Question
Morning,
Anyone with working knowledge of .png graphics files -
using Photoshop 6.0 I need to know exactly how to save these for use on the web ... I've never used this format before and have a potential client with real strict parameters on a project that includes the need for a set of graphics in .png
Help me pleeeeease? ...
I'll be yer best friend!
~ gem ~
Answer
I found this already and read it but ...
http://user.fundy.net/morris/redirec...otoshop3.shtml
The test image I did,
a) not sure if I saved it correctly, and
b) when I uploaded it and test-viewed it - came up with a Quicktime load?
Hubba wha?!
~ gem ~
Answer
Gem, you can use Irfanview to save any graphic in PNG format. To check I just changed one of my jpg pics to a png and it opened just fine.
Irfanview is a free editor and handy for a lot of things that you can't do with others.
Joan
Answer
Hi Joan,
I use PSP and Photoshop which both support the .png format, turns out -
(didnt know that until I started researching for this project this morning!)
I have to actually create these graphics from new and send them to the client in several formats, so while I know Irfanview is a great tool, it wouldn't work for what I need to do here.
I was hoping for a more technical run-down of the file format itself I guess (like, is it becoming more widely supported across browsers, when and why would it be a better choice than JPG or GIF, are there certain defaults to use when saving to optimize the image, etc) But I think I got it figured out!
Thanks for the response, though
~ gem ~
Answer
I was hoping for a more technical run-down of the file format itself I guess (like, is it becoming more widely supported across browsers, when and why would it be a better choice than JPG or GIF, are there certain defaults to use when saving to optimize the image, etc) But I think I got it figured out!
Gem, I was over my head just posting in your thread. I was really just trying out those blue balls.....LOL
Joan
Answer
I was over my head just posting in your thread.
No way!
urgh Really hope I wasnt sounding like a know-it-all, in my response up there! Didn't mean to! Cuz considering it was my question, and I myself am breathing through a straw stickin' outta the water half the time - don't feel bad, you're in good company.
.: scoot scoot :. wanna share my liferaft?
I just hate it when I feel like I should know what I'm doing 100% of the time then someone throws me a curveball and I'm all -
"Daaaarrrr! I dunnoooo"...drooldrool...
I just appreciate that you responded at all!
Answer
Typical Usage
The Portable Network Graphics (PNG) format was designed to replace the older and simpler GIF format and, to some extent, the much more complex TIFF format. (See the main page or the history page for background information.) Here we'll concentrate on two major uses: the World Wide Web (WWW) and image-editing.
For the Web, PNG really has three main advantages over GIF: alpha channels (variable transparency), gamma correction (cross-platform control of image brightness), and two-dimensional interlacing (a method of progressive display). PNG also compresses better than GIF in almost every case, but the difference is generally only around 5% to 25%, not a large enough factor to encourage folks to switch on that basis alone. One GIF feature that PNG does not try to reproduce is multiple-image support, especially animations; PNG was and is intended to be a single-image format only. (A very PNG-like extension format called MNG was finalized in mid-1999 and is beginning to be supported by various applications, but MNGs and PNGs will have different file extensions and different purposes.)
For image editing, either professional or otherwise, PNG provides a useful format for the storage of intermediate stages of editing. Since PNG's compression is fully lossless--and since it supports up to 48-bit truecolor or 16-bit grayscale--saving, restoring and re-saving an image will not degrade its quality, unlike standard JPEG (even at its highest quality settings). And unlike TIFF, the PNG specification leaves no room for implementors to pick and choose what features they'll support; the result is that a PNG image saved in one app is readable in any other PNG-supporting application. (Note that for transmission of finished truecolor images--especially photographic ones--JPEG is almost always a better choice. Although JPEG's lossy compression can introduce visible artifacts, these can be minimized, and the savings in file size even at high quality levels is much better than is generally possible with a lossless format like PNG. And for black-and-white images, particularly of text or drawings, TIFF's Group 4 fax compression or the JBIG format are often far better than 1-bit grayscale PNG.)
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Compression
PNG's compression is among the best that can be had without losing image information and without paying patent fees, but not all implementations take full advantage of the available power. Even those that do can be thwarted by unwise choices on the part of the user.
PNG supports three main image types: truecolor, grayscale and palette-based (``8-bit''). JPEG only supports the first two; GIF only the third (although it can fake grayscale by using a gray palette). The impact on compression comes from the ability to mix up image types in PNG. Specifically, forcing an application to save an 8-bit palette image as a 24-bit truecolor (or ``RGB'') image is not going to result in a small file. This may be unavoidable if the original has been modified to include more than 256 colors (for example, if a continuous gradient background has been added), but many images intended for the Web have 256 or fewer colors.
On the programmer's side, one common mistake is to include too many palette entries in a PNG image. This error is most noticeable when converting tiny GIF images (bullets, buttons, etc.) to PNG format; these images are typically only 1000 bytes or so in size, and storing 256 three-byte palette entries where only 50 are needed would result in over 600 bytes of wasted space.
Another common programmer mistake is to use only one type of compression filter, or to vary them incorrectly. Compression filters are described below and can make a dramatic difference in the compressibility of the image. In general this is not a feature that users should be forced to experiment with.
Finally, the low-level compression engine itself can be tweaked to compress either better or faster. Often ``best compression'' is the preferred setting, but an implementor may choose to use an intermediate level of compression in order to boost the interactive performance for the user. Usually the difference in file size is small, but there are cases where such a choice can make a big difference.
See the zlib home page for further details on PNG's compression engine and the CRC-32 algorithm, the 7-Zip home page for an alternative implementation of the deflate algorithm, and Vince Sabio's Compression Primer for an overview of compression in general. For tools to optimize the compression of PNG images, see the converters page (especially Glenn Randers-Pehrson's pngcrush and Ulead's SmartSaver).
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Compression Filters
Compression filters are a way of transforming the image data (losslessly) so that it will compress better. Each horizontal line in the image can have one of five filter types associated with it; choosing which of the five to use for each line is almost more of a black art than a science. Nevertheless, at least one reasonably good algorithm is not only known but also described in the PNG specification and implemented in freely available software. Other algorithms are likely to perform even better, but so far this has not been an active area of research.
By way of example--admittedly an extreme and unrealistic case*--a 512 x 32,768 image containing all 16,777,216 possible 24-bit colors compressed over 300 times better with filtering than without. The uncompressed image was 48 MB in size; the compressed-but-unfiltered version was around 36 MB; but the filtered version is only 115,989 bytes (0.1 MB). Yow. (A 4096 x 4096 version, created by Paul Schmidt, is a mere 59,852 bytes--more than 600 times better than the unfiltered version, at an overall compression ratio of 841:1. Ted Samuels ran it through Ken Silverman's PNGOUT utility--see the converters page for links to it and other optimizers--and trimmed it to 57,549 bytes, for an overall 875:1 ratio. See this page for a downloadable version and further info.)
A more realistic example is the oceanography data at NASA's Ocean ESIP site. Digital maps displaying various physical measurements can be generated dynamically in either GIF or PNG format; the PNG versions are invariably one-fifth the size of the GIFs, thanks to PNG's compression filters. For example, a map showing the surface height of the northeastern Pacific Ocean on 1 August 1997 (during a major El Niño) is 70,090 bytes in GIF format but only 13,880 bytes in PNG format.
See the Filter Algorithms chapter of the PNG specification for details.
* As a measure of just how unrealistic, note that these seemingly hyper-compressed PNG images can themselves be compressed by an additional factor of anywhere from 21 to 97 or so (depending on which image) simply by applying gzip to them. Of course, a gzip'd PNG is not terribly useful in most contexts, and MNG is the best of all--it drops the size to 456 bytes.
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Alpha Channels
Also known as a mask channel, an alpha channel is simply a way to associate variable transparency with an image. Whereas GIF supports simple binary transparency--any given pixel can be either fully transparent or fully opaque--PNG allows up to 254 levels of partial transparency in between for ``normal'' images (or 65,534 levels for the special ``deeply insane'' formats, but here we're concentrating on image depths that are useful on the Web).
All three PNG image types--truecolor, grayscale and palette--can have alpha information, but it's most commonly used with truecolor images. Instead of storing three bytes for every pixel (red, green and blue), now four are stored: red, green, blue and alpha, or RGBA. The variable transparency allows you to create ``special effects'' that will look good on any background, whether light, dark or patterned. For example, a photo-vignette effect can be created for a portrait by making a central oval region fully opaque (i.e., for the face and shoulders), the outer regions fully transparent, and a transition region that varies smoothly between the two extremes. When viewed with a Web browser such as Arena, the portrait would fade smoothly to white when viewed against a white background, or smoothly to black if against a black background. Drop-shadows are another ideal application for alpha transparency; in the images below, the same toucan image is displayed against a colorful background and against another copy of itself:
...article continued and much, much more here: http://www.libpng.org/pub/png/pngintro.html