WORK IN PROGRESS - Atari port (parody) of C64 1nvader
Game Screen Development
27 Dec 2020 -- Game Screen Startup
The Game Screen fires up after the countdown. The transition from Title screen to Game screen is instant and seamless -- no redraw or obvious flicker. (Because very little is being redrawn. There are just a few pointers to screen memory being updated in the Display List.)
The Player does not move yet.
The panorama of distant mountains continues its motion since it is serviced by the exact same Display List segment, Display List Interrupts, and VBI service code.
The random flashing stars are not yet being serviced for the animation, but the Display List Interrupt to apply color is running to help work out the alignment.
30 Dec 2020 -- It's Full Of Stars...
The random flashing stars are functioning on the game screen. Note that the screen snapshot above is a cheat and presents stars merged from two different screen grabs. There are only 4 stars visible at any time, but since they flash so quickly it appears there are more visible.
There is only one line of star data. The 16 lines of stars all refer to the same screen data, so this section of the screen showing the stars takes roughly 100 bytes including all the display list instructions.
Positioning the star is done by horizontally scrolling the line to a random position. Fine scrolling is also used, so the stars can be horizontally positioned per pixel/color clock as if the stars are drawn in a pixel-graphics mode.
The Vertical blank interrupt manages choosing random star locations and setting the stars positions, and running the color fading transitions for each star. Each star is assigned a random base color.
Each line of stars is serviced by a Display List Interrupt that provides the horizontal fine scrolling control per line and applies the colors to the top/middle/bottom of the star.
21 April 2021 -- Alien Antagonist Animation Added
Demo code is added to do simple imitation of the mothership movement for the game.
The demo code starts the mothership from the top of the screen and animates the movement row by row until it gets to the bottom row.
When the mothership reaches the bottom row the limiting code prevents further movement down, but the bounce logic keeps it moving back and forth.... forever.
A difference from the original game is that there is a transition animation that slides the mothership down to the next row when it reaches the horizontal limit.
Mothership speed and score display are not yet functioning. (Neither are the gun movements.)
25 May 2021 -- We Need More Guns (again)
The guns are now moving and bounce against each other and the bumpers. At the start of the game the guns assume a random direction.
Other tweaks have occurred. Altirra indicated a timing/opverlap conflict between the DLI for the stars v the DLI for the scrolling mountains. This resulted in intermittent crashing on Altirra. (Though, the atari800 emulator on linux is tolerant of this bad behavior.) To fix this I subtracted one line of stars and replaced with the equivalent in blank lines, and the Game screen is now stable on Altirra.
Altirra also showed that setting the status text to black to make it invisible doesn't work as intended. Black text on a black background for ANTIC Mode 2 is still visible over the Player graphics. This is how the real hardware works, so the fault was my own Stupid Programmers' Trick. To really make the status text invisible it has to actually be removed/cleared out on the Title screen and this change was added.
24 June 2021 -- Bang Bang Shoot Shoot
Lasers are working. Sort of.
Pressing the joystick button fires the laser. If the laser is within the bottom half of the screen it cannot be restarted. If the laser is within the top half of the screen, then pressing the button will remove the laser and restart it.
There is a bug here. OCCASIONALLY, for one frame the laser will disappear from its current path and appear in the horizontal overscan area, and then on the next frame it is back where it belongs. Have not been able to track that down yet. It is intermittent, and it SEEMS that it may stop misbehaving after a few minutes of run time. Confirmed that this is not a weird behavior of atari800 as I saw it do the same thing in Altirra which is more trustworthy for timing.
I was thinking it was possible the problem is timing between the VBI code and the main code -- maybe the main code was taking so long that the main line was overrunning the frame. But, on further diagnostics the VBI code plus the main code take so little time that they complete before the scan line has reached the playfield.
Weird, still digging looking for the logic issue.
28 June 2021 -- Baseline Stable Behavior
After suffering from self-induced premature optimization several rounds of debugging appear to have everything working now. Also, code size is currently reduced to 8,539 bytes where not too long ago this was around 11K.
Testing so far looks like everything for joystick input, gun movement, and laser shooting are all stable for one player (either one) or both players at the same time.
Remaining TO-DOs -- Collision detection, explosion, mothership adjustments when shot and the mothership speed up behaviors, and scoring.
11 July 2021 -- Scores And Stats
A litany of stupid programmer tricks have been resolved. Finally got many display items working...
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The math to add to the player scores, the high score comparisons, and the gfx update to display these thngs.
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Also, the statistics information for the line counter, points to award for hitting the mothership on the current line, the hit counter, and the gfx update to display these things.
Remaining TO-DOs -- mothership speed up behaviors, game over logic.
14 July 2021 -- Motherhip Pushes Guns
The mothership speedup every 10 hits seems to be working.
The bottom row mechanics are working. When the mothership enters the bottom line and hits the player it pushes them along off the screen. Either direction, to the left or the right, works. There is obvious redundancy in the crash mechanics, so this code will endure some optimization. (and I'll probably break it a couple times in the progress of that.)
This completes almost all the required operating parts of the Game screen.
The remaining work is triggering the end screen, and then showing the end screen/end of game text.
29 Aug 2021 -- Multi-color Mothership
Playtester Level42 suggested making the mothership multi-color.
Reworked the Player/Missile use to make a multi-color mothership on the title screen and in the game. Also, the motherships have animated running lights and windows.
Due to the way Player/Missile multi-color players work, there is an unavoidable difference between the PAL and NTSC rendering of the motherships. The overlay color produced by the two players' color is based on OR'ing the binary value of the two registers together -- NOT merging the aparent color displayed. As a result, the PAL values used to make the two red colors for the mothership combine to result in nicely light-blue lights and windows. In NTSC different values produce the reds, and the binary OR combining these values is still in the red area of the palette, so the lights are red.
DD MMM 2021 -- TO-DO
WIP