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Feature: Two-Dimensional BASIC Scrolling
Posted by Nick on 8 March 2000, 01:23 GMT
Brandon Green of SiCoDe Software is the first person to take the mic in this feature concerning two-dimensional scrolling in a BASIC program. Hopefully it can teach you a thing or two about BASIC programming and how strings work.
SiCoDe Software has recently discovered a method to scroll the entire screen at four frames per second. One example of this technology is the TI-83 BASIC game Frogger. The method envolves using a seldom exploited function called a string. [Strings are frequently used in BASIC programs, just not in this specific way. -ed.]
A tutorial of how we did it follows:
ADVANCeD.BASiC Tutorial:
Scrolling the Entire Screen From Left To Right
Part 1: Strings Make it Possible
String?
Suprisingly enough, perhaps one of the most advanced functions of the TI-83 is also the least known. For those of you who dont know a string is used to store a, well, string of charachter data. It is very similar to lists, and with the many advanced string operations you can mainpulate it just as you would a list.
A string can be any length, it can be infinitely long, or when you run out of memory. As every one knows, when you use the Disp command and you try to disp a message that is wider than the screen you get those annoying little dots(...) and it cuts it off. However, when you use the Output( command to output a string, it automatically wraps the message in the string to the next line. This will come in very handy down below.
String Operations
Since string operations arent well known, I will list them here. All these operations can be found in the catalog on your calculator: [note that all these operations can be found in your manual:), if you have any syntax questions, consult your manual, believe it or not, you can learn something from reading a book]
->-(storing into a string)
In order to put a message into a string you put what you want to input in quotes, like so: "STRINGS ARE FUN!"->Str1. You could then do a Disp Str1, and STRINGS ARE FUN! would be put on the homescreen. You can also do Input Str1 and then whatever the user typed in would be stored into the string.
+-(concatenation)
Suppose you had , "HELLO_"->Str1, and , "BOB"->Str2. If you wanted to add these together to create Str3, "HELLO_BOB", you would just type in: Str1+Str2->Str3.
Length(string)
If you had a string and you wanted to know how many charachters were in "HELLO BOB", then you would type Length(Str3) and it would return a 9. You could store this number into a variable for later use by Length(Str3)->S.
Sub(string,begin,length)- This is used if you want to take just a part of a string and store it into another string.
For example:
"ABCDEFGHIJKLM"->Str1
Sub(Str1,4,3)->Str2
Disp Str2
[displays]
DEF This is a way to substract parts from a string. You'll understand more after you read the rest of the tutorial.
Part 2: Moving the String
Scrolling
The basic idea of scrolling is that you have a 'map' that is bigger than the screen. You can then move the point that the screen is centered on to see different parts of the 'map'. By moving the point the screen is centered on over and over 1, you generate the illusions that the 'map' is scrolling by on the screen. Here is an example program that will scroll a string across the screen:
This is written with the TI-82/83/83+ in mind, if your screen is not 16 charachters wide, you will have to change the code.
1:ClrHome
2:"THIS_STRING_IS_MORE_THAN_16_CHARACTERS_LONG."->Str1
3:"________________"+Str1-
>Str1 //16
spaces long
4:For(a,1,45)
5:Output(1,1,sub(str1,a,16)
6:End 2: Sets the 'map'(in this case its just a one dimensional 'map') into string 1;
3: Since we want to scroll the entire string ALL the way across the string, we have to add this. If we didn't when the for loop got to 30, it would try to take the next 16 charachters to output, but it isn't 46 characters long, so we would get an error. This prevents that.
4: This is the loop that keeps track of the point at which the screen is looking at the 'map'.
5: What makes this fast and possible is that it outputs everything in one output statement, it takes the position of the screen on the 'map', gets the next 16 characters, and outputs them.
6: Ends for loop.
As you can see, there is no need to clear the first line because the output statement overwrites it , with a space, every time. By using this same principle you can scroll the entire screen. Here is some sample code I came up with. I didn't provide documentation so you'll have to figure it out on your own. ClrHome
"*_+_*++*_+_*++*_+_*++*_+_*++*_+_*++*_+_*++*"->Str1
"________________"->Str2 //16 spaces long
6->A
While 1
0->K
While not(getKey
End
(Ans=26)-(Ans=24)+A->A
If A>27:A-1->A
If A<1:A+1->A
sub(Str1,A,16)+Str2->Str3
Output(1,1,Str3+Str3+Str3+Str3
End As you can see, with this you can move the entire screen at about 3-4 FPS! This is an incredible speed in a basic program.
Don't forget that it auto-wraps the string to the next line when you use strings, this is what is makes it so fast, you output the entire screen in one statement!
For some reason there seem to be no basic games out there that use this technology effectively. Perhaps no knew about it. But dont worry, SiCoDe is working on some very impressive side scrollers... Check the site in the next few months (2/2000 to 5/2000) to get the side scroller when it is released.
Part 3: Conditionals and Strings
Let's say you were working on a game - a frogger game - and you already know how to scroll the cars across the screen. So if you put in the loop where you could control a character, and you had one whole list outputed to the screeen. Then you check to see if the character hit something by: If (sub(Str1,(16(y-1))+x,1))!="_" //for those nonC++ programmers, != means does not equal
disp " you hit something!"The (16(y-1))+x converts the two coordinates of the charachter to a position along the screen, it follows this pattern:
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16 row 1
17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32 row 2
33,34,35,36,37,38,39,40,41,42,43,45,46,47,48,49 row 3
....
113,114,...,128 row 8 (Each number represents a character position along the screen.)
This formula would only work if your string was outputed at 1,1. But it would be easy to change the formula to whatever you wanted.
In this way you could have hit detection using strings. And here is a sample program so that you can understand this better. Note that this is not optimised or anything, I threw it together in 3 minutes, so yes, it could be made a heck of a lot better. "_____(---)__(++)_____(**)__(---)___(///////)__(----)_____(+++)"->Str1 //you could put
//any string here that is more than 20 characters long
"________________"->Str2 //16 spaces
5->X:8->Y
1->P:1->H
length(Str1)-16->L
While H
Output(Y,X,"*"
sub(Str1,A,16)->Str3
Str2+Str3+Str2+Str3+Str2+Str3+Str3+Str2->Str4
Output(1,1,Str4
getKey->K
If K:Then
(K=26)-(K=24)+X->X
(K=34)-(K=25)+Y->Y
End
If (sub(Str4,(16(Y-1))+X,1))!="_" //does not equal a space
0->H
A+1->A
If A=L
1->A
End
Output(Y,X,"X"
There is also a 2-D scrolling engine which SiCoDe has invented. You can expect more advanced scrolling games from SiCoDe in the future.
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The comments below are written by ticalc.org visitors. Their views are not necessarily those of ticalc.org, and ticalc.org takes no responsibility for their content.
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Re: Feature: Two-Dimensional BASIC Scrolling
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PacMak 
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wow, thats incredible! i wish i had thought of it!
-pac
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8 March 2000, 01:52 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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Adam Vore
(Web Page)
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hey, I was fooling around w/ the person next to me's calc in 2nd period today..
and I thought of this... guess I wasnt fast enough... then I came home, and I see this... I was like "WOW!" we were thinking of the same thing...
I'll program a good side scroller tomorrow in 2nd period... then If I can ever get it uploaded to my computer, I'll post it...
well, see ya'll
Peaze (-<)
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8 March 2000, 01:52 GMT
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Re: Re: Re: Feature: Two-Dimensional BASIC Scrolling
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Adam Vore
(Web Page)
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ya, this was a really great idea... unfortunately I dont own a TI-83, just a stinkin TI-82, the BASIC language on the 82 is much more limited, so its really hard to create good programs... but it can be done... I messed around with writing dec code to a matrix and writing a routine that decoded it to a string, but that was impractical and very SLOW! so I deleted it... the best language for the TI-82 is asm... but for the 83, BASIC and asm are very comparable and I love it... I forgot that tomorrow I am having a test in 2nd period, so I cant program anything :( but when I get some time, I'll make a game... my next production for the 82 is Oregon Trail... I'll see how it turns out, and if its good enough, I'll release it.. soon I will release BASeBALL 2000 in BASIC which is one of my better accomplishments. also, I am working with a ti82 without link capabilities... so bear with me while I type line for line, the code for BASeBALL into my computer and upload it... sorry about the length.. I get a little carried away here sometimes...
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8 March 2000, 08:31 GMT
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Re: Re: Re: Re: Re: Feature: Two-Dimensional BASIC Scrolling
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EvanMath
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Actually, I personally prefer to use ASM for the display, and BASIC for the rest of the program. For example, in a puzzle game I wrote called Boulders, I used a list (actually, it worked almost exactly like the way strings are used in the feature, maybe I'll change it to use a string instead...) to store the level, and two variables, X and Y, to give the coordinates of the screen. Then I wrote an ASM routine to fetch the variables X and Y, and used a ROM call to read that element of the list, and display the corresponding graphic in the upper left corner. Then it moved on to the next element, and so on until it displayed the whole screen. This way, I had a two dimensional side-scroller in BASIC with a superfast display.
But that's just my way of doing things.
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9 March 2000, 23:13 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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James abba shalaka Rubingh
(Web Page)
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Why is the 'i' and 'e' small in the title of the tutorial?
also, nice work fellas...
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8 March 2000, 02:01 GMT
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Re: Re: Re: Re: Feature: Two-Dimensional BASIC Scrolling
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Jim Haskell
(Web Page)
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You can get n, a, b, c, d, e, r, p, z, t, and s from the var:stats menu. combined with i, u, v, w, and the degree sign, you can get a lot of lowercase characters without using ASM =)
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9 March 2000, 23:45 GMT
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