Why does BitmapFactory.decodeResource scale my image?

问题

I have an image that is 120x120px located in my /res/drawable/ directory. This is the size required for all devices.

To load this Bitmap, I am using the following:

Bitmap tmpBmp = BitmapFactory.decodeResource(getResources(), R.drawable.myimage);

The issue is that when I measure tmpBmp, the size is 360x360px. I know how to scale this back down to the 120x120 that I need, but it is a waste of processing time, although minimal.

I am assuming that this has something to do with screen densities, since my resource is in a density-less directory.

What is the reason for decodeResource scaling my image? And how do I stop it from doing so?

回答1:

The default drawable directory assumes that images need to be scaled from the default mdpi size. Put your images in drawable-nodpi if you want to disable resource scaling.

Do note that a 120x120px image, if displayed on the screen, will be 3x smaller on a xxhdpi device compared to a mdpi device (as there are three times as many pixels per inch).

回答2:

That's because the density of your screen and your image are different. Then if you do not specify Options system will do it for you. At the source of BitmapFactory you could see this:

public static Bitmap decodeResourceStream(Resources res, TypedValue value, InputStream is, Rect pad, Options opts) {

    if (opts == null) {
        opts = new Options();
    }

    if (opts.inDensity == 0 && value != null) {
        final int density = value.density;
        if (density == TypedValue.DENSITY_DEFAULT) {
            opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
        } else if (density != TypedValue.DENSITY_NONE) {
            opts.inDensity = density;
        }
    }

    if (opts.inTargetDensity == 0 && res != null) {
        opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
    }

    return decodeStream(is, pad, opts);
}

Therefore to prevent scaling you need to specify Options param with inScaled=false param. Or put your image to the res/drawable-nodpi folder.

来源：https://stackoverflow.com/questions/23047748/why-does-bitmapfactory-decoderesource-scale-my-image