Bug 9426 – image/gif preferred over image/png and image/tiff

• Component: Images
• Status: NEW

[Nicholas Shanks]

When requesting a file for <img> elements Safari sends "Accept: */*", which means that by default Apache will send 8-bit GIF files in preference to 24-bit files in other formats, as the GIFs are smaller. All other browsers send a preference list, with GIFs given a low q-value.

A knowledgeable webmaster can set the quality of the GIF images on the server end to something less than that of the others, but many will not. I have set up the above URL to demonstrate this. Check it in Firefox and Safari and compare the last image.

[Alexey Proskuryakov]

See also: bug 9572 (not a duplicate).

Nick, could you please post exact Accept strings from other browsers here, and maybe suggest what WebKit should send?

[Nicholas Shanks]

With all relavent prefs reset to their defaults, and my AppleLanguages = ("en-GB", "en-AU", "en-CA", fr)
Strings are User-Agent, Accept, Accept-Language, Accept-Charset, Accept-Encoding

Retrieving a URL given in the Address Bar:
Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en-us) AppleWebKit/420+ (KHTML, like Gecko) Safari/417.9.3
    */*
    en-us
    -
    gzip, deflate
Opera/9.00 (Macintosh; PPC Mac OS X; U; en)
    text/html, application/xml;q=0.9, application/xhtml+xml, image/png, image/jpeg, image/gif, image/x-xbitmap, */*;q=0.1
    en
    iso-8859-1, utf-8, utf-16, *;q=0.1
    deflate, gzip, x-gzip, identity, *;q=0
Mozilla/5.0 (compatible; iCab 3.0.2; Macintosh; U; PPC Mac OS X)
    image/png;q=0.7, image/jpeg;q=0.6, image/pjpeg;q=0.6, image/gif;q=0.5, image/x-xbitmap;q=0.4, image/xbm;q=0.4, text/html;q=0.9, application/xhtml+xml;q=0.7, text/plain;q=0.6, */*;q=0.1
    en, *;q=0.5
    utf-8;q=0.9, utf-16;q=0.5, utf-7;q=0.2, ISO-8859-1;q=0.7, *;q=0.6
    -
Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en-US) AppleWebKit/125.4 (KHTML, like Gecko, Safari) OmniWeb/v563.66
    image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, image/tiff, multipart/x-mixed-replace, */*;q=0.1
    en-gb, en-au;q=0.8, en-ca;q=0.6, fr;q=0.4
    iso-8859-1, utf-8, iso-10646-ucs-2, macintosh, windows-1252, *
    bzip2, gzip, deflate, identity
Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-GB; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4"
    application/xhtml+xml, text/html; q=0.9, text/plain; q=0.8, image/png, */*; q=0.1
    en-gb,en;q=0.5
    UTF-8,*
    gzip, deflate
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)
    */*
    en-gb
    -
    gzip, deflate



All browsers send the same string at other times too, except Firefox and WinIE, which also send:

When retrieving a document via an <img> tag with no type:
Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-GB; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4
    image/png,*/*;q=0.5
    en-gb,en;q=0.5
    UTF-8,*
    gzip, deflate
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)
    image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/x-shockwave-flash, application/vnd.ms-excel, application/msword, */*
    en-gb
    -
    gzip, deflate

When retrieving a document referenced via a <link rel="stylesheet" type="text/css"> tag:
Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-GB; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4
    text/css,*/*;q=0.1
    en-gb,en;q=0.5
    UTF-8,*
    gzip, deflate

When retrieving a document via a <script type="text/javascript"> tag:
Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-GB; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4
    */*
    en-gb,en;q=0.5
    UTF-8,*
    gzip, deflate

[Timothy Hatcher]

This could be important with HiDPI and multi-representation tiffs or JPEG 2000 files...

[Alexey Proskuryakov]

Preferring PNG over GIF can break color consistency - see <http://hsivonen.iki.fi/png-gamma/> for a long sad story. WebKit renders most PNGs differently from Firefox (only ones without any color space information at all look the same).

[Nicholas Shanks]

Well that's rather irrelevant, because (a) it'll get fixed, and (b) GIF doesn't do colour correction anyway.

[Alexey Proskuryakov]

> (b) GIF doesn't do colour correction anyway.

This is exactly why it is better for compatibility (please read the provided link). The issue with PNG gamma/color correction is a well known problem, I certainly wouldn't denounce it as irrelevant.