Ultima VII Game Structure

This article was written by Gary Thompson. It can be found in the documents section of the Exult source code here.

Introduction
In this file, I will attempt to explain the structure of the data files that accompany Ultima 7 - The Black Gate. I am currently in the process of writing a game editor. At this current moment, I am still trying to figure out the data files. As I go along and learn more and more, I will be adding to this text file.

I originally wrote this text file structuring the data using the concept of "layers". After making significant progress decoding the data files, I have abandoned it. There is no concept of "layers" whatsoever in Origin's files.

I have received some serious help from several people around the world. I feel I MUST credit them since they helped me so much.


 * Matthias Ho 
 * Michael A. Cornelius (Newt) 
 * Troy Forrest 

GRAPHICS
The graphics that you see in Ultima 7 are stored in the data files in a very strange way. It makes me wonder what kind of programmers they have working there. I could have devised a much better data structure.

The graphics are split apart and strewn all through many data files. All of the graphics themselves are stored in the SHAPES.VGA file. All other files structure the data pointing to shapes and frames stored in the shapes.vga file. There are several major sets of data files that structure the graphics... U7MAP, U7CHUNKS, U7IFIX, and U7IREG files.

No one set of files stores a certain type of graphic data as I originally thought. Moveable objects seem to be majorly stored in the IREG files, But semi-immovable objects like walls and trees are stored both in the U7MAP file and the U7IFIX files. First story graphics are stored in both IFIX and IREG files.

U7MAP
The basic U7 map is fairly easy to understand. The map is broken down on a large scale into a 12*12 grid of what's called "superchunks". This large scale grid makes up the entire map in the game. Each superchunk is broken down into a 16*16 grid of chunks. Each chunk is a single piece of land data.... such as road, water, grass...etc. The U7MAP file contains all the data for the entire map down to the chunk level. The U7MAP file contains a list of chunk numbers, each chunk number taking up 2 bytes in the file. These chunk numbers are arranged in order on the grid moving left to right, top to bottom by chunk grid then by superchunk grid. So, the first two bytes in the file refer to the upper left chunk on the map.... like this... Cn is chunk 'n'. Sn is superchunk 'n'.

It's fairly easy to understand. This gives you a total grid of 192*192 chunks or 36864 total chunks. Of course there are not that many different chunks, a great number of them are replicated. You can use the 192*192 grid and a simple formula to calculate the chunk and superchunk numbers as well as the offset in the U7MAP file.

U7CHUNKS
Each chunk is broken down into a grid of 16*16 8*8 pixel chunk bits. These are also arranged from L to R, T to B. The U7CHUNKS file contains the palette data for each chunk. AKA: Which chunk bits go where in each chunk. If you take the chunk number out of the U7MAP file, and multiply it by 512 (512 bytes per chunk palette)... you will get the offset in the U7CHUNKS file where the palette data for that chunk is stored. Each chunk palette in the U7CHUNKS file is also arranged L-R, T-B. Each chunk bit takes up 2 bytes also. The first byte combined with the lower 2 bits of the second byte make up the shape number in the shapes.vga file. The upper 6 bits of the second byte make up the frame number.

SHAPES.VGA
The SHAPES.VGA file contains all of the graphics you see on the screen. The top half of the file contains an index of where the graphics are located and how long each entry is. Offset 0x54 tells how many total entries there are in the file. Offset 0x80 starts the index. Each entry in the index is offset (4 bytes) and length (4 bytes). Each entry corresponds to the shape number found in the U7CHUNKS file. At each offset starts either a header or the picture data. If it's a header, the first four bytes will contain the length of the data again as in the index. If these numbers are not identical, then it's one of the 8x8 chunk bits. The 8*8 chunk bit data is not encoded or compressed in any way.

If it's a header, the second 4 bytes tell the length of the actual header. The header length will always be at least 8 bytes long. This includes one frame. Adding 4 bytes to the header length for each additional frame. Each 4 byte frame offset (offset from start of header), is stored next. Following that are 4-2 byte entries... W2 W1 H1 H2. Take W1+W2 and H1+H2 and you will get the uncompressed width*height of the bitmap. Following that is the actual compressed picture data.

The compressed picture data uses a modified simple RLE encoding. The first 2 bytes are the length of this scan line, the next four tell the X/Y offset on the screen where the pixel is. The next byte is odd or even tells whether to copy that many next bytes out, or copy the next byte out that many times.

Drawing the map using the above data, I got very good results. It seems very strange to me that they stored all the walls, trees and such in the MAP file since they actually don't make up the map itself. Only first story walls are stored in the U7MAP and U7CHUNKS files. Second story walls, roofs, and outdoor 1st story semi-immovable objects (like the Shrine of Compassion in Cove) are stored in the IFIX files.

You may be wondering why they went through all that crap just to draw a map. Well... look at it this way. As I mentioned earlier, there is a map grid of 192*192 chunks...and each chunk is a grid of 16*16 chunk bits. And each chunk bit is a 8*8 grid of pixels.... well, multiply all that together and you get an entire map of 24576*24576 pixels. And that relates to a raw picture data file size of over 600 megs. Even GIF compressions will only reduce that to maybe 100 or 200 megs.... which is still too large to be practical. And they couldn't use gif compressions because it would slow down the display of the game greatly.

U7IFIX
Since there are 144 superchunks, there are also 144 IFIX files. Each IFIX file contains all necessary data for 256 chunks. The file is broken down similar to the shapes.vga file. In that it has an index half and a data half. The index half is exactly the same. The data half is different though. If there are no items to be drawn on that chunk location, the offset and length in the index half for that chunk will be 0.

The length found in the index will always be a multiple of 4, since there are always 4 bytes per item entry. There can be MANY item entries. Starting at the offset found in the index, you will find a series of 4 byte entries. These entries are drawn on the screen in REVERSE ORDER, (I have no idea why) so you have to read the entire data portion first, then display the last 4 bytes first, then the previous 4...etc. You have to draw it this way so all items are displayed in the correct order. Try it in forward order and you'll see what I mean.

The bytes are broken down thusly... Byte 1 is a combined byte. It is split in half. The upper half is the X offset, lower is the Y. Since there are only 16*16 chunk bits in the chunk, this will work. Each item in the list is displayed on the screen "anchored" to it's lower right corner. The lower right corner corresponds to the lower right corner of one of the chunk bits. The X and Y offsets are the chunk bit grid offsets where this item will be anchored. The second byte is split into 2 halfs also. The lower half I haven't figured out yet (I will..) The upper half contains what I call the "lift factor". This lift factor shows how much the item is to be raised off the ground. You multiply the lift factor by 4 and subtract the X and Y screen pixel locations where to draw it, by this much. The third and fourth bytes are combined bytes. The third byte, combined with the lower two bits of the fourth byte make up the shape number in the SHAPES.VGA file. The upper 6 bits of the fourth byte make up the frame number.

One thing I can't seem to figure out is some frame numbers in some items are shorter than others. There seems to be no corresponding bit to tell which are which. For example: the shrine in cove, the lowest platform in the upper right corner is shorter than the other three left of it. So, this platform must be drawn one pixel left and up from the other three in order to align with them. I can't figure out how to detect which items fall under this category. So, I checked the shapes.vga file... it seems frame numbers 0-3,12-15,& 18 in shape number 1014 fell under this category. So, I took all items with those frame numbers and shifted them 1 pixel left and up... they aligned correctly in the shrine, but I don't know if other shapes will align properly using this method in other locations.

U7IREG
Like the IFIX files, there are 144 IREG files, one for each super chunk. These files are not as well structured as the IFIX files. There is no index, no headers. Just a list of entries. And even these entries seem to follow no order. But they DID structure each entry so it's easily enough to tell of that entry should belong in the chunk you're working with.

Each entry corresponds to a single item to be drawn on the screen, such as a desk, gold bar, bed...etc. Each item will start with a one byte length byte and either 6 or 12 bytes following that. A 6 byte entry refers to simple objects like a gold bar or chair. A 12 byte entry refers to either EGGS or containers like a chest or bag. 12 byte eggs will always have a 01 as the last byte. 12 byte containers will have a 00 at the end and then be followed by a series of 6 byte simple entries corresponding to each item INSIDE that container. After all the 6 byte objects in the container there will be a 01 to denote the end of the container list.

All items in the list that belong in the working chunk also need to be displayed in reverse order as in the IFIX files. Displaying this type of data in reverse order is no easy task. What I did was I set up a dynamicly memory allocated doubly-linked list. This works great for this. I read in the first item, decode it into shape, frame, lift..etc. Then I create a new node for the list and store the data in it. I set the appropriate node pointers then read the next item. When I've read all the data, I just use the node pointers to read it back out in reverse order.

Each 6 byte entry has the following format: The upper halfs of bytes one and two refer to the X and Y of the chunk inside the superchunk where that particular object is to be stored. The lower halfs of those same bytes refer to the X and Y of the chunk bit inside that chunk. Bytes 3 and 4 hold the shape and frame numbers in the same format as above. Byte 5 holds the Lift Factor as above. Byte 6 holds the "Quality" of the item. The quality refers to extra things like the number of charges in a magic wand, how many coins in a pile of gold...etc.

12 Byte entries have a slightly different format. The first four bytes hold the same format as the 6 byte entries. Bytes 5 thru 9 I haven't figured out yet. Byte 10 is the Lift Factor byte as in byte 5 of the 6 byte entries. Bytes 11 and 12 I haven't figured out yet either.

Now, 6 byte items that appear in the container list also have a slightly different format... but for the most part is the same as the standard 6 byte entries. The only exception bieng bytes 1 and 2... which I don't know what they do.

DISPLAYING
Displaying all the data in the above files is no easy task either. You cannot simply draw all the data in the U7MAP, then the IFIX then IREG. The data must be drawn all at the same time. I haven't designed a sequencing routine yet but I have theories.

I'm thinking I might be able to sequence from L to R, T to B, drawing all data in each chunk bit one by one. For example, chunk bit 0*0, I would draw the data for that bit from the U7MAP file first, then the IFIX file, then the IREG file. Then I would move to chunk bit 1*0 then draw the U7MAP, IFIX then IREG...etc. I'm hoping this will work... We'll see.

I tried it, and it didn't work. I've realized Origin must be using some kind of layering technique. If you run the game with the cheat command and add "spam s", during the init, one of the lines says "z-buffer". A z-buffer is a method of drawing layered images, determining which objects have priority over others by their Z value. The formula should be simple, but I ran into a wall.

Look at it this way, the graphics screen U7 is using is 320x200 pixels, which is 64,000 pixels. Drawing the graphics on the screen using a z-buffer, I'll need to calculate a separate z-value for every pixel on the screen. For each pixel, I'll need to store a z-value (int) and the color of the pixel (int). Since INTs are 2 bytes, that's 256,000 bytes.

I've never used a buffer that size before and I'm having problems allocating it. I've pretty much resigned myself to using extended memory. And extended memory is not the easiest thing to work with.