A Summary of the Differences Between Sierra's AGI and SCI

General technical information about Sierra & classic games. Please do not ask game support questions here unless it directly relates to an existing thread.

A Summary of the Differences Between Sierra's AGI and SCI

Postby Collector » Wed Jun 04, 2014 2:54 am

A Brief Summary of the Differences Between Sierra's AGI and SCI Engines

Note: for simplicity, this post is limited to ports of AGI's and SCI's main target platform, the PC.

Many of the Sierra On-Line games before AGI used the then standard 4 color CGA graphics and single voice sound via the PC speaker.

WinnieCGA.png
Winnie the Pooh
WinnieCGA.png (7.05 KiB) Viewed 4768 times

In 1984 Sierra introduced the original "King's Quest: Quest for the Crown" to showcase the expanded abilities of IBM's new computer, the PCjr. In fact, the first couple of releases were published by IBM. The Adventure Game Interpreter (AGI) was developed as the new engine for this task. AGI's graphics were 160px x 200px by 16 colors (quadrupedal that of CGA's 4 colors) using vector graphics backgrounds. It also had three voice with noise channel sound. This was quite a technological advancement and even cutting edge at the time. AGI was designed for portability (easy to port to other platforms.) It became Sierra's main game engine until the advent of Sierra's next engine, SCI. Most of Sierra's famous series began in the AGI era.

https://www.youtube.com/watch?v=qdbdGJBwpgs
King's Quest : Quest for the Crown Intro (AGI)

Hardware began to advance beyond what AGI could utilize, so Sierra began development on its next engine, SCript Interpreter or later Sierra Creative Interpreter (SCI). The previous year before the last official AGI game, "Manhunter 2: San Francisco" in 1989, Sierra released the first SCI (SCI0) game, "King's Quest IV: The Perils of Rosella" in 1988. In keeping with Moore's Law, SCI could take advantage of the constantly increasing capabilities of the new hardware. As with AGI in its day, SCI was a big leap forward in technology. The higher resolution allowed for better graphics. Even though 200px x 320px may not seem that much over AGI's 160px x 200px, it is very noticeable. While still just 16 colors, but SCI0 introduced dithering, which mixed pixels of different colors into the same fill areas that gave the effect of more than 16 colors. Most importantly, support for the new sound cards. With SCI, music became an inseparable part of Sierra games. Even though the first exposure most had was with AdLib's hoots and honks, it also supported the MPU-401, which allowed Sierra's composers to create some amazing Roland MT-32 sound tracks.

AGIvSCI.png
AGI vs SCI

https://www.youtube.com/watch?v=HHYRgh226Is
King's Quest I: Quest for the Crown SCI Demo

Sierra co-developed King's Quest IV in both AGI and SCI. If the hardware requirements were too high for a user's PC, he could send in his SCI game for the AGI version. Here are side by side comparisons of the two versions:

RosellaGenesta.png
AGI Rosella and Genesta vs SCI

https://www.youtube.com/watch?v=n3XyMSwXcrQ
King's Quest IV: The Perils of Rosella AGI Intro

https://www.youtube.com/watch?v=TrMQw6OwEGo
King's Quest IV: The Perils of Rosella SCI Intro

Technologically, another huge advance was SCI's object oriented scripting language (AGI was a procedural language), which allowed for much more complex scripting, opening possibilities the developers could have only dreamt of earlier. Sierra was able to continually expand SCI's capabilities as hardware advanced. Slightly before the transition from SCI0 to SCI1, digital sound was added, and the characters began to speak.

https://www.youtube.com/watch?v=8a6OP9JWNxk
Roger Speaks for the first time!

With King's Quest V: Absence Makes the Heart Go Yonder!" Sierra introduced VGA to SCI (SCI1), giving SCI games a 256 colors with bitmapped or raster backgrounds.

KQ5.png
King's Quest V
KQ5.png (41.19 KiB) Viewed 4768 times

During the transition to VGA, Sierra released both EGA and VGA version of the same games. Some of these had artwork that were completely done for both versions. The end result was better optimized for the EGA users, as in Jones in the Fast Lane.

JonesEGA-VGA.png
EGA and VGA versions of Jones in the Fast Lane

Later VGA games simply took the VGA graphic and reduced the palette.

RobimEGA-VGA.png
EGA and VGA versions of Conquests of the Longbow

Besides more detailed images and more accurate colors, the expanded color depth allowed palette switching to simulate even more colors and being able to modify an existing picture resource by changing the palette it used. They could simulate a day/night cycle by merely changing the palette, while using the same PICTURE resource. The same method was used to add dawn and dusk scenes, as well.

QfG1VGA_DayNight.png
Quest for Glory I VGA Day/Night Cycle

Note the KQ4AGI day/night cycle had to be done with two distinct PICTURE resources.

KQ4AGI_DayNight.png
King's Quest IV AGI Day/Night Cycle

Like KQ4AGI, KQ4 SCI version 1.000.111 used two distinct PICTURE resources, too.

KQ4SCI1_000_111DayNight.png
King's Quest IV SCI version 1.006.004 Day/Night Cycle

KQ4 SCI version 1.006.004 used additional partial PIC resources overlayed on top of the normal daytime PIC resource, such as a dark sky with stars to add a darker sky with stars and covering other lighter colors with darker ones, etc. to create a composite for the night PIC. SCI0 generally did not use a complete second resource the way AGI did. Note that the teal color in the overlay PICTURE resource is set to be transparent. Anything in the base Pic under the teal would show.

KQ4SCIDayNight.png
King's Quest IV SCI & Pic Overlay for Day/Night Cycle
version 1.006.004

With SCI2, around the time of "Gabriel Knight: Sins of the Fathers", VESA support was added, giving SCI a 640px x 480px resolution. The first VESA games used a mixture of VGA and VESA graphics. Using the VESA driver or a Windows interpreter, the 320px x 200px graphics were scaled up, overlaying higher resolution graphics for things like GUI items on top. The VGA to VESA transitional games would have two sets of resources, one lower res for VGA and a higher one for VESA.

GK1withGUI.png
Gabriel Knight 1 with GUI

Later versions of SCI32 used Windows SVGA and/or DOS VESA at 640px x 480px only.

Shivers.png
Shivers

Note that while the pixel dimensions mentioned are not proportional, the aspect ratio for these games is 4:3. The monitors from the different eras had differently shaped pixels, but the same overall dimensions

https://www.youtube.com/watch?v=4J1ZhRMtNkA
Leisure Suit Larry 7: Love for Sail Opening Credits

Often the interpreter was modified, improved or features added to meet the needs of a game in development. This could cause the engine to become so specific to that game that the the source code for the engine would be stored with the source for the game.

SCI lasted until the switch to the 3D engines used for Mask of Eternity, Gabriel Knight 3 and Quest for Glory V. Sierra as we like to remember it, the Ken Williams era, did not last much longer than SCI.


AGI
Graphics:
  • 160x200 by 16 colors
Audio:
  • 3 voice plus noise channel (PCjr/Tandy)
  • 1 voice PC speaker (PC)
Interface:
  • Parser without mouse support
List of the Sierra AGI games

SCI
Graphics:
  • 320x200 by 16 colors EGA (SCI0 - Early SCI1)
  • 320x200 by 256 colors VGA (SCI1 and later)
  • 320x200 by 256 colors VGA with video support (SCI1.1 and later)
  • 640x480 by 256 colors VESA (SCI2 and later)
Audio:
  • AdLib/MPU-401/Roland MT-32 music (SCI0)
  • AdLib/MPU-401with Roland MT-32 music with digital audio (SCI0.1 - Early SCI1)
  • AdLib/MPU-401with Roland MT-32/General MIDI music with digital audio (SCI1.1 and later)
  • Digital audio (SCI1 and later)
Interface:
  • Parser with mouse support (SCI0 - Early SCI1)
  • Point and Click (SCI1 and later)
List of the Sierra SCI games
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby DeadPoolX » Wed Jun 04, 2014 4:11 pm

That was pretty cool. I mean, I actually knew most of it, but even so, it was very informative and easy-to-read. :)
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Mostly » Wed Jun 04, 2014 8:11 pm

^ Ditto.

Thanks for putting that together!
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Maxor127 » Thu Jun 05, 2014 1:42 am

The Apple IIgs versions of AGI games (and probably Amiga and Atari ST versions) had mouse support. Apple IIgs AGI games also had enhanced audio.
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Collector » Thu Jun 05, 2014 2:25 am

If you read the note at the very top you will see that I was purposely limiting the discussion to just the PC ports of the interpreters for simplicity and because it was the primary platform for which the games were written. Things quickly become a lot more complicated if you start throwing in different platforms from different eras as their capabilities do not evolve evenly. Apple becomes replaced by the Mac, Amiga and Atari disappear completely.

Yes, AGI did take advantage of some features of other platforms that were more limited on the PC, by the time you get to the VGA era, the Mac was the only thing that could give the PC a run for the money and Mac ports of SCI games often left something to be desired. The Mac version of KQ6 was not even SCI, rater used Revolution Software's Virtual Theatre engine.
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Tawmis » Thu Jun 05, 2014 8:29 pm

As I said on the FB page, this is an amazing thread.
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Collector » Thu Jun 05, 2014 9:20 pm

I thought it important to be able to add examples, which would have been a lot harder on the FB group. These are all captures and recordings that I made specifically for this thread. Getting all of the captures and uploading to the SHP YouTube channel took more time than it took to write the post. I have added a couple of new examples (the GK1 with GUI screenshot and Roger uttering his first words to the world) since I originally posted this.
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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Constantin35 » Sat Jun 07, 2014 1:28 pm

"Note the KQ4AGI day/night cycle had to be done with two distinct PICTURE resources. KQ4SCI used overlays to add a darker sky and other colors."


I think the KQ4SCI Version 1.006.004 did, the KQ4SCI Version 1.000.111 actually also has two distinct PICTURE resources.
One question: Is there any difference in the SCI Engines of the games, that had distinct VGA and EGA versions? Between them?

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Re: A Summary of the Differences Between Sierra's AGI and SCI

Postby Collector » Sat Jun 07, 2014 10:28 pm

Though I did not have an example, I did note that the SCI version of KQ4 used a 2nd partial PIC resource overlayed on top of the daytime PIC resource, such as a dark sky with stars to change the sky to a night sky, etc. to create a composite for the night PIC. It generally did not use a complete second resource the way AGI did. When I get a chance I'll edit the entry to make it more clear and add a couple of examples.

Edit, when I first answered your post, I was in a hurry and misread it. You are right about 1.000.111. There are other differences in the Pic resources between the versions as well. You can see a few screenshot comparisons on the game's SCI Wiki page: http://sciwiki.sierrahelp.com/index.php ... of_Rosella

The floppy versions of KQ5 are SCI1 and the MPC (CD) version was SCI1.1. I am unaware of an EGA version of the MPC KQ5.
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