
This is great feedback!
Reading that my work entertained you for more than an hour made my day!
- Great suggestion! I might add a 1.5x gravity challenge.
- I have the source code for loading the original levels, in c++. I'll have a crack at using that. If someone reads this and wants to take the challenge: let me know
- VERY valuable feedback. I'll look into giving all pickups their own sound. In this case a turbo whoosh. I think you've been playing an older version, because the sprites for pickups are redone, and also the HUD icon for speed blinks and gets replaced by a fast forward symbol. Please let me know whether this is clear to you.
Crash: I would like to get to the bottom of this. Can you make sure you are using version 0.5 and tell me the steps to reproduce? If it is not too much effort, I'd like to know whether you can reproduce it in the simulator and what the error message is in the console.
@metafish I will upload a pdx this evening.
Here's the zipped pdx of v0.5:
btw, I noticed I forgot to actually prevent you from starting a locked level 
Anyways, that would be trivial to add later. For now it's convenient for playtestesting.
@metafish @karljpsmith
cgpd_0_5_1.pdx.zip (2.5 MB)
edit: actually made a change to properly reset the game state when selecting retry, which might possibly fixes the crash. Calling this v0.5.1.
1 Like
Progress on loading the original Crazy Gravity level files in Love2D. This is actually going to work 
For posterity: Reading binary files in lua 5.1 is a terrible idea.
1 Like
... and another one.
I've hit a roadblock here though, as the documentation I found is incomplete 
That project seems to be a complete "open" implementation of Crazy Gravity, so I could glean the rest from the .c files. I'm not used to reading C tho, it might be a bit too much of a hurdle for me / manual labour for me. So I'm stumped here. I might still try to compile that project on linux. It would mean I get the entire level in a C data structure that I might be able to write to lua, which could be worth it.
2 Likes
Hope you get it working!
1 Like
I did indeed!
And also the collection of 278 fan-made levels uses the same binary format. That's a big relief.
There is a challenge of differing tile sizes, where the original level uses 4x4 pixel units and I use 8x8. There is some anti aliasing needed. What do you do with a 4x4 block? Make it an 8x8 or delete it? I think I found a fair solution.
2 Likes
The project to auto-convert 278 fan-made Crazy-Gravity levels is complete!
With some pretty impressive - and sometimes touching - results.
For example this level by "Myriam and Esther", probably made in 1999, now resurrected to be enjoyed once more.
Unfortunately, many of these levels are huge and don't fit in the playdate's 16MB memory. We can also blame my inefficient memory format and lua's 32-bit (or 64-bit?) number format where 8-bits would have been enough to store most data in these gargantuan 400x400x5-numbers sized tables.
I need your help to sift through this massive pile of levels!
Please join the crowd-sourcing effort at Gravity Express: Fan-made levels playtesting
1 Like
I ran into some out of memory issues on lots of user levels, which isn't that strange; considering this game apparently was the minecraft of the nineties.
Also had this with level 13 of the official set tho.
So I devised a memory compression approach, which I will share more about later, if it turns out it doesn't hurt performance too much.
Would someone please test this on hardware and tell me if a solid 30fps is still achieved (and whether it loads in the first place)
If needed, grab a previous build for comparison
Please check levels 1, 3, 13, and 20.
13 being the largest one, that would crash with out of memory without this compression technique.
In other news, I commissioned some start-screen and loading screen art which is being made right now.
cgpd.pdx.zip (3.1 MB)
2 Likes
These all load fine on device, and play fine. I didn't complete them though.
Looks fine, one of the levels had 27-29fps, the others seemed pretty solid at 30.
1 Like
Memory "compression" explained
So the memory compression approach turns out to work even better than I'd hoped. It's a bit hard to measure what level of compression is achieved because the data is now in metatables, and I don't think the Simulator's memory profiler can read those. I'm estimating the gains to be about three quarters though.
It relies heavily on string packing
The idea here is that storing small numbers in tables is very inefficient because lua has only a single number type. Whether it's the number 1, 35435636, 3.133333333, 5.13E12 or 202432451351634, it all takes the same amount of memory, depending on the system that would be 32 or 64 bits.
The numbers I use for my tilemap are all positive and smaller than 256. So, 8 bits should be enough. Considering a tile map could have dimensions of 300x300 tiles, with 5 properties each, that does add up to several megabytes. Add to that the overhead of storing it in nested tables instead of a binary array, and I was looking at 7MB of tile data. That's laugably inefficient, that's why I wrote "compression" in quotes: de-bloating might have been a better term.
To the rescue comes string packing, which lets you define a data format and convert the data to raw binary. So my 5 numbers in the range of 0-255 could be stored as 5 bytes written to a file.
When provided with the same format string, the data can be read just as easily. But, you can't read it all in memory, or it'll take up just as much memory as the original table would (remember Lua's inefficient number format).
The solution, then, is to keep these packed strings in memory, and unpack them whenever a tile is needed. The memory requirements for this are the size of all packed data, plus the size of a single unpacked tile.
One problem remains: the existing game logic heavily uses the tile map, and expects it to be a 3 dimensions ([column][row][tile property]) table. Rewriting all that would be a gruesome task.
To solve this last problem, we can turn to metatables . With metatables, you can define the indexing operation. And in that indexing operation, we can unpack a tile. The implementation of loading a file is:
local brickT = {}
local format = levelProps.packFormat
local packSize = string.packsize(format)
local packSizeOffset = packSize-1
local unpackMeta = {
__index = function(tbl, idx)
-- tbl["compressed"] contains the packed string.
-- idx*5-4: each tile entry is 5 bytes long,
-- for idx 1 we should start reading at pos 1. So, 1*packSize-packSizeOffset = 1
return {unpack(format, tbl["compressed"], idx*packSize-packSizeOffset)}
end
}
local brickFile = file.open(path..".bin")
for x = 1, levelProps.sizeX do
brickT[x]= setmetatable({compressed = brickFile:read(5*levelProps.sizeY)}, unpackMeta)
end
Suppose our code needs the second property of the tile at x=5,y=3. It will be accessed as brickT[5][3][2]. What happens under the hood?
brickT is an ordinary table. brickT[5] returns a table containing all data for a column of tiles. This ,however, is a metatable without numerical entries. The only entry it has is called compressed and it is a string of 5*sizeY bytes representing a column of tiles. When we index it with [3], that entry cannot be found, and the metatable is queried. This has the __index method which calculates the byte-offset of the 3rd tile in the column-string, unpack a tile there and returns it. The returned value is a regular indexed table of size 5*, and the second entry [2] can be accessed in the regular manner. We're done.
The great thing about the approach above is that the game logic is not aware that the table data is compressed. It requests brickT[5][3][2] and gets what it expected. Note that if the code needs more of the tile than just entry [2], the tile should be stored in a local variable to prevent unpacking multiple times.
My last remaining fear was about performance: how much cpu time would the unpacking consume?
Luckily this turned out to be very minimal, perhaps 6% of the total cpu time according to the (Simulator!) profiler. There will be a compounding effect of earlier optimisation, because I already optimised the amount of redraw per frame for the tile-map. Not a lot of tile data is needed every frame.
@fosterdouglas and others have confirmed solid fps even with this technique applied; so I couldn't be happier 
!
*the actual size is 6, as the unpacking operation inserts and extra number at the end, representing the index of the next byte in the packed data. We don't mind it being there; removing it costs cpu time while it does not bring us any advantage.
4 Likes
That's really cool! There is always more to learn about Lua. (Does this mean all those biggest user-built levels now work? Or are some of the extreme cases still not possible?)
Can't wait for this to release!
(when viewing the video above, make sure to select the highest quality (720p), Youtube seems to make weird quality decisions for this content)
I like the sound of chiptune / midi music, but when loading a midi into a Sequence, no sound is produced. That's because a midi file only contains the score/notes and no instruments.
I wrote MIDI Master as a tool to program instruments for midi files. It is being used by someone on the community discord to compose and "master" music for Gravity Express.
2 Likes
Camera Controller
To avoid flickering, camera position has to be updated by multiples of two pixels. Still, we want smooth camera movement.
This camera controller uses an algorithm incorporating speed, plane orientation and speed-hold to prevent erratic movement.
Graph shows horizontal camera movement speed. Set video quality to 720p to see the graph.
Gravity was disabled for this test.
Note that I chose a worst case scenario for this test, a 2x2 dither pattern for the bricks.
4 Likes
Now this is what I'm talking about!
Game Launch animation
4 Likes
Flying the ship for the UI is a cool idea! Might be tricky for newbies, but I'm used to these kinds of games. And it's a built-in low-stakes flight tutorial.
1 Like