On the merits of Excel and running out of space

Monday, 12th September 2005

Latest video. [2.29MB AVI]

You chaps probably don't know what a major headache it is trying to get clear, non-wobbly video... rolleyes.gif


I gave up in the end and went for the "propped-up against a wall with camera on stack of books" approach. The reason I cannot, therefore, appear to play the game very well is because I cannot see the screen. All I can see is a faint, inverted glow and have to guess where I am. Not fun.

Here are the major updates since last time:

  • Removed Blackadder theme rolleyes.gif
  • Altered colour contrast on title screen, added wipe to clear it in imitation of original, restructured code for better fades, changed timing, added sound. Title screen is now complete.
  • Rewrote entire code for checking bullet collisions with buildings on the ground from scratch. It now works perfectly.
  • Levels now "start" correctly, so the ship moves in from the bottom, the start platform moves towards you very fast until they are in the correct positions, at which point gameplay commences (much smoother than just dumping you at the start position with a full level already there).
  • All levels now mapped correctly, in order, with correct palettes for backgrounds AND sprites. Levels that end a zone with a platform rather than with the face are flagged as such and the correct end is substituted in now.
  • Each level can be pointed at a 'replacements' table, which lists "replace tile X with tile Y". Tiles are swapped around properly so that levels appear more varied.
  • Explosions are now fully implemented (2 kinds, background and foreground - with very slightly different animations) and working.
  • Path-based enemy attacks are now in full working order, using paths generated with a combination of my graphical calculator and Excel.
  • A couple of other new enemy attacks have been added.
  • The game now sports a BBC Micro font which can be used for proper text display.
  • The "epilogue" to the game is fully coded (type-writer effect for final message copied from original game, including sound effects and fades).
  • Each level now begins with a summary screen (sprite limits meant that displaying all your lives at the start of the level was impossible, so is now done on an interval screen). The screen displays the zone name and a number of lives, the number of lives being surrounded by orbiting space-ships (the number of those being the number of lives left).
  • The game now fully works on hardware (should now work on old hardware - not checked, as I do not have an old BIOS-less Game Gear) and in all emulators (I was not initialising the display correctly).
  • Destroyed terrain tiles are not updated live to the name table in the fairly intesive loop - they are saved to a list of destroyed tiles, which are then read off directly and updated when needed in the intensive graphics bit.

I've probably forgotten a lot of stuff. Bear with me.

There have been a number of "WTF" moments reading back some of the source. Take, for example, this gem:
Some explanation is needed here - this is from the code that handles the special case when you have shot a 2x2 square building and it lies out of bounds of the name table. The name table is my grid (32x28) of tiles indices that each point to an 8x8 tile in the VRAM. As the view point moves up this map, new tiles indices are written to it, or old ones are adjusted when something is blown up. The worst case scenario here is if the 2x2 tile lies just above the top of the name table - which is what the above code has to fudge. Look at this:

(I love debugging emulators!)
This shows all of my 32x28 tilemap. That funny white rectange is my view rectangle (it's currently half-off the display, and it loops around). If you look at the top and bottom edges of the tilemap, you'll see a ring has been shot out. Naturally, the bottom will have been hit first. What happens in this case is the address of the tiles that have been erased is decremented by a whole row of map points (there are 2 bytes to a spot on the name table, so I go backwards by 64). In this case, I find that the address is above the start of the tilemap so I add on 64*28 (which is the entire size of the table) and add it back to get back into the bounds of the table itself. As you can see, it was a successful operation in this case, but only because I was writing back HL this time and NOT the accumulator! No wonder my sprites were becoming destroyed as I was writing in completely the wrong part of the VRAM...

This is the screenshot from the emulator of the screen display for the above tilemap (the sprite layer is not displayed in the tilemap, for obvious reasons).

I'm still not getting very far with my quest for music. sad.gif
The only bit of the game that has music is the title screen, and that's not so much music as a weird noise (here is an MP3 of my Game Gear's rendition, this is the original from the BBC Micro). Even though I have specified that the noise channel (the one that produces the high beeping noises) to use the highest frequency, it's still lower than the Beeb. All the other notes are correct, though.

Latenite has got yet another update - project support. (It looks more and more like VS.NET every day!) Rather than create messy proejct files and stuff which makes distrubution tougher, I thought I'd go the easier route of just allowing people to stick directives in the file with the ;#latenite:main directive. You can now ;#latenite:name="name_of_project" and ;#latenite:project="file_name"[,"project_folder"].

Click for larger image

Skybound Visualstyles (which I'm using to fix the weird .NET XP interface bugs) looks lovely but hugely slows down the app - only really noticeable with so many hundreds of nested controls like me and when moving large windows on top of it forcing repaints, so now the XP styling option is saved away (so you can revert to boring standard grey if you like). Latenite now saves window/splitter positions when you close - a first for an app by me! grin.gif
In fact, Latenite is also my first app that needs a splash screen, it takes so long to get going... sad.gif It's only about 2 seconds, though, for it to track down all the tools, load all the documentation, add all the compiler scripts and present itself, so it's not too bad.

One overlooked piece of software is Microsoft Excel. People seem to develop a hatred of it, especially in people who end up trying to use it as a database application (it appears that this is what 9 out of 10 smallish (and some not so small!) companies do).
I have been using Excel to create those smooth curving paths for some of the enemies to follow. First, I prototype them on my graphical calculator (lots of trial and error involved!):


..before transferring into an Excel spreadsheet.

Click for legible!

The Excel spreadsheet is set up so that column A is the steps (usually 0 to 256) in ascending order. Column B is the X coordinate, column C is the Y coordinate.
Columns E and F serve a special purpose - they contain the differences between each element in columns B and C (so E3=B3-B2, E4=B4-B2 and so on). Columns G and H "fix" the values - if it's negative, make it positive and set the fourth bit (just add 8). This is for the internal representation of the paths. Column I contains the data I can just copy and paste into the source files - it starts with the initial (x,y) coordinate, and then has a list of bytes. Each byte signifies a change in (x) and change in (y), with the lower four bits for (x) and the upper four bits for (y).If the nibble is moving the object left or up (rather than right or down), then the most significant bit of that nibble is set. For example:

%0001 - move right/down 1 pixel
%0011 - move right/down 3 pixels
%1001 - move left/up 1 pixel
%1100 - move left/up 4 pixels

...and therefore...

%11000001 = move right one, up four.

Poor Excel has suffered a little bit of neglect from me - I used to use it to generate my trig tables. WLA-DX, the assembler I learn to love more and more every day, supports creation of a number of different tables - including trig tables - with a single directive. Just specify .dbsin (or .dbcos) followed by the start angle, how many bytes of trig table you want, the step between each angle, the magnitude of the sine wave and the DC offset and away it goes.
People still using TASM, STOP! Use WLA-DX. It's ace (and free, not shareware!).

You can never have enough pictures in a journal (hey, you should see this page load on our 700 bits/sec phone line at home!)
Here's the full list of sprites:


The last ship being after the explosions is a mistake, but it's too much hassle to change. I'm really pleased the way they've come out - I'm no pixel artist. smile.gif

I'll finish off with a few assorted screenies.


...I would have done. I should have done. However, when taking those screenshots I saw a glitch in the graphics. As an exercise for the reader, can YOU spot where the bug is?

(and it's not in one of those external functions I wrote - this is why copy-paste coding is bad, 'kay?)

Oh, (possibly?) final last thought popping into my head: one thing I really miss from the BBC Micro era is the comfortable keyboard layout. I mean, look at this small photo of a Master 128 keyboard:
Click for bigger

You might think that that blocky monstrosity would knacker your fingers, but that's not the reason. It's the fact that rather than squash one hand's fingers onto a small cursor pad (or the even sillier WASD), you balance movement between your two hands - Z and X for left and right, : and / for up and down. (On a modern UK layout, that'd be ZX and '/). The space bar is now conveniently accessible by both thumbs, and you can reach around most of the rest of the keyboard with your other fingers.
The Game Gear is an ergonomic handheld (unlike the uncomfortable Game Boy), so I'm reckoning that I can balance the controls between hands - ← and → on the d-pad can control left and right, and the 1 and 2 buttons can control up and down. I will offer multiple layouts, though, as otherwise that might confuse some people. wink.gif

Anyway, I hope you find this journal interesting. It seems a bit all over the place - video, pictures, rambling, odd bugs, nostalgia... The only thing that's an issue is my rapidly shrinking GDNet+ account web space!

Fire Track

Friday, 9th September 2005

When people ask, in the future, what the greatest feature of Fire Track is, they will not say the graphics. Nor the gameplay. In fact, they will... well, see for yourself. (Sorry, it's a bit quiet). smile.gif
There is one more attack pattern. I have redesigned the attack system so that the enemies do NOT have their clocks auto-incremented (though they can call a function in their update code to do so if they really need it). This is so I can use the clock to store states instead. The new attackers slide in from the top-right, move down until they are level with the player then slide right again.
One peculiarity of gameplay in Fire Track is that you are permanently firing. You can never let go of the fire button. Also, the enemy ships do not fire at you - they just try to crash in to you.
I ran out of room. The game refused to compile. So, with a bit of tweaking in the map editor and a rewritten scrolling routine, I have doubled the amount of free space (without the music, about 10000 bytes are free). I'm using RLE compression on each 20 bytes of the level. If I get even tighter for space, I could probably pack the 8 bit level bytes into 6 bits.

Here's an awful sprite of an explosion. It looks really ropey, but it's as close as I can get to the authentic Fire Track (ship) explosions. I don't know how good it'll look in action!



Thursday, 8th September 2005

They claim a picture says a thousand words.
In this case, with these two pictures and one video [549 frames] I estimate that I have saved myself from having to write over half a million words in this journal entry.
enemies_1.gif enemies_2.gif
There is, of course, the usual shonkily shot video footage to watch: 416KB AVI.
The collision rectangles need a little bit of fine-tuning and something better than just vanishing sprites needs to be implemented, but the modular and simple system is there.
Each level contains a pointer to an "attacks" table.
Each "attacks" table entry contains a $FF terminated list of attack patterns, in order.
Each attack pattern specifies -

  • Which sprite number to use.
  • A pointer to the routine to call to initialise the enemy sprite locations.
  • A pointer to the routine to call to move each enemy sprite each frame.
  • An attack time.

Each enemy is held in a table containing an X,Y coordinate and a "ticks" counter that is incremented each frame automatically.
The "attack time" timer is primed and decremented each frame, and once it hits zero the system moves to the next attack pattern from the level's list.
This means that I can have a number of different patterns. So far, I've implemented a simple sine-wave attack and an even simpler random-positions-and-drift-past attack (both from the original game), but some attacks are more complex, relating to paths or drifting towards the player. This system I have in place should make having a wide variety of different types of attack simple.
I got so far, compiled, ran on my Game Gear and - oh dear. Lots of nasty name table corruption. To save space, I had decided to check the bullets against the enemy ships inside the sprite-writing routine, which was obviously too slow (much popping, pushing and use of the index registers) and as such the sprites were being written after the screen had started to be displayed. I have moved the code outside the loop and restructured parts of the code, but I hope that I have enough time/space to implement other things properly, without being restricted too badly by the low CPU speed.

I'm not getting far with this elsewhere, so I'll ask here - anybody here with BBC Micro skills?
I really need help with the music side of things - not so much writing a sound/music engine for the game, but more the actual "porting". The BBC Micro has the EXACT SAME PSG in it, so I could probably write a modification for a BBC emulator (BeebEm, for example, comes with source) which logged writes to the PSG hardware port (and time-since-start-of-program), but I'd still have the piou-piou-piou-skrrch sound effects over the top. Does anyone have a BBC Micro disk with a copy of the Fire Track music on it? I remember seeing/using music collection disks which featured game music on them a decade ago, and SOMEONE must have a "raw" copy of the Fire Track music (how else is there a recording in the STH archives?) but nobody on the STH fora seem to be able to or want to help.

Finished the levels! Wahey!

Wednesday, 7th September 2005

...and here is level 6. This, however, worries me:


Hopefully I can cram in all the rest of the code, tables, graphics and music into 5K. Failing that, I might have to start compressing my levels...

I don't believe I've introduced you to my enemies yet? Here's the complete sprite table - don't worry, the letters P, A, U, S, E and D do not feature as enemy craft!


I spent some time adding a new command systen to QuadPlayer, so you can now set a song to loop back X times (or infinitely) to a particular spot. It'll only loop from the end, sadly. Nested repeats will be too evil to try and implement. I'd like to experiment more with white noise - maybe a command to set up certain channels as being white noise rather than tones? Not sure how I'd generate the white noise, as the register R (which is the source of my random number generators) isn't going to be very random if accessed in a short loop!

Sample song, from a Kirby game (I added the fade at the end manually, that's not produced by the calculator!) Not bad, seeing as it has been generated by a calculator with no proper sound hardware. smile.gif

ASCII Madness II and Tutorials

Friday, 5th August 2005

Well, it's been a long time - here are two updates in one!

First up, ASCII Madness II. It's a text-mode scene demo that runs in the Windows console using characters from the extended ASCII set to produce the effects.
You can download it (and the source) from here.

Next up are some simple tutorials. Oldschool graphics stuff, I'm afraid, but hopefully someone will find them of interest. smile.gif They are presented in the form that I would have liked to have read... and I know that I'm not very conventional in my learning style.
I'd be interested to see if the demo apps (blobs, tunnel) work fine - I've had one complaint that they can't find a particular DLL, which is a bit odd.

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