Best approach to storing image pixels in bottom-up order in Java
- by finnw
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/or SampleModel interfaces from scratch (but I have a hunch that I will be unable to avoid this.)
