Passing Image data through JNI?

Question

Hello,
I am trying find the fastest way to pass image data from Java through JNI. Passing Java images through JNI seems to be a very common thing to do so I expect there's an oft-used approach. Can anyone give me a good explanation of this or point me to some references?
I need to pass the image data to a C++ function which operates on this data, writing the results to a destination buffer which goes back to Java and gets put back into an image.
Currently, I'm using BufferedImages and getRGB to get an int array from my source image, passing this through JNI along with another int array to be used as the destination buffer, then calling setRGB with my destination buffer after the JNI function returns. getRGB() preserves the alpha channel of png files, which is functionality I need. The problem with this approach is that getRGB is extremely slow. It can take up to two seconds with a large image on my 1.2 Ghz Athlon. It seems to be performing a copy of the entire source image's data. Maybe there is a way to decode a jpg or png directly into an int array?
-Aaron Dwyer

Answer 1

Here's how you could do it for TYPE_4BYTE_ABGR
BufferedImage img=new BufferedImage(13,10,BufferedImage.TYPE_4BYTE_ABGR);
img.setRGB(0,0,0xff0000);     
img.setRGB(5,1,0x00ff00);        
img.setRGB(1,1,0xcafebabe);
DataBuffer db=img.getRaster().getDataBuffer();
int dataType=db.getDataType();
if (dataType!=DataBuffer.TYPE_BYTE)
     throw new IllegalStateException("I can do it only for TYPE_BYTE");
int imgType=img.getType();
if (imgType!=BufferedImage.TYPE_4BYTE_ABGR)
     throw new IllegalStateException("I can do it only for TYPE_4BYTE_ABGR");
DataBufferByte dbi=(DataBufferByte)db;
byte[] array=dbi.getData();
SampleModel model=img.getSampleModel();
if (!(model instanceof PixelInterleavedSampleModel))
     throw new IllegalStateException("I can do it only for PixelInterleavedSampleModel");
PixelInterleavedSampleModel pisModel=(PixelInterleavedSampleModel)model;
if (pisModel.getPixelStride()!=4)
     throw new IllegalStateException("I can do it only for pixel stride of 4");
if (dbi.getNumBanks()!=1)
     throw new IllegalStateException("I can do it only for 1 band");
int scanlineBytes=pisModel.getScanlineStride();
// Access the green Pixel on Position 5,1
System.out.println( (int)array[5*4 + scanlineBytes] &0xff);     // Alpha        
System.out.println( (int)array[5*4+1 + scanlineBytes] &0xff);     // Red        
System.out.println( (int)array[5*4+2 + scanlineBytes] &0xff);     // Green        
System.out.println( (int)array[5*4+3 + scanlineBytes] &0xff);     // Blue     I've added some checks to make sure we're accessing the data the right way.