Friday, 12 April 2013

Virtual tile grid

So I have been busy with quite a lot of other stuff lately and haven't really been looking at duskz much for a while. Partly there are some biggish problems to do with tooling and game-building that i'm not sure how to address off the top of my head, partly my mind is too full of NEON/Android/OpenCL and takes too long to context switch, and partly I just have other distractions.

This long weekend (every weekend for me is a long one now! yay for me!) the weather is too unseasonably nice to be stuck inside and I need to do some work in the yard too, so I probably wont do any more soon either.

However the other evening I had a short look at addressing one of the problems - how to find out where interactive things are set up within the map. Oddly enough looking at coordinate pairs and using command line globbing isn't a very friendly way to navigate through the game ...

To that end I have had the idea of a JavaFX fork of Tiled, just forking Tiled as is, or creating something more task specific ... that idea is still on the back-burner (one gets a little over-excited at times) but I did investigate how JavaFX would go about displaying the map layers in such a tool. Possibly it should just be a 'creator' mode within the game itself, but a separate tool has it's advantages too.

JavaFX scalability

I tried loading the Dusk map into a Pane using ImageViews, and it just exhausted memory after a very long pause. Yes the map is a very impractical 700x700 tiles which is not necessary anymore with the multi-map support, but if it would be nice if it coped.

JavaFX isn't really 'lightweight' as is mentioned in places - just looking at Node would tell you this.

It would be nice if there were Array versions of some of the basic Nodes that could manage state in a much more efficient manner. e.g. an imageview that could draw thousands of images, but tracks the coordinates or other adjustables in arrays of primitives which are more compact and can be processed efficiently. i.e. by using a structure of arrays rather than an array of structures. Such an approach has some pretty big benefits for things like parallelisation and animation too, but might be hard to fit into a general purpose object-oriented api such as JavaFX.

But even for something like this an Array implementation would need to virtualise it's coordinate generation to be efficient - 16 bytes per coordinate is a lot of memory particularly when the coordinate can be calculated faster on the fly from the tile location than it could ever be loaded from memory, even without an object dereference.

But since no such mechanism exists, one is left with ...


So basically you get to throw out all that nice scene-graph stuff and have to manage the viewable objects yourself. This management of the viewport is kind of the whole point of scene-graphs (or at least a very major part of it), so for a scene-graph implementation to require manual virtualisation is a bit of a bummer. But what can you do eh? Fortunately for 2D the problem is relatively simple but it will fall down completely for 3D.

I started by trying to read through the ListView source to see how it virtualised itself. After half an hour trolling through half a dozen classes spread across a few namespaces I can't say i'm much the wiser. It seems to be based on the layout pass, but how the layout is requested is a mystery ... Although I did gather that unlike Swing (which to be honest was confusing and difficult to understand) which allows Gadgets to render virtually within any ScrollView (as far as i can tell), JavaFX's ListView just does the scrollbar management itself.

I figured that perhaps I could just use a TableView too, but as i'm not familiar with it at this point I thought i'd give it a pass. It didn't quite seem to be the right approach anyway as as far as I can tell the data still needs to be added as fielded rows, whereas my virtual data 'model' is a 1D array of shorts and it would be easier just to access it directly as 2D data.

As I couldn't really work out how the ListView was working I just took a stab with what I could figure out. I do everything in the layoutChildren() function. First I ensure there are enough ImageView objects just to cover the screen, and then update the tile content to match when the location changes. Per-pixel scrolling is achieved by a simple modulo of the location if you really must have it (personally I find it annoying for lists).

    Pane graphics;

    ScrollBar vbar;
    ScrollBar hbar;

    int vcols, vrows;
    double oldw, oldh;
    protected void layoutChildren() {
        if (oldw != getWidth() || oldh != getHeight()) {
            vcols = (int) (getWidth() / 32);
            vrows = (int) (getHeight() / 32);
            oldw = getWidth();
            oldh = getHeight();

            ObservableList<Node> c = graphics.getChildren();

            for (int y = 0; y < vrows; y++) {
                for (int x = 0; x < vcols; x++) {
                    ImageView iv = new ImageView();
                    iv.relocate(x * 32, y * 32);

If the size has changed, it creates enough ImageView's to cover the screen (actually it needs to do one more row and column, but you get the idea).

updateMapVisible(), well updates the visible map oddly enough.

    private void updateMapVisible() {
        int y0 = (int) (vbar.getValue() / 32);
        int x0 = (int) (hbar.getValue() / 32);

        // Set per-pixel offset
        graphics.setTranslateX(-((long)hbar.getValue() & 31));
        graphics.setTranslateY(-((long)vbar.getValue() & 31));

        for (int y = 0; y < vrows; y++) {
            int ty = y + y0;
            for (int x = 0; x < vcols; x++) {
                int tx = x + x0;
                ImageView iv = (ImageView) graphics.getChildren().get(x + y * vcols);

                int tileid = map.getTile(0, tx, ty);
                data.updateTile(iv, tileid, tileSize, tileSize);

Initially I just created new ImageViews, but just updating the viewport and/or the image was faster. Obviously updateMapVisible could optimise further by only refreshing the images if the tile origin has changed, but it's not that important.

There is only one extra bit required to make it work - manage the scrollbars so they represent the view size.

    protected void layoutChildren() {

        ... other above ...

        vbar.setMax(map.getRows() * 32);
        hbar.setMax(map.getCols() * 32);

It's only an investigative bit of prototype code so it doesn't handle layers, but obviously the same is just repeated for each tile or whatever other layer is required.

It's NOT! magic ...

And I gotta say, this whole thing is a hell of a lot simpler to manage than any other virtually scrollable mechanism I've seen. General purpose virtually scrollable containers always seem to get bogged down in how to report the size to the parent container and other (unnecessarily) messy details with scrolling and handle sizes and so on. A complete implementation would require more complication from selection support and so on, but really each bit is as simple as it should be.

One thing I do like about JavaFX is that in general (and so far ...) it doesn't need to rely on any weird 'magic' and hidden knowledge for shit to work. The scenegraph is just a plain old data structure you can modify at whim without having to worry too much about internal details - the only limitation is any modifications to a live graph needs to be on a specific thread (which is trivially MT-enabled using message passing). I've you've ever worked with writing custom gadgets for any other toolkit you're always faced with some weird behaviour that takes a lot of knowledge to work with, and unless you wrote the toolkit you probably will never grok.

Although having said that ...

What I didn't understand is that simply including a ScrollBar inside the view causes requestLayout() to be invoked every time the handle moves. I'm no sure if this is a feature (some 'hidden' magic) or a bug. Well if it is at least it's a fairly sane bit of magic. The visibleAmount stuff above also doesn't really work properly either - as listed it allows the scrollbar to scroll to exactly one page beyond the limit in each direction. If i tried adjusting the Max by the viewport size ... weird shit happened. Such as creating much too big handle which didn't represent the viewable area properly. Not sure on that, but it was late and I was tired and hungry so it might have been a simple arithmetic error or something.

I suspect just using a WritableImage and rendering via a Canvas would be more efficient ... but then you lose all the animation stuff which could come in handy. The approach above will not work well for a wide zoom either as you may end up needing to create an ImageView for every tile anyway which will be super-slow and run out of memory. So to support a very wide zoom you'd be forced to implement a Canvas version. i.e. again something the scene-graph should handle.

I'm still struggling a bit with general layout issues in JavaFX - when and when not to use a Group, how things align and resizing to fit. That's something I just need a lot more time with I guess. The tech demo I wrote about in yesterday's post was one of the first 'real' applications I've created for my customer that uses JavaFX so I will be getting more exposure to it. Even with that I had a hard time getting the Stage to resize once the content changed (based on 'opening a file') - actually I couldn't so I gave up.

Update: Well I worked out the ScrollBar stuff.

When you set VisibleAmount all it does is change the size of the handle - it doesn't change the reported range which still goes from Min to Max. So one has to manually scale the result and take account of the visible amount yourself.

e.g. something like this, which scales the maximum from 0-(max-visible) linearly, where Max was set to the total information width, and VisibleAmount was set to the width, in pixels.

    double getOriginX() {
        return hbar.getValue() * (hbar.getMax() - getWidth()) / hbar.getMax();

TBH it's a bit annoying, and I can't really see a reason one would ever not need to do this when using a ScrollBar as a scroll bar (vs a slider).

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