Please provide feedback. For the next one, I'll do priority map issues.
What I'd really like is for someone (Christoph) to provide a tutorial for
tracking down the real cause of the bug and how to determine a fix. This
can be summarized and I can document the tutorial, but I think that is the
next step.
Hope this helps!
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Problem: In some games, ego can't walk to certain areas, other actors get
stuck, or I'm always too far away from an object to do something with it.
Sometimes, FreeSCI doesn't draw lines or fill boxes on the control map
properly, which sometimes results in these kinds of bugs. The first thing
to do is to compare the control map as FreeSCI renders it, and the way Sierra SCI
renders it for the room where you are observing the problem. This requires
a DOS/Windows machine (or emulator) to run the original game on, a hex
editor, and a copy of FreeSCI. Having the game is helpful, also. In this
example, we'll use bug #331 as the basis for our investigation. Bug #331
affected Police Quest 2 version 1.002.011 in room 15.
First, get farmiliar with activating the debugger in Sierra SCI (SSCI) and
FreeSCI (FSCI). After starting the game, we press both Shift keys and the
minus ("-") key on the keypad at the same time. The game should pause, and
a dialog box with a title of "Debug" should appear in the upper right-hand corner.
In FreeSCI, pressing the Control key and the tilde ("~" or "`") key
activates the debugger. In FreeSCI, the game should halt and the text
window give some information about where the game has stopped.
>From here, we "teleport" to the room where the problem is. In SSCI,
we break into the debugger, then press "g" key. (For a list of SSCI
debugger commands, press shift+? while the debugger is active.)
When prompted for which global variable you wish to edit, enter "13" and
press enter. Global variable 13 stores the current room number, changing
it and breaking out of the debugger will "teleport" ego to that room.
Note that this isn't a very good way to cheat, since some game flags won't
be set -- it serves our purpose of just seeing the room, though.
In the case of bug #331, we want to teleport to room 15. After choosing
global variable 13, we see a new window in SSCI that displays the current
value of the variable we are editing. Use backspace to erase the current
value, type "15", and press enter.
If you don't know what the room number is, restore your savegame where the problem
occurs. Break into debug modein SSCI, edit the global variable 13, and
note the current value.
In FSCI, break into the debugger. At the debugger prompt in the text
window, type "vmvars g 13 15" to change global variable 13 to the value of
15. (For a list of FSCI debugger commands, type "list cmds" at the
debugger prompt in the text window.) To see what room you are currently
in, type "vmvars g 13" and the debugger will output the value in decimal
(base 10) and hex (base 16) of global variable 13.
After changing the value of the global variable 13, resume the game. In
SSCI, press both Shift keys and "D" together to resume executing the game.
In FSCI, type "go" at the debugger prompt. The game should now resume, but
magically in room 15 (in PQ2 this is the airport). Note that entering a
room that doesn't exist can cause the virtual machine to crash.
Now that we know how to get the room in both interpreters easily. We'll
need to compare the control maps in both interpreters. FSCI's debugger has
a built in command to do this, but SSCI will require some hex editing on
the SCIV.EXE/SIERRA.EXE binary. We'll start with the FSCI debugger first.
Once in the room with the bug in FSCI, break into the debugger. Make sure
the window that is diaplying game graphics is fully visible, as you won't
be able to move the window while broken in the debugger. Once in the
debugger, type "gfx_show_map 2". Map 1 is the priority map, map 2 is the
control map. The priority map says which images go in front of which other
images, giving the game a sort of 3D effect. The control map is used by
scripts to determine behaviour and to limit the movement of actors in the
room. For instance, standing in a box on the control map would tell a
script that the actor is close enough to push a button. If the box on the
control map is drawn incorrectly, the script might think the actor is
never close enough to push the button. This is the case with bug #331.
To make the display normal, type "gfx_show_map 0" in the debugger and press
enter.
To display the control map in SSCI is a bit more complicated. The SSCI
debugger doesn't have the functionality to change which map is being
displayed, so hex editing of the interpreter's executable is necessary.
On Windows, I like "Hex Workshop" for this step. Make a backup of the
original executable. Load the SCIV.EXE or
SIERRA.EXE into the hex editor and search for "01 00 00 00 01 00" in hex.
When it is found, edit the second 01 to be 04 and save the executable.
When you run the interpreter now, only the control map will be displayed.
(When done, change the 04 back to 01 to regain normal functionality.)
Enter the debugger and teleport to the room as you normally would.
Now that you can see the control map in both interpreters, note any
obvious differences like shapes missing entirely, shapes that are not
filled with the same color, etc. To get assistance from other FSCI
developers and testers, it is usually a good idea to get a screenshot of
the control map in SSCI if possible. To do this in Windows, run the
interpreter in a DOS shell. When at the point where you want to take a
screen shot, press Alt+Enter. With the DOS window selected, press
Shift+Print Screen. (Print Screen is usually next to the Scroll Lock key.)
Click Start->Run, then type mspaint. In mspaint, press Shift+Insert
(Control+V also works) and answer Yes to resize the bitmap. The image from
the DOS window should appear in MSPaint, select File->Save As to save the
bitmap to a file (16 colors preferred).
When reporting the bug, mention the room and the obsreved differences in
the control map (if any). Giving a URL to the screenshot or privately
mailing it to one of the developers is also a good idea. Please note the
version of the game you are reporting against! In the example of bug #331
(any many other bugs), the bug is only in one version of the game and not
the other.
Next tutorial: Once seeing the control map is wrong, what happens next?
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http://www.clock.org/~matt
