Subtleties of Console Development

I've been trying to write my own sprite management code as a sort of exercise in C++ as well as the DS. I was having trouble getting things displayed correctly and thrashed back and forth changing one thing and then another, writing directly to memory rather than using DMA, etc. etc. etc. trying to figure out what was wrong. I'm a pretty careless programmer, so I assumed that it was either my mistake or the result of developing on an only partially documented system. Not having figured out a good way to do debugging obviously doesn't help either.

And, I finally figured it out. It was in code I wasn't even looking at; apparently I was trying to do something the system hadn't been initialized to do.

You can pass a number of flags to videoSetMode() when you are initializing the display, and apparently DISPLAY_SPR_1D_BMP is necessary in order to display 16-bit, non-palettized sprites. It seems obvious given that the ATTR0_BMP flag must be set in the sprite's OAM entry, but I didn't even know I should be looking for it.

Further, apparently you can't mix ATTR0_BMP and non-BMP sprites. At least, I can't get both to display at the same time. Perhaps I need another flag....

Lua, Lua... Oh no! Me gotta go

Been messing around with Lua a bit the last few days. Can't remember how I came across it but it's performance against other languages (1, 2) made me decide to give it a serious look. Particularly given how well it did against the dark lord Perl and my personal favorite, Python. Found several other things where people remarked how easy it was to embed in C/C++ programs so I figured it was worth the time.

Downloaded the 5.1.1 source from www.lua.org and had it compiled under cygwin in a matter of seconds without any problems. Found a decent selection of tutorials, and an online version of Programming in Lua but found they were a bit thin when it came to embedding Lua in C. Thought I hit the jackpot with this writeup at debian.org but was amazed to find out that it no longer worked despite being less than a year old.

After an hour or two of (re-)reading the sources above, looking at header files, etc. I was able to figure out enough to get a demo working. It's pretty slick. You can write a C program like:

#include <stdio.h>
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
int
main( int argc, char ** argv ){
int value = 1;

// Initialize Lua
lua_State *L = lua_open();

// Load various Lua libraries
luaopen_base( L );
luaopen_table( L );
//luaopen_io( L ); // Not sure why but I get a PANIC if i try to load this
luaopen_string( L );
luaopen_math( L );

luaL_loadfile( L, "3.lua");
lua_pcall( L, 0, 0, 0 ); // Have to parse it once or getglobal() will fail

printf( "C: value is %d\n", value );

lua_getglobal( L, "test" );
lua_pushnumber( L, value );
lua_pcall( L, 1, 1, 0); // 1 argument, 1 result
value = (int)lua_tonumber( L, -1 ); // Lua arrays are 1-indexed /boggle

printf( "C: value is %d\n", value );

// Cleanup
lua_close( L );

return 0;
}

And then you create a script called 3.lua like:

function test (x)
return (x + 1)
end

And your output should be:

C: value is 1
C: value is 2

Pretty slick. Doesn't make the C program terribly huge, either. The luaopen_*() functions load libraries which naturally increase the size of executable. The plain C executable was about 8k, with the four libraries above about 130k, or with just "base" alone it was 114k. OK, OK, so the executable is about 12x larger... it's still neat seeing as how you now have full access to a fairly powerful scripting engine without having to recompile.

I'm positive I can put this to good use in at least one of my projects. Why do I mention it here? Saw a bunch of comments about it being used for scripting, etc. in various video games.... good stuff.