Drawing an image using sub-pixel level accuracy using Graphics2D

Question

I am currently attempting to draw images on the screen at a regular rate like in a video game.

Unfortunately, because of the rate at which the image is moving, some frames are identical because the image has not yet moved a full pixel.

Is there a way to provide float values to Graphics2D for on-screen position to draw the image, rather than int values?

Initially here is what I had done:

BufferedImage srcImage = sprite.getImage ( );
Position imagePosition = ... ; //Defined elsewhere
g.drawImage ( srcImage, (int) imagePosition.getX(), (int) imagePosition.getY() );

This of course thresholds, so the picture doesn't move between pixels, but skips from one to the next.

The next method was to set the paint color to a texture instead and draw at a specified position. Unfortunately, this produced incorrect results that showed tiling rather than correct antialiasing.

g.setRenderingHint ( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );

BufferedImage srcImage = sprite.getImage ( );

g.setPaint ( new TexturePaint ( srcImage, new Rectangle2D.Float ( 0, 0, srcImage.getWidth ( ), srcImage.getHeight ( ) ) ) );

AffineTransform xform = new AffineTransform ( );

xform.setToIdentity ( );
xform.translate ( onScreenPos.getX ( ), onScreenPos.getY ( ) );
g.transform ( xform );

g.fillRect(0, 0, srcImage.getWidth(), srcImage.getHeight());

What should I do to achieve the desired effect of subpixel rendering of an Image in Java?

Answer 1

You can use a BufferedImage and AffineTransform, draw to the buffered image, then draw the buffered image to the component in the paint event.

    /* overrides the paint method */
    @Override
    public void paint(Graphics g) {
        /* clear scene buffer */
        g2d.clearRect(0, 0, (int)width, (int)height);

        /* draw ball image to the memory image with transformed x/y double values */
        AffineTransform t = new AffineTransform();
        t.translate(ball.x, ball.y); // x/y set here, ball.x/y = double, ie: 10.33
        t.scale(1, 1); // scale = 1 
        g2d.drawImage(image, t, null);

        // draw the scene (double percision image) to the ui component
        g.drawImage(scene, 0, 0, this);
    }

Check my full example here: http://pastebin.com/hSAkYWqM

Answer 2

You can composite the image yourself using sub-pixel accuracy, but it's more work on your part. Simple bilinear interpolation should work well enough for a game. Below is psuedo-C++ code for doing it.

Normally, to draw a sprite at location (a,b), you'd do something like this:

for (x = a; x < a + sprite.width; x++)
{
    for (y = b; y < b + sprite.height; y++)
    {
        *dstPixel = alphaBlend (*dstPixel, *spritePixel);
        dstPixel++;
        spritePixel++;
    }
    dstPixel += destLineDiff; // Move to start of next destination line
    spritePixel += spriteLineDiff; // Move to start of next sprite line
}

To do sub-pixel rendering, you do the same loop, but account for the sub-pixel offset like so:

float xOffset = a - floor (a);
float yOffset = b - floor (b);
for (x = floor(a), spriteX = 0; x < floor(a) + sprite.width + 1; x++, spriteX++)
{
    for (y = floor(b), spriteY = 0; y < floor (b) + sprite.height + 1; y++, spriteY++)
    {
        spriteInterp = bilinearInterp (sprite, spriteX + xOffset, spriteY + yOffset);
        *dstPixel = alphaBlend (*dstPixel, spriteInterp);
        dstPixel++;
        spritePixel++;
    }
    dstPixel += destLineDiff; // Move to start of next destination line
    spritePixel += spriteLineDiff; // Move to start of next sprite line
}

The bilinearInterp() function would look something like this:

Pixel bilinearInterp (Sprite* sprite, float x, float y)
{
    // Interpolate the upper row of pixels
    Pixel* topPtr = sprite->dataPtr + ((floor (y) + 1) * sprite->rowBytes) + floor(x) * sizeof (Pixel);
    Pixel* bottomPtr = sprite->dataPtr + (floor (y) * sprite->rowBytes) + floor (x) * sizeof (Pixel);

    float xOffset = x - floor (x);
    float yOffset = y - floor (y);

    Pixel top = *topPtr + ((*(topPtr + 1) - *topPtr) * xOffset;
    Pixel bottom = *bottomPtr + ((*(bottomPtr + 1) - *bottomPtr) * xOffset;
    return bottom + (top - bottom) * yOffset;
}

This should use no additional memory, but will take additional time to render.

Answer 3

I successfully solved my problem after doing something like lawrencealan proposed.

Originally, I had the following code, where g is transformed to a 16:9 coordinate system before the method is called:

private void drawStar(Graphics2D g, Star s) {

    double radius = s.getRadius();
    double x = s.getX() - radius;
    double y = s.getY() - radius;
    double width = radius*2;
    double height = radius*2;

    try {

        BufferedImage image = ImageIO.read(this.getClass().getResource("/images/star.png"));
        g.drawImage(image, (int)x, (int)y, (int)width, (int)height, this);

    } catch (IOException ex) {
        Logger.getLogger(View.class.getName()).log(Level.SEVERE, null, ex);
    }

}

However, as noted by the questioner Kaushik Shankar, turning the double positions into integers makes the image "jump" around, and turning the double dimensions into integers makes it scale "jumpy" (why the hell does g.drawImage not accept doubles?!). What I found working for me was the following:

private void drawStar(Graphics2D g, Star s) {

    AffineTransform originalTransform = g.getTransform();

    double radius = s.getRadius();
    double x = s.getX() - radius;
    double y = s.getY() - radius;
    double width = radius*2;
    double height = radius*2;

    try {

        BufferedImage image = ImageIO.read(this.getClass().getResource("/images/star.png"));

        g.translate(x, y);
        g.scale(width/image.getWidth(), height/image.getHeight());

        g.drawImage(image, 0, 0, this);

    } catch (IOException ex) {
        Logger.getLogger(View.class.getName()).log(Level.SEVERE, null, ex);
    }

    g.setTransform(originalTransform);

}

Seems like a stupid way of doing it though.