Mipmap drawables for icons

问题

Since Android 4.3 (Jelly Bean) we can now make use of the res/mipmap folders to store \"mipmap\" images.

For example, Chrome for Android stores its icons in these folders instead of the more normal res/drawable folders.

How are these mipmap images different from the other familiar drawable images?

I see that in my manifest, we use the @mipmap/ qualifier, instead of @drawable/, which makes sense given the resource folder name:

<activity
    android:name=\".MipmapDemp\"
    android:icon=\"@mipmap/ic_launcher\" />

References:

The Android 4.3 APIs document has the following to say:

Using a mipmap as the source for your bitmap or drawable is a simple way to provide a quality image and various image scales, which can be particularly useful if you expect your image to be scaled during an animation.

Android 4.2 (API level 17) added support for mipmaps in the Bitmap class—Android swaps the mip images in your Bitmap when you\'ve supplied a mipmap source and have enabled setHasMipMap(). Now in Android 4.3, you can enable mipmaps for a BitmapDrawable object as well, by providing a mipmap asset and setting the android:mipMap attribute in a bitmap resource file or by calling hasMipMap().

I don\'t see anything in there that helps me to understand.

XML Bitmap resources have an android:mipMap property:

Boolean. Enables or disables the mipmap hint. See setHasMipMap() for more information. Default value is false.

This does not apply to launcher icons as far as I can see.

The question was raised on Google Groups (The purpose of resource name \"mipmap\"?!), to which Romain Guy replied:

It\'s useful to provide an image at a larger resolution that would normally be computed (for instance, on an mdpi device, Launcher might want the larger hdpi icon to display large app shortcuts.)

I feel like this almost makes sense of it, but not quite.

I\'m still inclined to go with Randy Sugianto\'s follow up:

What are the advantages of this? Is there any guide how to use mipmaps, probably for better launcher icons?

Of course, Wikipedia has a page for \"Mipmap\", which refers to an older technique invented in 1983, that I can\'t quite relate to the current Android implementation.

Should we be storing all our app icons in res/mipmap folders these days, and what are the guidelines for these mipmap images?

Update #1

Here\'s a blog post that tries to explain it a bit.

Mipmapping for drawables in Android 4.3

But the image used in that blog post shows what looks like one file with many logos in it. This is not what I see in Chrome\'s mipmap folder.

Chrome\'s mipmap-hdpi folder contains three images. One is the Chrome logo, on its own.

$\"Chrome$

Strangely, it is 72x72, not 48x48 which I would expect to see.

Perhaps that is all there is to this - we just need to keep bigger icons in the mipmap folders?

Update #2

The Android Developers Blog post of 23/10/2014 again confirms the idea of using the mipmap folders for application icons:

Getting Your Apps Ready for Nexus 6 and Nexus 9

When talking about the Nexus 6 screen density, the author writes:

It’s best practice to place your app icons in mipmap- folders (not the drawable- folders) because they are used at resolutions different from the device’s current density. For example, an xxxhdpi app icon can be used on the launcher for an xxhdpi device.

Update #3

Note that Android Studio creates the ic_launcher.png icons in the mipmap... folders rather than the drawable... folders that Eclipse used to create them in.

回答1:

There are two distinct uses of mipmaps:

For launcher icons when building density specific APKs. Some developers build separate APKs for every density, to keep the APK size down. However some launchers (shipped with some devices, or available on the Play Store) use larger icon sizes than the standard 48dp. Launchers use getDrawableForDensity and scale down if needed, rather than up, so the icons are high quality. For example on an hdpi tablet the launcher might load the xhdpi icon. By placing your launcher icon in the mipmap-xhdpi directory, it will not be stripped the way a drawable-xhdpi directory is when building an APK for hdpi devices. If you're building a single APK for all devices, then this doesn't really matter as the launcher can access the drawable resources for the desired density.
The actual mipmap API from 4.3. I haven't used this and am not familiar with it. It's not used by the Android Open Source Project launchers and I'm not aware of any other launcher using.

回答2:

It seems Google have updated their docs since all these answers, so hopefully this will help someone else in future :) Just came across this question myself, while creating a new (new new) project.

TL;DR: drawables may be stripped out as part of dp-specific resource optimisation. Mipmaps will not be stripped.

Different home screen launcher apps on different devices show app launcher icons at various resolutions. When app resource optimization techniques remove resources for unused screen densities, launcher icons can wind up looking fuzzy because the launcher app has to upscale a lower-resolution icon for display. To avoid these display issues, apps should use the mipmap/ resource folders for launcher icons. The Android system preserves these resources regardless of density stripping, and ensures that launcher apps can pick icons with the best resolution for display.

(from http://developer.android.com/tools/projects/index.html#mipmap)

回答3:

How are these mipmap images different from the other familiar drawable images?

Here is my two cents in trying to explain the difference. There are two cases you deal with when working with images in Android:

You want to load an image for your device density and you are going to use it "as is", without changing its actual size. In this case you should work with drawables and Android will give you the best fitting image.
You want to load an image for your device density, but this image is going to be scaled up or down. For instance this is needed when you want to show a bigger launcher icon, or you have an animation, which increases image's size. In such cases, to ensure best image quality, you should put your image into mipmap folder. What Android will do is, it will try to pick up the image from a higher density bucket instead of scaling it up. This will increase sharpness (quality) of the image.

Thus, the rule of thumb to decide where to put your image into would be:

Launcher icons always go into mipmap folder.
Images, which are often scaled up (or extremely scaled down) and whose quality is critical for the app, go into mipmap folder as well.
All other images are usual drawables.

回答4:

The Android implementation of mipmaps in 4.3 is exactly the technique from 1983 explained in the Wikipedia article :)

Each bitmap image of the mipmap set is a downsized duplicate of the main texture, but at a certain reduced level of detail. Although the main texture would still be used when the view is sufficient to render it in full detail, the renderer will switch to a suitable mipmap image (...) when the texture is viewed from a distance or at a small size.

Although this is described as a technique for 3D graphics (as it mentions "viewing from a distance"), it applies just as well to 2D (translated as "drawn is a smaller space", i.e. "downscaled").

For a concrete Android example, imagine you have a View with a certain background drawable (in particular, a BitmapDrawable). You now use an animation to scale it to 0.15 of its original size. Normally, this would require downscaling the background bitmap for each frame. This "extreme" downscaling, however, may produce visual artifacts.

You can, however, provide a mipmap, which means that the image is already pre-rendered for a few specific scales (let's say 1.0, 0.5, and 0.25). Whenever the animation "crosses" the 0.5 threshold, instead of continuing to downscale the original, 1.0-sized image, it will switch to the 0.5 image and downscale it, which should provide a better result. And so forth as the animation continues.

This is a bit theoretical, since it's actually done by the renderer. According to the source of the Bitmap class, it's just a hint, and the renderer may or may not honor it.

/**
 * Set a hint for the renderer responsible for drawing this bitmap
 * indicating that it should attempt to use mipmaps when this bitmap
 * is drawn scaled down.
 *
 * If you know that you are going to draw this bitmap at less than
 * 50% of its original size, you may be able to obtain a higher
 * quality by turning this property on.
 * 
 * Note that if the renderer respects this hint it might have to
 * allocate extra memory to hold the mipmap levels for this bitmap.
 *
 * This property is only a suggestion that can be ignored by the
 * renderer. It is not guaranteed to have any effect.
 *
 * @param hasMipMap indicates whether the renderer should attempt
 *                  to use mipmaps
 *
 * @see #hasMipMap()
 */
public final void setHasMipMap(boolean hasMipMap) {
    nativeSetHasMipMap(mNativeBitmap, hasMipMap);
}

I'm not quite sure why this would be especially suitable for application icons, though. Although Android on tablets, as well as some launchers (e.g. GEL), request an icon "one density higher" to show it bigger, this is supposed to be done using the regular mechanism (i.e. drawable-xxxhdpi, &c).

回答5:

One thing I mentioned in another thread that is worth pointing out -- if you are building different versions of your app for different densities, you should know about the "mipmap" resource directory. This is exactly like "drawable" resources, except it does not participate in density stripping when creating the different apk targets.

https://plus.google.com/105051985738280261832/posts/QTA9McYan1L

回答6:

Since I was looking for an clarifying answer to this to determine the right type for notification icons, I'd like to add this clear statement to the topic. It's from http://developer.android.com/tools/help/image-asset-studio.html#saving

Note: Launcher icon files reside in a different location from that of other icons. They are located in the mipmap/ folder. All other icon files reside in the drawable/ folder of your project.

回答7:

There are two cases you deal with when working with images in Android:

You want to load an image for your device density and you are going to use it “as is”, without changing its actual size. In this case you should work with drawables and Android will give you the best fitting image.

You want to load an image for your device density, but this image is going to be scaled up or down. For instance this is needed when you want to show a bigger launcher icon, or you have an animation, which increases image’s size. In such cases, to ensure best image quality, you should put your image into mipmap folder. What Android will do is, it will try to pick up the image from a higher density bucket instead of scaling it up.

SO

Thus, the rule of thumb to decide where to put your image into would be:

Launcher icons always go into mipmap folder.

Images, which are often scaled up (or extremely scaled down) and whose quality is critical for the app, go into mipmap folder as well.

All other images are usual drawables.

Citation from this article.

回答8:

res/
mipmap-mdpi/ic_launcher.png (48x48 pixels)
mipmap-hdpi/ic_launcher.png (72x72)
mipmap-xhdpi/ic_launcher.png (96x96)
mipmap-xxhdpi/ic_launcher.png (144x144)
mipmap-xxxhdpi/ic_launcher.png (192x192)

MipMap for app icon for launcher

http://android-developers.blogspot.co.uk/2014/10/getting-your-apps-ready-for-nexus-6-and.html

https://androidbycode.wordpress.com/2015/02/14/goodbye-launcher-drawables-hello-mipmaps/

回答9:

If you build an APK for a target screen resolution like HDPI, the Android asset packageing tool,AAPT,can strip out the drawables for other resolution you don’t need.But if it’s in the mipmap folder,then these assets will stay in the APK, regardless of the target resolution.

回答10:

When building separate apks for different densities, drawable folders for other densities get stripped. This will make the icons appear blurry in devices that use launcher icons of higher density. Since, mipmap folders do not get stripped, it’s always best to use them for including the launcher icons.

回答11:

The understanding I have about mipmap is more or less like this:

When an image needs to be drawn, given we have different screen sizes are resolutions, some scaling will have to take part.

If you have an image that is ok for a low end cell phone, when you scale it to the size of a 10" tablet you have to "invent" pixels that don't actually exist. This is done with some interpolation algorithm. The more amount of pixels that have to be invented, the longer the process takes and quality starts to fail. Best quality is obtained with more complex algorithms that take longer (average of surrounding pixles vs copy the nearest pixel for example).

To reduce the number of pixels that have to be invented, with mipmap you provide different sizes/rsolutions of the same image, and the system will choose the nearest image to the resolution that has to be rendered and do the scaling from there. This should reduce the number of invented pixels saving resources to be used in calculating these pixels to provide a good quality image.

I read about this in an article explaining a performance problem in libgdx when scaling images:

http://www.badlogicgames.com/wordpress/?p=1403

来源：https://stackoverflow.com/questions/23935810/mipmap-drawables-for-icons

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android

android-drawable

mipmaps