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Author Topic: Alien Invaders, Sprites and 'Hi!'  (Read 415 times)
Mux
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« on: April 25, 2019, 05:43:11 AM »

Hi everyone!

Bit of a background and sort of an intro...

I've been working in the games industry for the last 30+ years or so my first EVER games console was a Philips G7000 (grew up in Holland). Started off in basic and then assembler on a vic20, I moved on with the whole industry. That said, I *love* constraints on hardware figured I'd see if I can create some games for the O2!

Anyway, the question I have is about Alien Invaders. If you look at the the image below:



I can kinda see a few possibilities in terms of sprite allocation:

- The spider (obviously) is 2 sprites. The bunkers are filled grid and the scores are most likely (up to) two digits using the four of the twelve 'fixed' sprites. The 4 quad texts can be used for the men and the circles below it... The bullets are then likely the remaining two sprites, doubled.

So the obvious question then is: How the hell are the shields under the men done and the bullets? Looking at the character set the shields are most likely done using the 'tree' with the two top lines color changed to blue but I'm curious to see if anyone has another theory!

-Mux
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Sorex
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« Reply #1 on: April 25, 2019, 02:30:00 PM »

interesting post... I had some questions about it aswell a day or 2 ago.

I was wondering where those yellow mushrooms come from.

It appears that most of the enemies are 'chars' but that mushroom doesn't belong to the charset.

and as you can only have 4 custom sprites in a row that can't be it either.

Edit:

LOL, after re-reading your post I noticed I had the exact same question. (shield = mushroom)
« Last Edit: April 25, 2019, 02:55:48 PM by Sorex » Logged
Sorex
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« Reply #2 on: April 25, 2019, 02:42:48 PM »

Looking at the character set the shields are most likely done using the 'tree' with the two top lines color changed to blue but I'm curious to see if anyone has another theory!

you're right. you can display a char starting from pixel row 1 or 2 so that the lines are not blue but just not being displayed.
if you 'scroll' too far the next char shows up partially aswell. (atleast that's how it is in the emu, I should test this on real hardware aswell)

it's done with the 2nd & 3rd byte of the char definition (it's a pain to figure out how that exactly works)

Byte 2: (char * 8 ) - (Y / 2)
Byte 3: Bit 0:    9th bit of char
             Bit 1..3: color (321 = BGR)
« Last Edit: April 25, 2019, 02:57:18 PM by Sorex » Logged
Mux
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« Reply #3 on: April 25, 2019, 06:50:51 PM »

Hah! That's actually an angle I hadn't looked at!

So that's pretty cool and I will *definitely* be checking to see if I can take advantage of that. I've got a whole list of games I want to write for the O2 :-)

-Mux
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Sorex
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« Reply #4 on: April 28, 2019, 01:56:01 PM »

well, it's harder than it looks.

When looking at some old/basic game ideas I always see some issues to make it possible on the videopac.

The biggest culprits are

1. the 4 sprites of only being 8x8.
most other retro systems have sprites of atleast the double amount of pixels and more than 4 aswell.
if you want 2 colors in your sprite you only have 2 left if they could appear next to you.

2. the lack of a custom charset.
I'm not sure if there are options here to display other data than the 64 standard symbols?
If you could replace a few it would open a lot of doors to make some cool stuff but I don't think it's possible.
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Rafael
Pedit5
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« Reply #5 on: April 28, 2019, 05:19:26 PM »

You can combine characteres to create new objects, see for instance Killer Bees, where the enemies are combo of characteres. Super Bee is other nice example how to get
diferent "sprites". All fruits are combo of characteres.  In Nightmare ghosts youŽll see combo of sprites+characters etc. Good luck for you.
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Sorex
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« Reply #6 on: April 28, 2019, 07:16:31 PM »

I know, Rafael.

But I doubt you can do the same as with 2 real sprites with pixel freedom since you're limited to what there is already.

The cars in frogger also needs some thinkering as there are 3 next to each other sometimes and still multi color.
Maybe they (ab)used the minus character with a sprite on top of it.

Al this gives more respect to those who wrote the existing games.
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Mux
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« Reply #7 on: April 29, 2019, 05:39:57 AM »

The 8x8 characters isn't really that big of a problem as they're doubled during scan-out. Plenty of monochrome systems did great (sinclair spectrum comes to mind)

I always look at these games not so much about the graphics but more at how playable you can make it with those limited resources.
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Rafael
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« Reply #8 on: April 29, 2019, 02:23:03 PM »

I know, Rafael.

But I doubt you can do the same as with 2 real sprites with pixel freedom since you're limited to what there is already.


But that's what makes it fun. I see people today creating modules to "add" systems like Colecovision or Intv. Maybe someday, someone will create something for the Odyssey, multiple srpites. It would be cool? Yes, but I would still like to see a root game.
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Rafael
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« Reply #9 on: April 29, 2019, 02:27:03 PM »

The 8x8 characters isn't really that big of a problem as they're doubled during scan-out. Plenty of monochrome systems did great (sinclair spectrum comes to mind)

I always look at these games not so much about the graphics but more at how playable you can make it with those limited resources.

I find totally charming monochrome spirtes as long as you have beautiful designs. For example, I like much more of KC Munchkin than the Atlantis sprites, they seem a lot more fun to me. Another example is Demon Attack, particularly I'd rather see more monsters onscreen.
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Mux
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« Reply #10 on: April 29, 2019, 06:21:44 PM »

Totally agree!

One of the things that I like about the odyssey 2 is that you can put out a really decent game with minimal graphics. You're kinda forced to work within the framework and limitations of the console, so it's really all up to creating fun and challenging experiences!

-Mux
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