I have a problem which I am struggling to solve. I have a large map of around 100x100 tiles which form an isometric map. The user is able to move the map around by dragging the mouse. I am trying to optimize my game only to draw the visible tiles.

So far my code is like this. It appears to be ok in the x direction, but as soon as one tile goes completely above the top of the screen, the entire column disappears. I am not sure how to detect that all of the tiles in a particular column are outside the visible region.

    double maxTilesX = widthOfScreen/ halfTileWidth + 4;
    double maxTilesY = heightOfScreen/ halfTileHeight + 4;

    int rowStart = Math.max(0,( -xOffset / halfTileWidth)) ;
    int colStart = Math.max(0,( -yOffset / halfTileHeight));

    rowEnd = (int) Math.min(mapSize, rowStart + maxTilesX);
    colEnd = (int) Math.min(mapSize, colStart + maxTilesY);