Convert Mouse Coords into Isometric Tile ID

Question

Using GameMaker Language, I'm basing off their Adavnced Examples. How can i convert the current mouse position into a tile position?

The mouse coordinates are 0,0 from top-left

This is the function when it draws. I think this is what needs reversing. I'm tragically bad at Maths, so I'm sorry in advance.

/// DrawLevel()
var xx,yy,destx,desty, base_x,base_y,current_row_x,current_row_y,tile,o,sp, m,h,offy, tileset;

TileRowDX = 68;
TileRowDY = 34;
TileColumnDX = -68;
TileColumnDY = 34;

MapX = 0;
MapY = 0;
MapDrawX = 11;
MapDrawY = 11;

/// Draw the complete isometric level
tileset = global.tiles;


// End point in map drawing (map coordinates)
destx = MapX+MapDrawX;
desty = MapY+MapDrawY;

// Start drawing position - center in the middle of the screen - and just above a little
base_x = surface_get_width(application_surface)/2;
base_y = 150;

// X and Y loop to print all cells.
for( xx=destx-1;xx>=MapX;xx--) {
    current_row_x = base_x;
    current_row_y = base_y;
    for( yy=MapY;yy<desty;yy++)
    {
        // First draw the column of tiles in this cell, bottom to top.
        offy=0;        
        for(var h=0;h<global.mapMaxLayers;h++){
            m = Map[h];         // get 2D map grid from level array (Z axis)
            tile = m[# xx,yy];  // get tile from 2D grid (x,y axis)
            if( tile>=0 ){
                // if we have a tile, then draw it.
                draw_sprite(tileset,tile,current_row_x,current_row_y-offy);
            }
            offy+=27;
        }

        // Next, get the object in this cell, and draw it on top. 
        // If allowing height for the player, you'd need to add on 27 for each block up.
        o = Objects[# xx,yy];
        if( o!=0 ){
            // Get the object sprite to draw (could also do frames etc)
            sp=o.Sprite;

            // get a 0 to 1 value into the isometric cell.
            var dx = (1.0/15.0) * ((o.x&15));
            var dy = (1.0/15.0) * ((o.y&15));

            // Move to top left of Isometrix grid cell
            var ox = current_row_x - TileRowDX;
            var oy = current_row_y - TileRowDY;

            // Now convert into an isometric offset on X and Y
            ox = ox + (dy*TileRowDX);                   //Y axis first
            oy = oy + (dy*TileRowDY);
            ox = ox - (dx*TileColumnDX);                // now X axis
            oy = oy - (dx*TileColumnDY);

            // and draw this sprite
            draw_sprite(sp,0,ox,oy);

        }

        // Move down and right
        current_row_x += TileRowDX;      
        current_row_y += TileRowDY;    
    }
    // Back up to top, and move left and down.
    base_x += TileColumnDX;
    base_y += TileColumnDY;

}

I'm sure this is just mostly a mathematics question!

Answer 1

isoX = cartX - cartY;
isoY = (cartX + cartY) / 2;

I don't know game maker's langauge, so I can only give you a pseudocode.

This only works correctly, if your "north" is facing to the top right direction

If you don't have objects, then you maybe need to create 2 separate methods to get tge X and the Y coordinate.