theV0ID theV0ID - 10 months ago 67
Android Question

Android: fast bitmap blur?

I've been searching the past three days for a built-in, hardware-accelerated way of bluring a bitmap with android. I stumbled upon certain work-arounds like shrinking the bitmap and scaling it up again, but this method produced low quality results which were not suitable for my image recognition requirements. I also read that implementing convolution with shaders or JNI is a good way to go, but I cannot believe that there is no built-in solution in the Android framework for this very common purpose. Currently I've ended up with a self-written convolution implementation in Java, but it is awkwardly slow. My question is:

  • Is there really no built-in solution in the Android framework?

  • In case there is none: what is the most efficient way of accelerating the convolution with a still reasonable complexity of implementation and maintenance? Shall we use JNI, shaders or something completely different?

Answer Source

I finally found a suitable solution:

  • RenderScript allows implementing heavy computations which are scaled transparently to all cores available on the executing device. I've come to the conclusion, that with respect to a reasonable balance of performance and implementation complexity, this is a better approach than JNI or shaders.
  • Since API Level 17, there is the ScriptIntrinsicBlur class available from the API. This is exactly what I've been looking for, namely a high level, hardware-accelerated Gaussian blur implementation.
  • ScriptIntrinsicBlur is now a part of the android support library (v8) which supports Froyo and above (API>8). The android developer blog post on the support RenderScript library has some basic tips on how to use it.

However, the documentation on the ScriptIntrinsicBlur class is very rare and I've spent some more time on figuring out the correct invocation arguments. For bluring an ordinary ARGB_8888-typed bitmap named photo, here they are:

final RenderScript rs = RenderScript.create( myAndroidContext );
final Allocation input = Allocation.createFromBitmap( rs, photo, Allocation.MipmapControl.MIPMAP_NONE, Allocation.USAGE_SCRIPT );
final Allocation output = Allocation.createTyped( rs, input.getType() );
final ScriptIntrinsicBlur script = ScriptIntrinsicBlur.create( rs, Element.U8_4( rs ) );
script.setRadius( myBlurRadius /* e.g. 3.f */ );
script.setInput( input );
script.forEach( output );
output.copyTo( photo );