Category Archives: General

Sony Xperia Z2 Tablet with touch screen problems

Last week I decided to buy a tablet with Android. After looking a while I decided to go with the Sony Xperia Z2 Tablet, which in my opinion is an awesome tablet. It is powerful, light, high quality finish, even waterproof and incredibly thin. Being definitely in a high price range, this is a professional tablet, which cost me roughly 420 EUR.

Only after buying and using it at home for a while, I occasionally experienced erratic touch screen behaviour. Touches would be not recognized, sometimes swipe gestures would be detected as taps, multitouch gestures would stop and break in the middle and sometimes middle touches would only be detected, if pressed very hard. And even then the touches would not be detected reliably, resulting in random taps, performing undesired actions. This is usually with the tablet laying on a flat surface, quite often over night. I know that electric fields while charging a device quite often disturb the touch screen, but with the Sony Xperia Z2 this happened without the charger connected. I had the latest available software installed, the 17.1.1.A.0.402, which obviously did not fix the issue.

Turning the screen off and on usually fixed the situation temporarily, but that is not a solution as it would randomly happen again.

I downloaded a touchscreen test app and also found that the tablet has a very hard time recognizing touches, when two fingers are exactly in a horizontal or vertical line. It would simple drop both fingers. This is not acceptable multitouch behaviour, where one touch affects the other.

Confused I searched the Internet and found quite some people reporting the same issue, even a YouTube video demonstrating the problem, making me believe this is a systematic design flaw, not just a flawed device:

Sony Support recommended me to install the latest software (this was already the case) and then said, my device might need repair. This issue has apparently been around since several month and still Sony did not confirm the problem.

But I believe my device wasn’t actually broken but suffered from design flaws, I returned it for a refund. I am not willing to try different devices until I find one that actually works. Not with so many people complaining about the touch screen. Not for this price.

Sorry, Sony, you made the Xperia Z2 Tablet too thin and you have problems to get it to work properly. I’m sorry my first experience with a Sony mobile device had to be a negative one and with this issue not even publicly confirmed, I can’t recommend buying it. Too bad, it could have been such an awesome tablet. in google play

The client app for is available in Google Play. It is by far my most polished and stylish app, that I developed for the German start-up moosic. in Google Play in Google Play

With you can keep track of the music that’s running in your favourite location. Browse past playlists or see what’s playing right now, all together with previews and cover pictures.

In the backend, the moosic listener listens to music in the background and uploads the recognized songs as a playlist. Users can browse these playlists using either the nice web frontend or the native Android App.

Note that the technology is currently being tested and evaluated. Kodus to T O-12 in Stuttgart for being available for the beta test and to Gracenote for their music recognizing api. For questions, don’t hesitate to contact the guys at


Chicken Tournament for Android in development

After more than 10 years I decided to port Chicken Tournament to current Android smartphones. Due to the differences between the platforms, this will result in a new game and a new engine, but because of lack of time and resources, I will reuse the models and most of the textures. OpenGLES 2.0 though allows me to massively improve the quality of the graphics.

The PC version and the android version will not be compatible.

Please follow the official Chicken Tournament facebook page to receive more information and updates. A very early development version allows driving the harvester over a plain using the accelerometer to steer. The chicken are nicely animated using vertex shader.

Development screenshot of CT for Android
Development screenshot of CT on my S3

Freebloks-1.5 for PC available

While I have frequently improved and updated the network compatible Android version of Freebloks, I have always neglected the Windows version. The Android version gained features like player names and the ability to choose the colors to play with. While the Windows and Android version always stayed compatible, the PC version simply did not offer these features to connecting Android devices or when connecting to a dedicated server.

Now Freebloks-1.5 for Windows is available, which lets connecting Android devices use all features and offers the player to choose his colours as well as to choose a name (which is not displayed in the game but on connected Android devices). If you frequently play Freebloks over the Internet, it is recommended to download the new version here:

isatapd package for android

I created and published an android app that bundles my ISATAP client daemon and provides a configuration front end. To run the binary on android, it requires root access, i.e. a rooted device or phone.

Screenshot_2013-09-26-10-41-13On my Galaxy S3 with Cyanogenmod 10.2 I successfully get a IPv6 address using the app. 😀

If you have a rooted phone and an ISATAP infrastructure close by, please feel free to give it a try:

The app is open source, with code being available on GitHub:

The source tree also contains a pre-compiled isatapd binary to be used in your own distribution or scripts.

Freebloks VIP – donate version of Freebloks

Freebloks 3D for Android is free software, it’s open source, completely free of charge and you can modify it to any extent. Free software however does not mean it has no value. That’s why Freebloks comes with the feature of in-app donations to support the developers.

You can choose how much Freebloks is worth to you, you can donate any amount you like or you can purchase the new published version called Freebloks VIP, that is identical to the free version but paid to show the support to the developers. The only visual difference is the coin in the app icon indicating your support.

Please don’t take free software for granted.


In-App Donations in Freebloks 3D

Freebloks 3D for Android is the Android port of the PC version of Freebloks 3D for Windows and Linux. Like the PC version, the Android port is completely free software, available for free in the Google Play Store and the source code being available on GitHub.

I strongly believe in Open Source software and that it can help to make the world a better place by making knowledge and power available to everyone. While many hours of work went into the Android port, I feel good to completely open the software for others to study, to modify or contribute. A lot of my knowledge and skills come from the study of others work and my contribution to open source software is my attempt to give something back in return.

If you like Freebloks, please be encouraged to contribute, send be feedback, work on the code or support the developers with a donation. The recent update makes in-app donations available for users. These in-app items acknowledge the work of the developers and are completely voluntary. Freebloks will always be free but relies on your contribution!

So if you think, the game has some value for you, please consider a donation of your choice.


Dropshadows for Freebloks for Android

The latest update of Freebloks 3D for Android adds nice drop shadows to falling stones. Instead of “correct” shadows using shadow volumes in a stencil buffer, the android version renders a pseudo drop shadow texture on the board. The shadows are not always correct, but it is much easier to add individual tinting, alpha or scale effects, depending on the distance of the stones. This adds a more realistic look and is easy on the hardware, because there is no need to recalculate the shadow volume each frame.


Much improved AI speed for Freebloks for Android using jni

For the port of Freebloks 3D to Android I rewrote all code from C to Java. While that was working fine and resulted in greatly simplified networking code, the speed of the AI was not so great. It took up to 10 seconds on a fairly powerful SGS 2 for the computer to find a good move.

I was trying to move the CPU intense routines of the AI to C again, using jni as a bridge between Java and C. The simple network routines should stay in Java.

But the transfer of relevant game data to C and back to Java turned out to be very ugly, yet the solution was incredibly simple:

The Freebloks code was always split in two parts, the GUI/client part and the AI/server part, with the client and server always communicating using network sockets. Yes, even the single player version starts a network server and connects to localhost. The original source code always contained a package for running a dedicated server.

It was incredibly easy to copy the dedicated server code into my project, compile the C code with the NDK and connect it to Java with only a single jni call. It was running out of the box, with almost no change of the original C code at all! Since the server is running in a thread started from the native C code, there is no additional jni call neccessary and no data transfers except for the sockets.

The average duration for the AI to calculate a complete game dropped from 87 sec to 28 sec on my SGS 2. The version 0.0.2 in the Google Play Store supports ARMv5, ARMv7 and x86. Grab it now! You may also download a free apk file here.

And please don’t forget to give feedback.


USB write performance drop on Fritz!Box 7170

I want to attach a USB stick to the AVM Fritz!Box 7170 to use as USB storage and be able to write to it using the integrated ftp server. When writing a bunch of files, the write performance drops to under 50 kb/sec, while the stick can easily handle 512 kb/sec. Why the bad performance and why the drop?

I replaced the stock AVM firmware with Freetz but got similar results. What got my attention is a drop in performance after copying 4 files, that does not recover after time. The following tests were done using the Freetz modification with Linux kernel

Performance drop when writing

Look at these numbers when copying a bunch of files to the stick using scp:

$ scp tmp*
tmp1                          100% 2048KB 682.7KB/s   00:03    
tmp2                          100% 2048KB 512.0KB/s   00:04    
tmp3                          100% 2048KB 512.0KB/s   00:04    
tmp4                          100% 2048KB  55.4KB/s   00:37    
tmp5                          100% 2048KB  38.6KB/s   00:53

Each following transfer would then be at only 55KB/s. Issuing a sync command to flush out dirty buffers makes no difference, so the speed is not throttled by the USB stick being busy.

Let’s have a look at the VFS cache

The Linux kernel reveals some interesting cache and memory information in /proc/meminfo. These are numbers taken after a fresh boot:

# cat /proc/meminfo 
MemTotal:        30204 kB
MemFree:          9632 kB    # unused, completely free memory
Buffers:           280 kB
Cached:           6280 kB    # memory used for cached files
SwapCached:          0 kB
Active:           8652 kB
Inactive:         1524 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:        30204 kB
LowFree:          9632 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:               0 kB    # memory waiting to be written to disk
Writeback:          0 kB    #  memory actively being written to disk
Mapped:           8040 kB
Slab:             6028 kB
CommitLimit:     15100 kB
Committed_AS:     5724 kB
PageTables:        240 kB
VmallocTotal:  1048560 kB
VmallocUsed:      4056 kB
VmallocChunk:  1043636 kB

While copying the first files, the highlighted numbers read like this:

MemFree:          1716 kB
Cached:          13704 kB
Active:           8976 kB
Inactive:         8928 kB
Dirty:            6836 kB     # lots of data waiting to be written
Writeback:         444 kB    # lots of data being actively writting

We see that the cache is filled up quickly with buffers also marked to be written on the stick (marked dirty) and that the pdflush daemon already started to write out chunks of consecutive data to the usb stick. Remember that usb sticks have good performance when streaming out data chunks that fit into the physical structure but bad performance, when writing out small chunks because a lot of the flash memory keeps being reread and overwritten. The performance is good here, because there are a lot of dirty buffers the kernel can optimize the writing out.

Writing file ‘tmp1’

Let’s go back and look at the numbers exactly after tmp1 has been written (2048 kB):

MemFree:          7100 kB    # before: 9632 kB
Cached:           8456 kB    # before: 6280 kB
Dirty:               0 kB
Writeback:           0 kB

The buffers have all been flushed, so the stick is idle. Our cache grew by 2048 kB taken from the free memory, containing now also the file tmp1.

Writing file ‘tmp2’

Copying file tmp2 (2048 kB) is fast and the memory info after copying is no surprise:

MemFree:          5084 kB    # 2048 kB less
Cached:          10504 kB    # 2048 kB more
Dirty:               0 kB
Writeback:           0 kB

Neither is tmp3 (2048 kB), because there is still unused memory left. But now it’s getting interesting, because write performance with tmp4 drops drastically.

Writing file ‘tmp4’ with no free memory

While writing tmp4, and the performance dropping to 30 KB/sec, the numbers look like this:

MemFree:          1148 kB
Cached:          13988 kB
Dirty:              12 kB
Writeback:          36 kB

Of course free memory is useless, we’d rather have everying to into the cache. The cache stays filled (we have tmp1, tmp2 and tmp3 in the cache), but the values for Dirty and Writeback are too low.

Before, the file to be written was completely loaded into the cache first and marked dirty.The pdflush daemon was started deferred and found rich caches to be written to disk.

The number of blocks marked dirty now never seems to exceed 50 kB. The pdflush daemon can only flush out small chunks of up to 36 kB at once (usually less), resulting in a lot of USB operations and overhead and low performance.

Clearing the cache helps

The Freetz kernel unfortunately does not expose /proc/sys/vm/drop_caches to drop all cached buffers. But what happens, if we rm tmp1:

MemFree:          1604 kB
Cached:          14004 kB

Nothing. tmp1 is not in the cache anymore and most likely tmp4 has taken it’s place, because it is newer. But tmp2 is still in the cache, so let’s rm it:

MemFree:          3464 kB     # rm tmp2 frees up the cached memory
Cached:          12152 kB     # the rm'ed file is removed from cache

Now we have over 3 MB free and unused memory and the file is not in the cache anymore.

Writing tmp5

Now let’s copy tmp5 (2048 KB). These are numbers from during the copy to see the values of Dirty and Writeback, so the file is only partly transfered yet:

MemFree:          2204 kB
Cached:          12948 kB
Dirty:             152 kB
Writeback:         424 kB

We again see high numbers for Dirty and Writeback as parts of the copied file are moved to the cache and dirty. The pdflush daemon gets huge chunks of buffers again to be streamed to the medium and we get a fairly high transfer rate.

Broken kernel behaviour

This is the fairly old Linux kernel from Freetz. The behaviour of the VFS and pdflush seems to be broken and thus result in very poor write performance:

  • when there is no free memory available, why doesn’t the kernel free more old cache memory for the new buffers to be marked dirty?
  • it seems, the pdflush daemon is forced to write out as soon as there are dirty buffers and memory is low (= no free memory). Why does the kernel seem to prefer to free memory by writing out dirty buffers instead of clearing the read cache to make room for more dirty buffers?
  • allocating new buffers seems to stall while pdflush daemon is freeing up dirty memory
  • new buffers are still taken from old cached files, so after copying the whole file, it is completely in the cache. why not put if completely in the cache before starting to write out and stall allocation of new buffers?
  • rm’ing a file that is in the cache, frees up the cache, resulting in performance boosts, until that free memory is used by the cache again and the pdflush daemon writes out much smaller chunks. Practically that won’t happen and a normal Linux system should never have large amounts of free memory.

This is a kernel bug preventing Fritz!Box 7170 from ever achieving good write performance on my USB stick and other mediums.


  1. When copying files to the Fritz!Box, the kernel caches these files in it’s cache only when free memory is available.
  2. It writes out the cached files to storage, with good performance because there are big chunks to be written. The files remain in the cache in case they are read.
  3. With a full cache and no free memory, new files aren’t cached anymore but directly written to the medium, resulting in a lot of small writes with big overhead and bad performance. The file is still in the cache after writing is done.
  4. Clearing up the cache results in free memory and write performance boosts until the cache is saturated again.
  5. Because the cache is only freed up on unmount, the situation almost never happens, making writing data to USB sticks a pain.

External harddrives might work better, because of fast integrated hardware caches that can take lots of small chunks. But on a USB stick without hardware cache, performance is killed by the small writes.

It is unlikely that this bug will be fixed by AVM or by Freetz for the Fritz!Box 7170 because it seems to be a flaw in the used Linux kernel and AVM does not update the 7170 firmware anymore.

Is this a known bug and is this fixed in newer kernels?

Out of Memory in GLSurfaceView on resume

The Symptom

After publishing WordMix with the OpenGL accelerated 2D game view (using GLSurfaceView), I received weird crash reports from some devices, mostly out of memory from within the GL context:

android.opengl.GLException: out of memory
 at android.opengl.GLErrorWrapper.checkError(
 at android.opengl.GLErrorWrapper.glGenTextures(
 at [...]

From the very limited information the Google Play Developer Console gives me about crash reports, I assumed it only affects devices running Android version 3. Modifying the code only caused the out of memory exception to be thrown at random other places, even at GL10.glClear(…)!

I also found out, the crash only happens when the user finishes a subactivity that would leave to the activity containing the GLSurfaceView. Users were complaining about the crash happening before starting a second game, which puzzled me, because all my rendering code seemed to be working fine on all devices running Android 4. Everything worked fine without the GLSurfaceView as well.

Looking that the source code for GLSurfaceView, nothing interesting was changed between Android 3.2 to Android 4, so the GLSurfaceView was hardly to blame, but more the hardware, drivers or specific OpenGL implementation.

The problem

The actual problem was very hard to track down and took me several hours and was particularly hard because I did not have an Android 3 tablet for debugging:

Up to Android 2.3, views were drawn in software and later composited using the hardware. Android 3 introduced an alternative hardware accelerated drawing engine for everything that uses Canvas classes. This alternative render path is disabled by default in Android 3 and supposedly enabled by default in Android 4 (previous blog post).

When I found out, that the Samsung Galaxy S2 does not enable hardware acceleration by default, I did set

<application android:hardwareAccelerated="true" ...>

in the AndroidManifest.xml for all activities that should support hardware acceleration. Using hardware acceleration for the activity with the anyway hardware accelerated GLSurfaceView did not make much of a difference. But accelerating the results or preferences activity, for example, gave a nice performance boost on my SGS2.

It turns out that the crash happens in Android, when an activity, that contains a GLSurfaceView, is paused for a fullscreen activity, that is hardware accelerated. When that hardware accelerated activity is finished, the underlying GLSurfaceView is screwed up, throwing out of memory exceptions, even though the GL context is completely reinitialized correctly.

The solution

Yes, I should have tested more the effects of hardwareAccelerated=”true”.

Leaving that attribute entirely unset is recommended for Android 3, especially when you use a GLSurfaceView, and should not hurt Android 4 devices as well. Setting a reasonable default value is then up to the manufacturers.


  • If you use a GLSurfaceView in an activity
  • and suspend that activity by starting another fullscreen activity
  • and that activity is hardwareAccelerated by setting so in the AndroidManifest.xml
  • and you target Android 3 devices
  • expect weird behaviour like out-of-memory exceptions

Welcome to fragmentation. Just let hardwareAccelerated be unset.

OpenGL antialiasing in Android and transparent textures

I tried to replace the legacy 2D rendering code of WordMix, which uses the native Android canvas methods, with an OpenGL renderer to allow for fancy effects and animations.

First attempt

simple texture with full bleed image, no border

Because the tiles are simple rectangles with round corners, I created a texture with gimp and rendered a quad in OpenGL. The texture had no mipmaps and was filtered linear for both, minimizing and magnifying. When rotating that quad, I got the typical “staircase” lines, because I did not use anti-aliasing / multisampling. The result looks rather horrible:

no multisampling, no mipmaps, simple texture result in typical “staircase” borders

You can see two effects, one if it being the clear staircase borders, where the texture is not linear filtered, and you see the round corners of the texture with a grayish border, I’ll explain in the next paragraphs.

Multisampling emulation to remove “staircase”

So how to achieve multisampling in OpenGL ES 1.1? The answer I found is quite simple and easy on the hardware: use a texture with a transparent border and linear texture interpolation will do the rest. So I modified the texture to include a transparent border and rendered the quads slightly bigger to fill the same amount of pixels.

texture image with transparent border

The result looked better but I was not satisfied with the borders. I saw the interpolations but there is still a very visible “staircase”. Plus it seems, that the borders are blended with a black color, which can be seen on the overlapping tiles:

no mipmaps, texture with transparent border, still visible “staircase” and dark colored border

This is in fact due to my texture, which had the transparent pixels assigned the color black. The OpenGL interpolation would just average two neighbour pixels, which would calculate like

(argb(1, 1, 1, 1) + argb(0, 0, 0, 0)) / 2 = argb(0.5, 0.5, 0.5, 0.5)

which is a semi transparent gray color tone.

Monkeying with gimp for transparent pixel

So how to create a texture, where the transparent pixels have the color white? Gimp seemed to screw up the color of transparent pixels even though when exporting my work as png file, it offers to keep the color of transparent pixels.

The trick: combine all visible planes, create an alpha channel and change the color layer. If you have uncombined planes, the result is unpredictable and the colors are screwed up.

So now I had a texture with a white but fully transparent border (value 0x00FFFFFF) and I’d expect the calculation to be

(argb(1, 1, 1, 1) + argb(0, 1, 1, 1)) / 2 = argb(0.5, 1, 1, 1)

But I still got the same result:

texture with transparent + white border: still black border in Android

Bitmaps with transparent pixels in Android

So why is my border still black, while the texture has white transparent regions? I checked the loaded Bitmap with this code after loading the png resource:

Bitmap bmp = BitmapFactory.decodeResource(getResources(), R.drawable.stone);
Log.d("texture", bmp.getPixel(0, 0)); /* result: 0 */

Why is the result 0?? I’d expect a 0x00FFFFFF, but either Androids Bitmap loader premultiplies the alpha or recompresses the image file on compile, although I did place the image in the res/drawable-nodpi folder.

But apparently Bitmap and Canvas throw away all color information, when drawing with an alpha value of 0. This results in a fully transparent, but black canvas:

canvas.drawColor(Color.argb(0, 255, 255, 255), Mode.SRC);
Log.d("texture", bmp.getPixel(0, 0)); /* result: 0 */

while the following results in a white canvas, which is almost transparent (1/256):

canvas.drawColor(Color.argb(1, 255, 255, 255), Mode.SRC);
Log.d("texture", bmp.getPixel(0, 0)); /* result: 0x01FFFFFF */

Good to know, so now I create my texture with a border that is almost transparent, but not completely (alpha value 1/256) and the color white, which should be hardly visible, calculating like:

(argb(1, 1, 1, 1) + argb(0.01, 1, 1, 1)) / 2 = argb(0.505, 1, 1, 1)

I checked with above Log code and indeed got the value 0x01FFFFFF. So at least the Bitmap was loaded correctly now. But I still get a  black border and the result looks the same. Why?

Creating OpenGL textures with unmultiplied alpha

I found a post and bug report that apparently the GLUtils.glTexImage2D() screws with the alpha and colors too, creating texture values of 0x01010101, which gets blended with the nearby white pixels on linear filtering. What the…?

The post suggests a workaround to not use GLUtils to load the Bitmap into an OpenGL texture but use the original GL10.glTexImage2D(). While the code in that post is not very efficient, it does result in nice and smooth blended borders. Of course the use of mipmaps helps too to make the texture smooth when minified:

texture with 0x01FFFFFF almost-transparent border and use of original GL10.glTexImage2D method and mipmaps


Several culprits were found to make antialiasing work with an Android App using OpenGL ES 1.1:

  1. Create textures that have transparent borders, so linear filtering emulated oversampling at polygon borders
  2. Make sure the transparent border of your texture contains color values, which will “bleed” into the border pixels of the texture.
  3. If you use mipmaps, make sure you have enough transparent border pixels or set GL_TEXTURE_WRAP to GL_CLAMP.
  4. Double check result, because gimp does screw up when having multiple layers, that are merged when exporting as png image.
  5. Androids Bitmap loader and Canvas code seems to zero out the color values when alpha is 0. The workaround to keep the color values on load: Use colored pixels with alpha value of 1 (of 255).
  6. The GLUtils.glTexImage2D implementation premultiplies alpha values with color values, resulting in very dark color, instead of the white I wanted. Use the GL10.glTexImage2D directly (example in this post).

Using mipmaps and adding a nice shadow texture results in a screen, that looks very similar to the original, but which is much faster:

final result with all workarounds, mipmaps and shadows