Canvas.drawText() doesn't render large emojis on Android

Question

Canvas.drawText() doesn't render emojis above a certain font size on Android.

Correct render at somewhere below 256 px:

Incorrect render at above 256 px:

(There is a similar question about Google Chrome, and just like Android, Chrome also uses the Skia graphics library, so this seems like a bug in Skia.)

Apparently emojis fail to render above 256 px font size on my devices. But I'm not sure if this is the limit everywhere.

Is there a way to learn the font size at which emojis disappear? Or is there a workaround for this?

Answer 1

I've come up with a test (an empirical estimation) for the maximum font size at which emojis can still be rendered.

The way this function works is it creates a 1x1 bitmap, and tries to draw the Earth Globe emoji (🌍) in the center of that. Then it checks that single pixel, whether it's still transparent, or colored.

I've chosen the Earth Globe emoji because I assume we can be fairly certain that no artist would ever draw the Earth with a hole in the middle. (Or we're in huge trouble anyway.)

The tests are done in a binary search - fashion, so the runtime should be logarithmic.

(Fun fact: the max font size on both my test phones turned out to be 256.)

public static float getMaxEmojiFontSize() {
    return getMaxEmojiFontSize(new Paint(), 8, 999999, 1);
}

/**
 * Emojis cannot be renderered above a certain font size due to a bug.
 * This function tries to estimate what the maximum font size is where emojis can still
 * be rendered.
 * @param   p           A Paint object to do the testing with.
 *                      A simple `new Paint()` should do.
 * @param   minTestSize From what size should we test if the emojis can be rendered.
 *                      We're assuming that at this size, emojis can be rendered.
 *                      A good value for this is 8.
 * @param   maxTestSize Until what size should we test if the emojis can be rendered.
 *                      This can be the max font size you're planning to use.
 * @param   maxError    How close should we be to the actual number with our estimation.
 *                      For example, if this is 10, and the result from this function is
 *                      240, then the maximum font size that still renders is guaranteed
 *                      to be under 250. (If maxTestSize is above 250.)
 */
public static float getMaxEmojiFontSize(Paint p, float minTestSize, float maxTestSize, float maxError) {
    Bitmap b = Bitmap.createBitmap(1, 1, Bitmap.Config.ARGB_8888);
    float sizeLowerLimit = minTestSize; // Start testing from this size
    float sizeUpperLimit = maxTestSize;
    Canvas c = new Canvas(b);
    float size;
    for (size = sizeLowerLimit; size < maxTestSize; size *= 2) {
        if (!canRenderEmoji(b, c, p, size)) {
            sizeUpperLimit = size;
            break;
        }
        sizeLowerLimit = size;
    }
    // We now have a lower and upper limit for the maximum emoji size.
    // Let's proceed with a binary search.
    while (sizeUpperLimit - sizeLowerLimit > maxError) {
        float middleSize = (sizeUpperLimit + sizeLowerLimit) / 2f;
        if (!canRenderEmoji(b, c, p, middleSize)) {
            sizeUpperLimit = middleSize;
        } else {
            sizeLowerLimit = middleSize;
        }
    }
    return sizeLowerLimit;
}

private static boolean canRenderEmoji(Bitmap b, Canvas can, Paint p, float size) {
    final String EMOJI = "\uD83C\uDF0D"; // the Earth Globe (Europe, Africa) emoji - should never be transparent in the center.
    can.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR); // clear the canvas with transparent
    p.setTextSize(size);
    { // draw the emoji in the center
        float ascent = Math.abs(p.ascent());
        float descent = Math.abs(p.descent());
        float halfHeight = (ascent + descent) / 2.0f;
        p.setTextAlign(Paint.Align.CENTER);
        can.drawText(EMOJI, 0.5f, 0.5f + halfHeight - descent, p);
    }
    return b.getPixel(0, 0) != 0;
}