
                                                REVIEW: A LOOK AT THE EFFECTIVENESS OF PNG
            Andy Dean                                                                                                                            adean@eleceng.ucl.ac.uk

[Well, it's mostly review. It's part plug, since the author of the article also wrote Image Studio, but hey...-Jason]

Dear Jason,

After reading the excellent article on GIF/PNG by Michael Console Battilana in AmigaReport 3.11, I thought you may be interested in the following article. We've recently added PNG support to ImageStudio and I've had a chance to test the new file format's performance against existing formats. I'd be delighted if you would consider the following article for inclusion in AmigaReport, it will of interest to anyone who deals with bitmap images.

At the start of 1995 the GIF file format was the centre of attention when it was revealed that the LZW compression algorithm used for its data compression was under patent from Unisys. This meant that authors of commercial and shareware software that wrote GIF files were required to pay Unisys (or Compuserve, the designer of the GIF format) royalties for every copy of their software sold.

This was deemed unacceptable by many users of GIF and so suggestions were put forward as to the creation of a royalty-free replacement for the GIF format. A few ideas have been suggested, but the most complete solution to date is the new PNG (Portable Network Graphics) file format.

PNG takes the following features from GIF:

Storage of palette based images of up to 256 colours.
Interlaced redraw for quick display on World Wide Web browsers.
Transparent colour, allowing for non-rectangular images.
Powerful lossless compression algorithm.
Extendable for future expansion.

and adds the following abilities:

Full colour images up to 48bits per pixel.
Full alpha channel.
Image pre/post filtering to further aid compression.
More levels of interlacing for faster display.

The full PNG specification has not yet been released (it is up to the tenth draft at the time of writing), but already a PNG support library is being written to aid software authors in loading and saving PNG files from within their programs.

As the co-author of the Amiga shareware image processing program "ImageStudio", I was very interested in this new format. So with the pre-release version of the PNG support library, the tenth draft of the PNG specs and the trusty SAS/C debugger at the ready, I wrote a PNG module for ImageStudio.

My aims were to find out exactly what PNG could be useful for. I was interested in:

How fast PNG files were to load and save.
How large a PNG file is compared to other popular formats.

I tested the PNG module with 3 images:

        "Bud"         - 256 colour, 640x806 scanned image of a Budweiser advert.
        "Colourface"  - 24bit, 574x815 scanned image of a woman's face.
        "Williams"    - 24bit, 640x480 raytraced image of a Williams Formula1 grand prix car.

With such a small number of images it is difficult to represent all types of image, but they represent perhaps the most popular types of image that are found in the 256 colour and 24bit formats.

The results of load/save times and file size are given below:

Image    Format         Load time       Save time       File size
-----    ------         ---------       ---------       ---------
Bud      IFF-ILBM       11s             11s             460K
         GIF            14s             17s             371K
         PNG            11s             52s             326K
         LHA(1)         4s              29s             411K

Colour-  IFF-ILBM24     32s             28s             1,195K
face     JPEG(2)        32s             30s             95K
         PNG            33s             5m26s           691K
         TIFF-LZW(3)    1m09s           1m15s           1,450K
         LHA(1)         10s             1m12s           1,099K

Williams IFF-ILBM24     16s             17s             354K
         JPEG(2)        16s             18s             48K
         PNG            15s             3m07s           104K
         TIFF-LZW(3)    27s             24s             178K
         LHA(1)         2s              19s             175K

Notes:

The computer used for all the tests was an Amiga 4000/030 with 10MB of RAM and a 25MHz 68030.
The software used to load and save the images was ImageStudio v2.1.1 (68020+ version).
(1) LHA is not an image file format. These values are the results of taking the IFF-ILBM and compressing it with LHA v1.38. The load time is the time taken to decompress the file, the save time is the compression time. This operation is a typical method of storing files to reduce their size.
(2) The JPEGs were saved with a quality setting of 85. JPEG uses lossy compression, so information was lost over the original 24bit images; this explains JPEG's ability to create much smaller files than any of the other formats.
(3) The TIFF-LZW files were loaded/saved by ImageFX v1.52 as ImageStudio cannot use TIFF-LZW yet.

The colour mapped "Bud" image showed the least of the improvements in compression, but it was still significantly smaller than any of the other formats. When you're paying per kilobyte of file downloaded, any reduction in file size is welcome.

It is the 24bit images where PNG excels. In both the scanned "Colourface" and raytraced "Williams" images, the PNG image was almost half the size of the nearest lossless format. JPEG still creates the smallest files, but at the cost of losing some of the image information. In the case of the "Colourface" image, the loss of information was almost imperceptible. With the clean raytraced "Williams" image however, the JPEG artifacts were quite noticible.

I'll leave it up to the individual to decided whether PNG looks like a useful format for their own use. As all the PNG images were smaller than the other lossless formats it seems a good idea to consider PNG for storing files for archiving in all cases. Using JPEG to store scanned 24bit images is possible, but for storing computer generated images (from a raytracer, landscape generator or fractal program) the need for lossless compression is paramount.

At the moment the creation of PNG files is substantially slower than with the other formats. This again suggests that PNG files lend themselves to applications where they are compressed once and read many times.

The ImageStudio PNG module used for these tests is now released on Aminet as:

gfx/conv ImageStudioPNG.lha

The code is based on a pre-release v0.6 of the PNG library and so should still be considered "work in progress". The PNG library loads the majority of the PNG files I've come across and saves PNG files correctly, including support for interlacing and a transparent colour. The above archive contains both 68000 and 68020+ versions of the PNG module. Unregistered users of ImageStudio will find that PNG files loaded with the module will be clipped to 512x512, registered users will have a fully functional module. The module will be uploaded on the 7th of June 1995 so please give it a couple of days to propagate around the Aminet network.

PNG is currently supported by both "ImageStudio" and Cloanto's "Personal Paint v6.3" on the Amiga, but as PNG becomes more popular I guess the format will be supported by many more packages.

For more information on the GIF saga, I suggest reading Michael Console Battilana's article in AmigaReport 3.11.

Excerpts are taken from the PNG specification, tenth draft.

PNG information and sample images can be obtained by anonymous ftp at ftp.uu.net /graphics/png