Parse a tile map into efficient horizontal and vertical bounding boxes - c++

Question

I'm working on a simple platformer game in C++, everything is working great except i want to group tiles into bounding boxes in order to have less passes of the physics function(i'm targeting both PC's and embedded devices so its all about speed).
What the following function does is load a simple map format:
"[char *header][char bytesize][bytesize w][bytesize h][char tw][char th][char *map]"
It then reads the array as a 2d map and groups vertical lines when possible.
The question is... could someone show me how to group these tiles into efficient bounding boxes on both vertical and horizontal axis?
Heres an image to help explain, i know my grammar and spelling is terrible: http://thetooth.name/dev/blocks_bic.png
void Environment::load_map(char* mapPath){
cl("Loading Map: %s ", mapPath);
FILE* mapFile = fopen(mapPath, "rb");
FILE* mapInfoFile = fopen(strcat(substr(mapPath, 0, strlen(mapPath)-3), "bmd"), "rb");
if (mapFile == NULL || mapInfoFile == NULL)
{
cl("[ERROR]\n");
throw KLGLException("Error loading map file!");
return;
}
size_t wordSize;
char tBuffer[8] = {};
int w = 0;
int h = 0;
int tileWidth = 0;
int tileHeight = 0;
fread(tBuffer, 1, 7, mapFile);
if (strcmp(tBuffer, "STME1.0"))
{
cl("[BADHEADER]");
}
fill_n(tBuffer, 8, NULL);
fread(tBuffer, 1, 1, mapFile);
if (!strcmp(tBuffer, "B"))
{
wordSize = sizeof(char);
}else{
wordSize = sizeof(int);
}
fseek(mapFile, 8, SEEK_SET);
fread(&w, wordSize, 1, mapFile);
fread(&h, wordSize, 1, mapFile);
fread(&tileWidth, 1, 1, mapFile);
fread(&tileHeight, 1, 1, mapFile);
#define lvLookup y*w+x
fill_n(mapData, (w*h)+1, '\0');
fill_n(mapMask, (w*h)+1, '\0');
// Read files into memory... back to front and inside out...
for(int y = 0; y < h; y++){
for(int x = 0; x < w; x++){
fread(&mapData[lvLookup], 1, 1, mapFile);
fread(&mapMask[lvLookup], 1, 1, mapInfoFile);
}
}
fclose(mapFile);
fclose(mapInfoFile);
// Parse map data into are geometry vectors
for(int x = 0; x < w; x++){
for(int y = 0; y < h; y++){
if(mapData[lvLookup] > 0){
int xl = x;
int yl = y;
while(mapData[yl*w+x] != 0/* && mapMask[yl*w+x] == 0*/){
yl++;
}
platforms->push_back(Platform(x*tileWidth, y*tileHeight, 1*tileWidth, (yl-y)*tileHeight, lvLookup, mapData, mapMask));
y = yl-1;
}
}
}
cl("[OK]\n");
}
Thanks in advanced!

Answer 1

This is a little ugly, but I think it should work as a starting point. It works by scanning left to right, top to bottom looking for to find whole columns that are alike. The challenging part is keeping track of consecutive complete vertical blocks and outputting "partial" runs correctly.
void find_bounds()
{
int startx = 0;
char last = mapData[0];
for (int x = 0; x < w; x++) {
int starty = 0;
for (int y = 0; y < h; y++) {
char c = mapData[x+y*w];
if (c != last) {
if (starty == 0) {
// finish last run of complete vertical blocks
if(startx != x) {
// it ran more than one column, output those first
span(last, startx, 0, x-1, h-1);
startx = x;
}
// and a run from the start of this column
span(last, x, 0, x, y);
} else {
// a run within a column
span(last, x, starty, x, y);
}
last = c;
starty = y;
}
}
// had a run withing this column or processing last column, finish it up
if (starty || x == w-1) {
span(last, x, starty, x, h-1);
startx= x + 1;
}
}
}
Test-suite (the first two should correspond to the second and third cases from your illustration):
#include <iostream>
#include <vector>
using namespace std;
const int w = 8, h = 8;
char mapData[w*h+1] =
#if 1
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
" xxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
#elif 0
"xxxxxxxx"
"xxxxxxxx"
"xxx xxxx"
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
#else
"xxxxxxxx"
"xx xxxxx"
"xxxxxxxx"
"xx xxxxx"
"xxxx xxx"
"xxxxxxxx"
"xxxxxxxx"
"xxxxxxxx"
#endif
;
void span(char type, int x0, int y0, int x1, int y1)
{
if (!(x0 == x1 && y0 == y1))
cout << type << " " << x0 << ", " << y0 << " -> " << x1 << ", " << y1 << "\n";
}
int main()
{
find_bounds();
}