Appendix d: Gamma correction

(This appendix is informative, not normative)

See the Gamma Tutorialin the PNG specification[12]if you aren't already familiar with gamma issues.

In the computation, UAs displaying on a CRT may assume an ideal CRT and ignore any effects on apparent gamma caused by dithering. That means the minimal handling they need to do on current platforms is:

PC using MS-Windows

none

Unix using X11

none

Mac using QuickDraw

apply gamma 1.39 [13] (ColorSync-savvy applications may simply pass the sRGB ICC profile [14] to ColorSync to perform correct color correction)

SGI using X

apply the gamma value from /etc/config/system.glGammaVal (the default value being 1.70; applications running on Irix 6.2 or above may simply pass the sRGB ICC profile to the color management system)

NeXT using NeXTStep

apply gamma 2.22

"Applying gamma" means that each of the three R, G and B must be converted to R'=R^gamma, G'=G^gamma, G'=B^gamma, before handing to the OS.

This may rapidly be done by building a 256-element lookup table once per browser invocation thus:

for i := 0 to 255 do

raw := i / 255;

corr := pow (raw, gamma);

table[i] := trunc (0.5 + corr * 255.0)

end

which then avoids any need to do transcendental math per color attribute, far less per pixel.

Appendix e: The applicability and extensibility of css1

(This appendix is informative, not normative)

The goal of the work on CSS1 has been to create a simple style sheet mechanism for HTML documents. The current specification is a balance between the simplicity needed to realize style sheets on the web, and pressure from authors for richer visual control. CSS1 offers:

• visual markup replacement: HTML extensions, e.g. "CENTER", "FONT" and "SPACER", are easily replaced with CSS1 style sheets.

• nicer markup: instead of using "FONT" elements to achieve the popular small-caps style, one declaration in the style sheet is sufficient. Compare the visual markup:

• <H1>H<FONT SIZE=-1>EADLINE</FONT></H1>

with the style sheet:

H1 { font-style: small-caps }

<H1>Headline</H1>

• various integration levels: CSS1 style rules can be fetched from external style sheets, included in the 'STYLE' element, or put into 'STYLE' attributes. The latter option offers easy transition from HTML extensions.

• new effects: some new visual effects have been added to offer users new toys. The typographical pseudo-elements and the extra values on the background property fall into this category.

• scalability: CSS1 will be useful on equipment ranging from text terminals to high-resolution color workstations. Authors can write one style sheet and be reasonably sure that the intended style will come across in the best possible manner.

CSS1 does not offer:

• per pixel control: CSS1 values simplicity over level of control, and although the combination of background images and styled HTML is powerful, control to the pixel level is not possible.

• author control: the author cannot enforce the use of a certain sheet, only suggest

• a layout language: CSS1 does not offer multiple columns with text-flow, overlapping frames etc.

• a rich query language on the parse tree: CSS1 can only look for ancestor elements in the parse tree, while other style sheet languages (e.g. DSSSL [6]) offers a full query language.

We expect to see extensions of CSS in several directions:

• paper: better support for printing HTML documents

• support for non-visual media: work is in the process to add a list of properties and corresponding values to support speech and braille output

• color names: the currently supported list may be extended

• fonts: more precise font specification systems are expected to complement existing CSS1 font properties.

• values, properties: we expect vendors to propose extensions to the CSS1 set of values and properties. Extending in this direction is trivial for the specification, but interoperability between different UAs is a concern

• layout language: support for two-dimensional layout in the tradition of desktop publishing packages.

• other DTDs: CSS1 has some HTML-specific parts (e.g. the special status of the 'CLASS' and 'ID' attributes) but should easily be extended to apply to other DTDs as well.

We do not expect CSS to evolve into:

• a programming language

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