I have an array of bytes representing an image in Windows BMP format and I would like my library to present it to the Java application as a BufferedImage
, without copying the pixel data.
The main problem is that all implementations of Raster
in the JDK store image pixels in top-down, left-to-right order whereas BMP pixel data is stored bottom-up, left-to-right. If this is not compensated for, the resulting image will be flipped vertically.
The most obvious "solution" is to set the SampleModel
's scanlineStride
property to a negative value and change the band offsets (or the DataBuffer
's array offset) to point to the top-left pixel, i.e. the first pixel of the last line in the array. Unfortunately this does not work because all of the SampleModel
constructors throw an exception if given a negative scanlineStride
argument.
I am currently working around it by forcing the scanlineStride
field to a negative value using reflection, but I would like to do it in a cleaner and more portable way if possible.
e.g. is there another way to fool the Raster
or SampleModel
into arranging the pixels in bottom-up order but without breaking encapsulation? Or is there a library somewhere that will wrap the Raster
and SampleModel
, presenting the pixel rows in reverse order?
I would prefer to avoid the following approaches:
- Copying the whole image (for performance reasons. The code must process hundreds of large (>= 1Mpixels) images per second and although the whole image must be available to the application, it will normally access only a tiny (but hard-to-predict) portion of the image.)
- Modifying the
DataBuffer
to perform coordinate transformation (this actually works but is another "dirty" solution because the buffer should not need to know about the scanline/pixel layout.) - Re-implementing the
Raster
and/orSampleModel
interfaces from scratch (because of the way compatibility checking is implemented (at least in the Sun JDK), requiring specific subclasses ofSampleModel
so a genericBottomUpSampleModel
wrapper class would not work.)