minicap

Minicap provides a socket interface for streaming realtime screen capture data out of Android devices. It is meant to be used as a component in a larger program and is therefore not immensely useful just by itself. For example, it is being used in STF for remote control.

Minicap works without root if started via ADB on SDK 28 (Android 9.0) and lower. The lowest SDK level we build for is 9 (i.e. Android 2.3). Minicap also works on Android Wear. Developer previews are, in general, supported once Google releases the source code for that release. Only the latest Developer Preview is supported, and only until there's a stable release. Emulators are not supported. Note that Android 3.x is not supported since those versions were never open sourced.

To capture the screen we currently use two methods. For older Android versions we use the ScreenshotClient, a private API in AOSP. For newer versions we use a virtual display, which also requires access to private APIs. The frames are then encoded using SIMD-enabled libjpeg-turbo and sent over a socket interface. A planned future improvement to allow for even higher FPS is to use MediaRecorder and friends to take advantage of hardware encoding.

In principle, every device should work. However, since minicap relies on private APIs, some may not. Please let us know by creating a GitHub issue about that device.

The project consists of two parts. There's the main binary that can be built using NDK alone. The other part is a shared library that's built for each SDK level and each architecture inside the AOSP source tree. We ship precompiled libraries in this repo, but any modifications to the code used by these shared libraries require a recompile against the corresponding AOSP branches. This can be a major pain, but we have several utilities to help with the ordeal. If you're interested in that, read the build instructions here.

Features

• Usable to very smooth FPS depending on device. Older, weaker devices running an old version of Android can reach 10-20 FPS. Newer devices running recent versions of Android can usually reach 30-40 FPS fairly easily, but there are some exceptions. For maximum FPS we recommend running minicap at half the real vertical and horizontal resolution.
• Decent and usable but non-zero latency. Depending on encoding performance and USB transfer speed it may be one to a few frames behind the physical screen.
• On Android 4.2+, frames are only sent when something changes on the screen. On older versions frames are sent as a constant stream, whether there are changes or not.
• Easy socket interface.

Requirements

• NDK, Revision 10e (May 2015)
• make

Building

We include libjpeg-turbo as a Git submodule, so first make sure you've fetched it.

git submodule init
git submodule update

You're now ready to proceed.

Building requires NDK, and is known to work with at least with NDK Revision 10e (May 2015). Older versions do not work due to the lack of .asm file support for x86_64. Important note: NDK 15 no longer supports Android platforms earlier than SDK level 14. This means that if compiled on NDK 15 or later, the binaries may or may not work on earlier platforms (e.g. Android 2.3, Android 2.3.3).

Then it's simply a matter of invoking ndk-build.

ndk-build

You should now have the binaries available in ./libs.

If you've modified the shared library, you'll also need to build that.

Running

You'll need to build first.

The easy way

You can then use the included run.sh script to run the right binary on your device. It will make sure the correct binary and shared library get copied to your device. If you have multiple devices connected, set ANDROID_SERIAL before running the script.

# Run a preliminary check to see whether your device will work
./run.sh autosize -t
# Check help
./run.sh autosize -h
# Start minicap
./run.sh autosize

The autosize command is for selecting the correct screen size automatically. This is done by the script instead of the binary itself. To understand why this is necessary, read the manual instructions below.

Finally we simply need to create a local forward so that we can connect to the socket.

adb forward tcp:1313 localabstract:minicap

Now you'll be able to connect to the socket locally on port 1313.

Then just see usage below.

The hard way

To run manually, you have to first figure out which ABI your device supports:

ABI=$(adb shell getprop ro.product.cpu.abi | tr -d '\r')

Note that as Android shell always ends lines with CRLF, you'll have to remove the CR like above or the rest of the commands will not work properly.

Also note that if you've got multiple devices connected, setting ANDROID_SERIAL will make things quite a bit easier as you won't have to specify the -s <serial> option every time.

Now, push the appropriate binary to the device:

adb push libs/$ABI/minicap /data/local/tmp/

Note that for SDK <16, you will have to use the minicap-nopie executable which comes without PIE support. Check run.sh for a scripting example.

The binary enough is not enough. We'll also need to select and push the correct shared library to the device. This can be done as follows.

SDK=$(adb shell getprop ro.build.version.sdk | tr -d '\r')
adb push jni/minicap-shared/aosp/libs/android-$SDK/$ABI/minicap.so /data/local/tmp/

At this point it might be useful to check the usage:

adb shell LD_LIBRARY_PATH=/data/local/tmp /data/local/tmp/minicap -h

Note that you'll need to set LD_LIBRARY_PATH every time you call minicap or it won't find the shared library.

Also, you'll need to specify the size of the display and the projection every time you use minicap. This is because the private APIs we would have to use to access that information segfault on many Samsung devices (whereas minicap itself runs fine). The run.sh helper script provides the autosize helper as mentioned above.

So, let's assume that your device has a 1080x1920 screen. First, let's run a quick check to see if your device is able to run the current version of minicap:

adb shell LD_LIBRARY_PATH=/data/local/tmp /data/local/tmp/minicap -P 1080x1920@1080x1920/0 -t

The format of the -P argument is: {RealWidth}x{RealHeight}@{VirtualWidth}x{VirtualHeight}/{Orientation}. The "virtual" size is the size of the desired projection. The orientation argument tells minicap what the current orientation of the device is (in degrees), which is required so that we can report the correct orientation over the socket interface to the frame consumer. One way to get the current orientation (or rotation) is RotationWatcher.apk.

If the command outputs "OK", then everything should be fine. If instead it segfaults (possibly after hanging for a while), your device is not supported and we'd like to know about it.

Finally, let's start minicap. It will start listening on an abstract unix domain socket.

adb shell LD_LIBRARY_PATH=/data/local/tmp /data/local/tmp/minicap -P 1080x1920@1080x1920/0

Now we simply need to create a local forward so that we can connect to the socket.

adb forward tcp:1313 localabstract:minicap

Now you can connect to the socket using the local port. Note that currently only one connection at a time is supported. It doesn't really make sense to have more than one connection anyway, as the USB bus would get saturated very quickly. So, let's connect.

nc localhost 1313

This will give you binary output that will be explained in the next section.

Usage

It is assumed that you now have an open connection to the minicap socket. If not, follow the instructions above.

The minicap protocol is a simple push-based binary protocol. When you first connect to the socket, you get a global header followed by the first frame. The global header will not appear again. More frames keep getting sent until you stop minicap.

Global header binary format

Appears once.

| Bytes | Length | Type | Explanation | |-------|--------|------|-------------| | 0 | 1 | unsigned char | Version (currently 1) | | 1 | 1 | unsigned char | Size of the header (from byte 0) | | 2-5 | 4 | uint32 (low endian) | Pid of the process | | 6-9 | 4 | uint32 (low endian) | Real display width in pixels | | 10-13 | 4 | uint32 (low endian) | Real display height in pixels | | 14-17 | 4 | uint32 (low endian) | Virtual display width in pixels | | 18-21 | 4 | uint32 (low endian) | Virtual display height in pixels | | 22 | 1 | unsigned char | Display orientation | | 23 | 1 | unsigned char | Quirk bitflags (see below) |

Quirk bitflags

Currently, the following quirks may be reported:

| Value | Name | Explanation | |-------|------|-------------| | 1 | QUIRK_DUMB | Frames will get sent even if there are no changes from the previous frame. Informative, doesn't require any actions on your part. You can limit the capture rate by reading frame data slower in your own code if you wish. | | 2 | QUIRK_ALWAYS_UPRIGHT | The frame will always be in upright orientation regardless of the device orientation. This needs to be taken into account when rendering the image. | | 4 | QUIRK_TEAR | Frame tear might be visible. Informative, no action required. Neither of our current two methods exhibit this behavior. |

Frame binary format

Appears a potentially unlimited number of times.

| Bytes | Length | Type | Explanation | |-------|--------|------|-------------| | 0-3 | 4 | uint32 (low endian) | Frame size in bytes (=n) | | 4-(n+4) | n | unsigned char[] | Frame in JPG format |

Debugging

You can use gdb to debug more complex issues. It is assumed that you already know how to use it. Here's how to get it running.

First, if you haven't already, make sure to build your project with the NDK_DEBUG option enabled, as follows.

ndk-build NDK_DEBUG=1

This will create gdb.setup and the gdb binaries themselves.

If you don't have NDK_ROOT set up yet, do it now like below. You may have to change the path, the sample path below works on OS X with Android Studio's NDK bundle (which you'll have to install as well).

export NDK_ROOT="$HOME/Library/Android/sdk/ndk-bundle"

Next, be sure that minicap is actually running. Just running ./run.sh autosize is enough.

Finally, you should be able to just run ./ndk.sh in a separate shell (you'll have to keep minicap running at the same time). It will pull all necessary libraries from the device to ease debugging, and you should end up at a functioning gdb shell. The ndk.sh script has been adapted from ndk-gdb for stand-alone binaries.

Improvements are welcome.

Contributing

As a small disclaimer, minicap was the first time the author used C++, so even any non-functional changes to make the code more idiomatic (preferably without introducing new dependencies) are also very welcome, in addition to bug fixes and new features.

See CONTRIBUTING.md.

License

See LICENSE.

Copyright © The OpenSTF Project. All Rights Reserved.