Android：在黑色背景上绘制位图数组列表

Question

it's my first time asking a question here so I'll try to stay on-topic. I'm trying to randomly generate a background by creating an appropriately-sized ArrayList of Bitmap objects, and drawing them in order. This implementation works fine loading a single Bitmap, by the way; it's just stumbling with a list.

Before I get to the code, I'd like to point out that Ideally I would make a single Bitmap by adding the individual pixels or tiles, and indeed have tried a few variations of that, but they all result in black screens; I'm starting to think it might be a problem with how I draw to the Canvas. Anyways, here's what I have:

First, I generate the random ArrayList, only using 3 colors right now. I'd make it return the list, but it's just a private method inside the thread referencing one of the thread's variables so it doesn't matter much.

    private void genMap(Resources res)
    {
        // Load basic tiles.
        Bitmap green = BitmapFactory.decodeResource(res, R.drawable.green);
        Bitmap red = BitmapFactory.decodeResource(res, R.drawable.red);
        Bitmap blue = BitmapFactory.decodeResource(res, R.drawable.blue);

                    // All tiles must be the same size.
        int tile_width = green.getWidth();
        int tile_height = green.getHeight();
        int num_x = mCanvasWidth / tile_width;
        int num_y = mCanvasHeight / tile_height;

        for (int j = 0; j < num_y; j++)
        {
            for (int i = 0; i < num_x; i++)
            {
                double r = Math.random();
                Bitmap tile;
                if (r <= 1/3) {tile = green;}
                else if (r <= 2/3) {tile = red;}
                else {tile = blue;}
                // Create a new Bitmap in order to avoid referencing the old value.
                mBackgroundImages.add(Bitmap.createBitmap(tile));
            }
        }
    }

So, that's how the random values are mapped to a pattern. The method is called in the thread's constructor, which is in turn called every time onCreate is called; for now, I'm just clearing the list and making a new random pattern each time:

...
Resources res = context.getResources();
mBackgroundImages = new ArrayList<Bitmap>();
genMap(res);
...

And finally, the draw method; it works fine loading a single Bitmap via BitmapFactory.decodeResources, but shows a black screen when doing this:

    private void doDraw(Canvas canvas)
    {
        /* Draw the bg.
         * Remember, Canvas objects accumulate.
         * So drawn first = most in the background. */
        if (canvas != null)
        {
            if (mBackgroundImages.size() > 0)
            {
                int tile_width = mBackgroundImages.get(0).getWidth();
                int tile_height = mBackgroundImages.get(0).getHeight();
                for (int y = 0; y < mCanvasHeight / tile_height; y++)
                {
                    for(int x = 0; x < mCanvasWidth / tile_width; x++)
                    {
                         // Draw the Bitmap at the correct position in the list; Y * XWIDTH + X, at pos X * XWIDTH, Y * YWIDTH.
                         canvas.drawBitmap(mBackgroundImages.get((x + y * (mCanvasWidth/tile_width))), x * tile_width, y * tile_height, null);
                    }
                }
            }
        }
    }

Any help would be appreciated, thanks.

Answer 1

Have you double-checked the order of precedence of the following line?

((x + y * (mCanvasWidth/tile_width)))

Interesting question though: are you drawing solid colors? I would also see if this is actually making a Bitmap:

mBackgroundImages.add(Bitmap.createBitmap(tile));

You might actually be able to just keep references instead of creating a lot of bitmaps, but that can come later