Java Glossary : PNG

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*.png (Portable Network Graphic) files are a replacement for *.gif files when you don't need animation. JPEG files have better compression for photographs but don't handle line art as well as gif and png formats. PNG have been one of the standard image formats in Netscape and Internet Explorer since 1997, though IE support is still only partial. Opera started native support with version 3.51. Apple's Quicktime supports the format. Unlike GIF files, there are no patent restrictions, and the file format is maintained in an open source environment. You can get on a mailing list if you want to be kept informed of the latest developments. One big advantage is that fades to transparent background and antialiasing look good no matter what the background colour. With GIFs, fades only work for one colour of background, usually white or black.

feature	GIF	JPG	PNG
colour resolution	8 bits	24 bits	8, 24, or 48 bits
grey resolution	8 bits	8 bits	8 or 16 bits
transparency	1 level	none	256 levels
lossy compression	no	yes	no
lossless compression	yes	unused	yes
patent restrictions	yes	no	no
animation	yes	no	no (see MNG)

PNG compression uses zlib, same as the gnu gzip and Java jar utilities. On most 8-bit images, the PNG form will be about 11% smaller than the GIF form. Java 1.4.1 supports PNG officially in the Advanced Imaging Kit. You can use PNG images with Toolkit.getImage.

You can test your browser's PNG support. You should see an image of a huge spiky red flower to the right of the table above, with proper fades into the transparent background, with no white halos. IE 5.5 does not show it at all. Netscape 4.79 shows it with a purple background. Netscape 7.02 shows it correctly. Opera 7.21 shows it correctly. You can also click here and here for more tests.

The PNG file header looks like this:

PNG Header Format
Field	Contents	Size
signature	\211PNG\r\n\032\n i.e. hex 89504E470D0A1A0A	8-bytes
chunksize	0x0000000D	4 bytes big-endian binary
chunkid	"IHDR" i.e. hex 0x49484452	4 bytes
width		4 bytes big-endian binary
height		4 bytes big-endian binary

The ImageInfo class the CMP Business classes will give you the width and height of png images (also gif and jpg) without loading them. It just read the first few bytes of the file.

GIF ¤ Image ¤ ImageInfo ¤ ImageIO ¤ Imagero reader ¤ Internet Explorer ¤ Introduction to PNG ¤ JPEG ¤ Masker ¤ MNG ¤ Netscape ¤ Opera ¤ WIF

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