Atari 8-bit guide for lr-atari800 and Retropie

Artifacting Introduction

High resolution graphics mode on Atari 8 bits was a one-color, two luminance mode. You could have one color for the background, and the same color at a different brightness for the pixels drawn.

However, similar to the Apple II, the Atari supported what is called artifacting. This is a literal “artifact,” a graphical glitch, caused by the way the chroma circuits in the original hardware worked. There are some great explanations of why and how it works on AtariAge and similar forums, but suffice it to say:

Draw one white pixel in an even column next to an empty pixel in an odd column, and it’d come out as color 1.

Draw one white pixel in an odd column next to an empty pixel in an even column, and it’d come out as color 2.

Draw two white pixels next to each other, and they’d come out as the true color.

Draw specific sorts of dithered patterns, and pull tricks with background colors (since the artifact colors blended), and you could get even more colors.

BUT — this only worked this way on NTSC. On PAL, the chroma logic is different. There are some PAL artifacts, but by and large, artifacting this way doesn’t work on PAL.

Bottom line? You may have seen a lot of Atari 8 bit games that look like this screenshot from archive.org:

But on an NTSC screen on actual hardware, it likely looks like this image from MobyGames:

The list of games that uses artifacting to get colors is HUGE and includes classics like Lode Runner, Flight Simulator II, the Ultima series, and more.

NTSC and PAL

lr-atari800 defaults to PAL. Which is why the Internet is full of images like the black & white first one, rather than color ones like the second one. These are all generated on emulators set to PAL or with artifacting turned off, by people who have never seen the games on actual hardware.

Now, there are PAL games that were created in Europe and therefore assumed there wasn’t any artifacting. Two good examples are 180, the darts game, and Head Over Heels.

Here’s 180 without artifacting on, the way it was meant to be:

And with it on, which is much worse:

Technically, to get the right version, we should also be in PAL. Here’s Head Over Heels in NTSC with artifacting in the emu:

Here it is in NTSC with no artifacting:

And here it is in PAL with accurate PAL blending turned on:

Note the different aspect ratio. If you, like me, have used overlays and a custom viewport size, you won’t see an aspect ratio difference, most likely.

So, not only do we have two different video systems to worry about, but games coded for one will tend to look wrong on the other. The first thing to do is set your lr-atari800 RGUI option to NTSC; the vast majority of well-known games that use this monochrome high-res graphics mode are NTSC games, not PAL games. Just like how BASIC is usually noted in the filename, PAL is also usually noted in the filename, so you can find PAL games and save options just like you did for BASIC games, because PAL vs NTSC is an RGUI option. EDIT: I recommend not doing this, and using the script below instead.

But here’s the bad news.

The four artifacting modes

The chroma circuit on the Atari 8 bits evolved a lot over the course of the decade plus that the computers were sold. They started out with a CTIA graphics chip. Then they went to a GTIA graphics chip. Then the circuit itself changed more than once.

The artifact colors are different on every single one of these.

In fact, there’s even an actual knob to turn on the actual hardware that lets you adjust the tint of the artifacts.

lr-atari800’s core supports

NTSC:

• no artifacting
• blue-brown 1
• blue-brown 2 (which should actually be called brown-blue)
• GTIA
• CTIA

PAL:

• no artifacting
• simple PAL blending
• accurate PAL blending

But these detailed options are not available in the RGUI. The libretro core is hardcoded to only use blue-brown 1, I checked the source.

However, you can use the in-game menu accessed via F1, go to Display Settings, and select

Video artifacts: old NTSC artifacts
NTSC artifacting mode: blue/brown 1 (or blue/brown 2, or GTIA, or CTIA).

And these will be saved in /opt/retropie/configs/atari800/atari800.cfg as

ARTIFACT_NTSC=NTSC-OLD
ARTIFACT_NTSC_MODE=1

where

ARTIFACT_NTSC_MODE=0  // off
ARTIFACT_NTSC_MODE=1  // blue/brown 1
ARTIFACT_NTSC_MODE=2  // blue/brown 2
ARTIFACT_NTSC_MODE=3  // GTIA
ARTIFACT_NTSC_MODE=4  // CTIA

(Sorry, haven’t done the same experiment for PAL to get the index numbers, but I suggest you just set those to on and accurate because they look fine that way).

Now, here’s the problem. First of all, you’ll have to change this by hand every time you put in a different game. And secondly…

• CTIA gives, more or less, green and blue-to-purple depending on your tint.
• GTIA gives, more or less, the reverse: what was green is blue, what was blue is green.
• Blue/brown 2 and blue/brown 1 are likewise inverse of one another. And brown is a misnomer; on a well-saturated screen, it looked closer to red.

The fact of the matter is that on real hardware, you don’t know what you will get, but at the same time, at given periods of the hardware existing, developers clearly assumed one set of artifacting colors or the other.

Ultima III with artifacting off, or in PAL:

CTIA, note the green flame from the dragon:

GTIA, now the flame is purplish red:

Brown/blue 2, flame is blue:

Brown/blue 1, flame is red:

If you want an accurate experience that is closest to what the developers intended, you need to know what it was meant for. Luckily, we can compare to games on other platforms, screenshots on the box, or even cover art, to figure that out in many cases. For example, It’s safe to say that in 3D Red Baron, the enemy plane is probably supposed to be red. In Ultima games, we probably want to aim for blue water and green grass, which is what the developers intended, but it’s hard to get that setting with what we have exposed in lr-atari800.

Other color adjustments

There are several video adjustment settings presented in the emulator menu, and all of them are broken. Again, the ones to not mess with even in config files: “New NTSC artifacts” renders incorrectly in all modes. “NTSC full filter” likewise renders incorrectly, and also doesn’t properly read and write settings to the atari.cfg file when you make adjustments under “NTSC filter settings.”

However, things that do work, sorta:

• The various “color preset” sliders are all broken; when you make a choice, they will jump to -1 or 1, which will be wrong. (Again, you can use Restore Default Colors to put these back). But you can edit the atari.cfg file, put in values, and they will be read correctly. There have been various posts around the forums on what makes for good values there. Note that the GTIA value here is, more or less, the same knob for adjusting the chroma circuit that the full NTSC filter provides. However, it interacts with the Tint slider in somewhat unpredictable ways.

• There are also a few default configs: Vibrant, deep black, and standard. vibrant is more saturated, deep black does what it says. Both are quite nice though a tad ahistorical, particularly when coupled with a palette.

• External palettes DO work, and will affect artifacting colors. Just note that the “Also adjust external palette” option needs to be set if you want to use both a palette and these controls at the same time.

One option for messing with the sliders and seeing what you are doing is to launch the standalone emulator, where these functions do work, and once you have found something you like, paste the values in. Alas, this does not work for the full NTSC filter, which is what we would totally all be using if we could.

NTSC filter and “New Artifacting”

There are additional options for artifacting visible in the F1 emulator menu. They don’t work in lr-atari800. The “new artifacting” settings simply render wrong. The “NTSC filter” settings have sliders that don’t work, turning it on will blow out the colors. (You can fix this by going to Display Settings->Restore Default Colors). Even though the NTSC filter settings will be written to the config, editing them there will still not work, so you should not bother turning this on.

What the standalone emu can do (version 4.1.0, though I am unsure when the full NTSC filter was implemented) is bypass the hacky “four choices of artifact settings” and go to a true analog system that emulates NTSC circuitry. This means it gains an analog slider called burst phase, which is the equivalent of that knob that adjusted the colors.

Its defaults are weird:

Supposedly, burst filter around -1, -.5, .5, and 1 are reasonable typical values. But by running the NTSC filter, I can get what Richard and Chuck probably actually meant for U3 by setting burst phase to around -0.76, which gives me green grass and blue water. It’s close to what we get with mode 1 (blue/brown 1).

The four settings we can use in lr-atari cannot, alas, quite replicate this.

Off
Mode 1
Mode 2
GTIA
CTIA

You will also find that some games have hacks to make them color by changing them to use a graphics mode 15 that only existed on XL/XE machines. Among the games you can find with this hack are Lode Runner, Choplifter, and Drol. Frankly, they don’t look as good, because they have half the horizontal resolution. But they tell us what the colors ought to be like.

Here’s my best stab at what you should pick from what’s available in lr-atari800:

Default for all games should be artifacting off, because having it on will mess up text display in color games. This is not at all how it was on the original hardware. This would affect games like

• Archon
• M.U.L.E.

Some games that you might think use artifacting but will look better with it off include

• Caverns of Callisto
• The Nightmares
• Transylvania (check out that font at the bottom!)

Where hi res text mode was used mixed in with the graphics.

Hi res PAL games should have game options files with PAL set, and accurate PAL blending on (mode 2).

• 180
• Head Over Heels

Artifacting games should have a game options file with artifacting turned on at the RGUI level, set to NTSC. In the atari800.cfg, you should have

ARTIFACT_NTSC=NTSC-OLD
ARTIFACT_NTSC_MODE=1

…and use the .opt file to turn artifacting on and off, which will leave these settings intact. EDIT: If you use the script below, this is not required anymore. This will cover most games looking right, such as

• AE by Broderbund
• Choplifter by Broderbund (cart and disk; the cart by Atari uses 4-color Gr. 15)
• Crossfire (disk version)
• Dark Crystal
• Das U Boot pinball
• David’s Midnight Magic by Atari (cart.)
• David’s Midnight Magic by Broderbund (disk)
• Diamond Mine
• Dragon’s Keep
• Drol
• Escape From Vulcan’s Island
• Gwendolyn
• Hard Hat Mack
• High Rise by Joe Calabrese (not the other one)
• Lode Runner by Atari
• Lode Runner by Broderbund (disk)
• Lunar Leeper
• Malpass
• Mission Asteroid
• Ogre
• Pensate
• Pinball Construction Set (and all pinballs created with it!)
• Pool 1.5
• Pool 400
• The Wizard and the Princess
• Ultima 2
• Ultima 3
• Ultima 4
• Ulysses and the Golden Fleece

Unfortunately, there’s no way I know of to save alternate atari800.cfg files for the games that look better in other modes. EDIT: The script below basically creates this alternate file on the fly.

Correct in mode 2:

• 221B Baker St (the bottom display uses artifacting; it’s correct when Sherlock’s colors here match his colors in the iso map above).
• 3d Red Baron (assuming you are fighting the Red Baron, his plane should be red not blue)
• Flight Simulator II (the control panel uses artifacting, and the level gauge is the clearest use of it; the sky in the level gauge should be blue)
• Fooblitzky (I went off the color of the roads and crosswalks)
• Marauder (based on box shots and other platforms)
• Mr. Do (Only for EXTRA and at bottom of screen — should match the color graphics parts of the screen)
• Night Mission Pinball (based on screenshots and box images)
• Threshold (based on screenshots from other platforms)

Correct in GTIA:

• AutoDuel (other platforms show the blocked hexes as red)
• Aztec (bit of a guess here based on the release date and the color on other platforms)
• Bug Off (green stems for the flowers)
• Twerps (the alien on the box is green; if you have the version with the title screen, you get a large full screen alien, and this is only mode he’s green in).
• Jawbreaker could go either GTIA or CTIA, I’m unsure.

CTIA:

• Micro League Baseball is supposed to have a green baseball field, not purple, red, or blue.

Adding the missing artifacting options

A request: if anyone can come up with a clever solution for changing the value of ARTIFACT_NTSC_MODE as part of the loading process for the game, please let me know. I tried setting up separate emulators for it, but they all end up referencing the same atari.cfg file. I suspect that the only fix might be to actually back up the old file, copy in one with the one line change (or maybe sed it in?) in a runcommand shell script. EDIT: done, see below

Otherwise, I think the only other choice is to hope that developers on the upstream lr-atari800 core add this specific option line to the RGUI interface. It’s currently there, but basically ignored and hardcoded, in the block starting at line 302 in libretro-atari800/libretro/libretro-core.c.

   var.key = "atari800_artifacting";
   var.value = NULL;

   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
     {
       if (strcmp(var.value, "enabled") == 0)
	 {
	   if (Atari800_tv_mode == Atari800_TV_NTSC)
	     {
	       ARTIFACT_Set(ARTIFACT_NTSC_OLD);
	       ARTIFACT_SetTVMode(Atari800_TV_NTSC);
	     }
	   else if (Atari800_tv_mode == Atari800_TV_PAL)
	     {
	       ARTIFACT_Set(ARTIFACT_NONE); // PAL Blending has been flipped off in config for now.
	       ARTIFACT_SetTVMode(Atari800_TV_PAL);
	     }
	 }
       else if (strcmp(var.value, "disabled") == 0)
	 {
	   if (Atari800_tv_mode == Atari800_TV_NTSC)
	     {
	       ARTIFACT_Set(ARTIFACT_NONE);
	       ARTIFACT_SetTVMode(Atari800_TV_NTSC);
	     }
	   else if (Atari800_tv_mode == Atari800_TV_PAL)
	     {
	       ARTIFACT_Set(ARTIFACT_NONE);
	       ARTIFACT_SetTVMode(Atari800_TV_PAL);
	     }	   
	 }
}```

Anyway... time for bed here, hope this is helpful to anyone else getting Atari 8 bit emulation going on their Retropie setups.

lr-atari800 = "/opt/retropie/emulators/retroarch/bin/retroarch -L /opt/retropie/libretrocores/lr-atari800/atari800_libretro.so --config /opt/retropie/configs/atari800/retroarch.cfg %ROM%"
lr-atari800-ctia = "/opt/retropie/emulators/retroarch/bin/retroarch -L /opt/retropie/libretrocores/lr-atari800/atari800_libretro.so --config /opt/retropie/configs/atari800/retroarch.cfg %ROM%"
lr-atari800-gtia = "/opt/retropie/emulators/retroarch/bin/retroarch -L /opt/retropie/libretrocores/lr-atari800/atari800_libretro.so --config /opt/retropie/configs/atari800/retroarch.cfg %ROM%"
lr-atari800-artifacting1 = "/opt/retropie/emulators/retroarch/bin/retroarch -L /opt/retropie/libretrocores/lr-atari800/atari800_libretro.so --config /opt/retropie/configs/atari800/retroarch.cfg %ROM%"
lr-atari800-artifacting2 = "/opt/retropie/emulators/retroarch/bin/retroarch -L /opt/retropie/libretrocores/lr-atari800/atari800_libretro.so --config /opt/retropie/configs/atari800/retroarch.cfg %ROM%"
default = "lr-atari800"

Dreadnought Megastars (1990)(Byte Back)(PAL)(GB)[k-file].atr  // this game will launch in PAL

Beer Shot (1994)(Bednar, Kamil)(PD)[BASIC].atr // this game will enable BASIC

Head over Heels (19xx)(Hit Squad)(GB)[h Homesoft][k-file].atr // this game will run with default of no BASIC and NTSC

Ice Cap (19xx)(MacIlwee, Steven)(GB)[req OSa].atr  // this game will run on a 400/800 with OSa

Galactic Quest (1981)(Crystalware)(US)[req OSa][BASIC].atr // this game will set the system to 400/800 OSa, and will also set BASIC to Rev. A 

ROM_BASIC_A=/home/pi/RetroPie/BIOS/BASIC Revision A (19xx)(Atari)(400-800).rom