#15171 - alek - Sun Jan 18, 2004 3:05 pm
I've been looking at the code for scrolling backgrounds larger than 512x512 in Mode0 found in the pernproject example "Scrolling both ways the not so easy way" and I have not been able to figure out a few things. I've been looking at this code a long time and the map viewer in VisualBoy Advance. I sent a email to dovoto but he hasn't answered so I hope you guys can give me a hand. So the questions I have are:
1. Why do you have to AND the x and y vule with 31?
| Code: |
for(loopx = 0; loopx < 31; loopx++)
{
//this is were the data from our map editor is copied to video memory
bgMap[((loopx+x)&31) + (y & 31) * 32] = data[(loopx+x) + (y) * dataw];
}
|
2. How does the oldy thing work?
| Code: |
static int oldy = -1;
//do we need to copy in new data?
if(y == oldy)
return(0);
oldy = y;
|
#15176 - LOst? - Sun Jan 18, 2004 4:50 pm
| alek wrote: |
I've been looking at the code for scrolling backgrounds larger than 512x512 in Mode0 found in the pernproject example "Scrolling both ways the not so easy way" and I have not been able to figure out a few things. I've been looking at this code a long time and the map viewer in VisualBoy Advance. I sent a email to dovoto but he hasn't answered so I hope you guys can give me a hand. So the questions I have are:
1. Why do you have to AND the x and y vule with 31?
| Code: |
for(loopx = 0; loopx < 31; loopx++)
{
//this is were the data from our map editor is copied to video memory
bgMap[((loopx+x)&31) + (y & 31) * 32] = data[(loopx+x) + (y) * dataw];
}
|
|
ANDing the x anf y value with 31 will make x and y loop between 0 and 31.
So if x is 32, then x = 0. And if x is 33, then x = 1.
#15179 - poslundc - Sun Jan 18, 2004 4:59 pm
| alek wrote: |
| 2. How does the oldy thing work? |
I haven't looked at the Pern code, but my guess is that it's checking to see if the value for your y-position has changed from one frame to the next, so that it will only update if it needs to.
This works because each frame oldy will contain the value of y from the previous frame. So if the current value of y is the same as the value of y from the previous frame, then no update is necessary. (Otherwise, y has changed from the last frame, so we need to update the map accordingly.)
Dan.
#15180 - YopYop - Sun Jan 18, 2004 5:08 pm
(x & 31) is equal to x%32
you can replace %nb by &(nb-1) if nb is a 2 powered number
static int oldy = -1; // static means that oldy is initialized to -1 only in the first function call (it's like a global variable for the compilo)
//do we need to copy in new data?
if(y == oldy) //for all function call he verify y
return(0); //if y has not change he have nothing to do
oldy = y; //but if it chang he update oldy
//here he may copy in the new data
I hope it will help you
#15183 - alek - Sun Jan 18, 2004 5:43 pm
So thats what static does. Thanks for the help.
Lostr:
| Quote: |
ANDing the x anf y value with 31 will make x and y loop between 0 and 31.
So if x is 32, then x = 0. And if x is 33, then x = 1. |
I hade figured that much out, why do you have to do it?
#15184 - LOst? - Sun Jan 18, 2004 6:10 pm
| alek wrote: |
So thats what static does. Thanks for the help.
Lostr:
| Quote: | ANDing the x anf y value with 31 will make x and y loop between 0 and 31.
So if x is 32, then x = 0. And if x is 33, then x = 1. |
I hade figured that much out, why do you have to do it? |
You don't need to copy more than 32 tiles on the bg. The rest of the bg will not be visible.
#15185 - alek - Sun Jan 18, 2004 6:17 pm
Thanks, now I can go on with my little project =)
#15189 - poslundc - Sun Jan 18, 2004 7:01 pm
| alek wrote: |
| So thats what static does. Thanks for the help. |
Actually, it depends on where the static keyword is used.
If a variable inside a function is declared as static, it means that the variable's value is retained from one function call to the next. It's like using a global variable, except the scope of the variable is still limited to the function.
If a global variable or function is declared as static, it is an indicator to the compiler that the variable/function is for internal use only (ie. only accessible by functions in the same C file) and should not be made available to the linker.
It's one of the wackier little quirks of C syntax that has evolved over the years...
Dan.
#15193 - alek - Sun Jan 18, 2004 8:39 pm
Appriciate all the help, it has really helped alot.