Contrast Analyser, Version 2.0

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Support Contrast Analyser development

Support has been provided in part by the National Institute on Disability and Rehabilitation Research via the Trace Center under grant # H133E030012

What is the Contrast Analyser?

It is primarily a tool for checking foreground & background colour combinations to determine if they provide good colour visibility. It also contains functionality to create simulations of certain visual conditions such as colour blindness.

Determining "colour visibility" is based on the Contrast Ratio algorithm, suggested by the World Wide Web Consortium (W3C):

Note: It's important to remember that the contrast ratio is not yet a W3C recommendation; it is suggestion by the WAI WCAG 2.0 working group to help determine whether or not the contrast between two colours can be read by people with colour blindness or other visual impairments.

Contrast Ratio

Measure the relative luminance of each letter (unless they are all uniform) using the formula:

• L = 0.2126 * R + 0.7152 * G + 0.0722 * B where R, G and B are defined as:
  • if RsRGB <= 0.03928 then R = RsRGB/12.92 else R = ((RsRGB+0.055)/1.055) ^ 2.4
  • if GsRGB <= 0.03928 then G = GsRGB/12.92 else G = ((GsRGB+0.055)/1.055) ^ 2.4
  • if BsRGB <= 0.03928 then B = BsRGB/12.92 else B = ((BsRGB+0.055)/1.055) ^ 2.4
  • and RsRGB, GsRGB, and BsRGB are defined as:
  • RsRGB = R8bit/255
  • GsRGB = G8bit/255
  • BsRGB = B8bit/255

The "^" character is the exponentiation operator.

Note: For aliased letters, use the relative luminance value found two pixels in from the edge of the letter.

Colour Brightness Formula

Colour brightness is determined by the following formula:

((Red value X 299) + (Green value X 587) + (Blue value X 114)) / 1000

The difference between the background brightness, and the foreground brightness should be greater than 125.

Note: This algorithm is taken from a formula for converting RGB values to YIQ values. This brightness value gives a perceived brightness for a colour.

Colour Difference Formula

Colour difference is determined by the following formula:

(maximum (Red value 1, Red value 2) - minimum (Red value 1, Red value 2)) + (maximum (Green value 1, Green value 2) - minimum (Green value 1, Green value 2)) + (maximum (Blue value 1, Blue value 2) - minimum (Blue value 1, Blue value 2))

The difference between the background colour and the foreground colour should be greater than 500.

Uses

The Contrast Analyser (CA) is useful to help determine, in particular, the legibility of text on a web page and the legibility of image based representations of text.

Assessing conformance with Guideline 1.4: Make it easier for people with disabilities to see and hear content including separating foreground from background, of WCAG 2 (May 2007 draft)

Guideline 1.4.2: Contrast (Minimum)- WCAG 2 (May 2007 draft) Text (and images of text) have a contrast ratio of at least 5:1, except if the text is pure decoration. Larger scale text or images of text can have a contrast ratio of 3:1. (Level AA) How to meet 1.4.2

Guideline 1.4.4: Contrast (Enhanced) - WCAG 2 (May 2007 draft) Text (and images of text) have a contrast ratio of at least 7:1, except if the text is pure decoration. Larger scale text or images of text can have a contrast ratio of 5:1. (Level AAA)

Assessing conformance with Checkpoint 2.2 of the Web Content Accessibility Guidelines 1.0

Checkpoint 2.2 of the WCAG 1.0 requires that "foreground and background colour combinations provide sufficient contrast when viewed by someone having colour deficits, or when viewed on a black and white screen."

Use the CCA to test colour combinations against the W3C's suggested algorithm for determining "sufficient contrast".

Using the CA

The documentation is in the process of being written, for the most part, using the CA is similar to the Luminosity Contrast Ratio Analyser and CCA 1.1 refer to using the CCA 1.1

The main differences are:

• The implementation of the updated contrast ratio algorithm
• The interface now has options to show and hide, the colour sliders, and result details.
• the colour picker sample size can be modified from 1 pixel to 8 x 8 pixels

Terms of Use Notice:

The Contrast Analyser 2.0 is Freeware.

By accessing or using the Contrast Analyser 2.0, you acknowledge that you have read, understood and agree to be bound by the Contrast Analyser 2.0 Terms of Use.

You may use the Contrast Analyser 2.0 Software (the "Software") for your personal or commercial use to help you determine the accessibility of your Web content.

THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED WARRANTY. IN PARTICULAR, THE WEB ACCESSIBILITY TOOLS CONSORTIUM (WAT-C) DOES NOT MAKE ANY REPRESENTATION ORWARRANTY OF ANY KIND CONCERNING THE RELIABILITY, QUALITY, OR MERCHANTABILITY OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR PURPOSE. ADDITIONALLY, WAT-C DOES NOT GUARANTEE THAT USE OF THIS SOFTWAREWILL ENSURE THE ACCESSIBILITY OF YOUR WEB CONTENT OR THAT YOUR WEB CONTENT WILL COMPLY WITH ANY SPECIFIC WEB ACCESSIBILITY STANDARD.

We value all feedback received. Please contact us about any functionality that may not work or other suggestions for improvement by emailing sfaulkner@paciellogroup.com.

This work is licenced under a Creative Commons Licence

Download and Installation Instructions

Please Note:

System Requirements

• Tested on Windows 2000, XP and Vista