proposed enhancement reduces load time for some image files by 40 percent
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Key: SANSELAN-56
URL: https://issues.apache.org/jira/browse/SANSELAN-56
Project: Commons Sanselan
Issue Type: Improvement
Environment: Tested in Windows, Linux, MacOS
Reporter: Gary Lucas
I have identified an enhancement that reduces the time required to load TIFF
image by 40 percent. I have tested a modified version of Sanselan under
Windows, Linux, and MacOS with consistent savings on each platform.
Additionally, I suspect that this technique may be applicable to other areas of
the Sanselan code base, including more popular image formats supported by
Sanselan such as JPEG, PNG, etc.
I propose to add the relevant code changes to the Sanselan code base. Once
these modifications are in place, there would be an opportunity for others to
look at the pro's and cons' of applying the techniques to other data formats.
The Enhancement
To load an image from a TIFF file, Sanselan performs extensive data processing
in order to obtain RGB values for the pixels in the output image. The code for
that processing appears to be well written and efficient. Once the RGB value
are obtained, they are stored in a Java BufferedImage using a call to the
setRGB() method.
Unfortunately, setRGB() is an extremely inefficient method. A much, much
better approach is to store the data into an integer array and defer the
creation of the buffered image until all information for the image has been
collected. Java has a nice (though somewhat obscure) API that lets memory in
an integer array be transferred directly to a BufferedImage so that the system
does not have to allocate additional memory for this procedure (a very nice
feature when dealing with huge images). This change virtually eliminated the
overhead for transferring data to images, which accounted for 40 percent of the
time required to load images. For TIFF files, this was a reasonable approach
because the TiffImageParser class always loads 4-byte image and the
getGrayscaleBufferedImage() method is never used. I have not investigated the
code for the other renders, but some refinement might be needed for the
one-byte grayscale images.
Steps to Integration
In sanselan.common, a new class called ImagePrep was created. ImagePrep
carries a width, height, and an integer array for storing pixels. It provides
its own setRGB() method which looks just like the one in BufferedImage.
Finally, it provides a method called getBufferedImage() which creates a
BufferedImage from its internal the integer array when the processing is
complete.
In the TiffImageParser classes, data is read from input stream and transferred
to pixel values in a series of classes known as PhotometricInterpreters. These
were modified to operate on ImagePrep objects rather than BufferedImage
objects. The DataReader and TiffImageParser classes were modified to pass
ImagePrep objects into the photometric interpreters rather than using
BufferedImages.
At the very last step, before passing its result back to the calling method
(the Sanselan main class, etc.), the TiffImageParser used the
ImagePrep.getBufferedImage() to convert the result to the expected form.
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