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Re: Chrome color contrast analyzer Pixel radius

for

From: Kelly Lupo
Date: Jan 11, 2016 6:28PM

Thank you!

Kelly

On Mon, Jan 11, 2016 at 5:33 AM, Chaals McCathie Nevile <
<EMAIL REMOVED> > wrote:

> So here is a description:
>
> if you put some bold black text on a white background, and select a pixel,
> it might be mostly surrounded by black, with some dark-grey pixels added to
> curves.
>
> This means the algorithm might see several black pixels nest to dark grey,
> and point out that this isn't enough contrast.
>
> But if you look at the average around a set of pixels, e.g. in a
> several-pixel radius, the black and dark-grey combined *do* give adequate
> contrast to the white, even with the odd light-grey pixel that is similarly
> added to make things look smoother.
>
> And regarding the algorithm itself:
>
> A bit of this depends on precisely how they select the pixels to check,
> and what the checking algorithm is. If you just move from the selected
> pixel until you meet pixels of a different colour, without calculating how
> they *look*, you might include the dark grey in the background. That would
> lead you to conclude there isn't enough contrast.
>
> Instead, it should be included in calculating the average foreground
> colour. Likewise, the light grey should be included in calculating the
> average background colour, rather than being used on its own. Having more
> pixels included makes the average better in general, except if two letters
> are very close together, or you check *too many* pixels, it breaks down.
>
> For example, where there is only one white pixel between the top
> right-hand point of an "r" and the top left-hand point of an "n" in a font
> that makes the strokes several pixels wide, it will be hard for people to
> pick the difference between "rn" and "m". On the other hand, if you select
> too many pixels from the middle of an "m", you might calculate that most
> are black, because you included the side strokes as part of the background
> colur.
>
> If you get a graphics program, but some text in using a large bold font,
> and then zoom to see individual pixels you'll normally see the way
> different colours are used to make it seem smoother.
>
> cheers
>
>
> On Sun, 10 Jan 2016 19:38:20 +0100, Kelly Lupo < <EMAIL REMOVED> >
> wrote:
>
> I don't think the pictures came through... :(
>>
>> Kelly
>>
>> On Sun, Jan 10, 2016 at 6:22 AM, Kevin Prince < <EMAIL REMOVED> >
>> wrote:
>>
>> If you have a block colour then pixel radius 1 is as good as any other
>>> however text is often presented in a smoothed manner by shading the
>>> absolute colour with lighter ones. This makes it both appear less blocky
>>> but also less intense. If you use 2 or 3 radius you'll get an average
>>> value
>>> which will be closer to the effective contrast between foreground and
>>> background.
>>>
>>> Here's a pic I prepared for explaining just this scenario. It's showing a
>>> spot colour of 83,83,83 but if you move the block just 1 pixel to the
>>> left
>>> you are still in the letter form but the average value will be less and
>>> the
>>> overall contrast reduced.
>>>
>>>
>>> Access1in5
>>> 0212220638
>>> 039290692
>>> Independent Accessibility and IT Consultancy.
>>>
>>>
>>>
>>> > On 5/01/2016, at 06:11, Alan Zaitchik < <EMAIL REMOVED> > wrote:
>>> >
>>> > Anyone know the answer to this one?
>>> > Using the Google Chrome color contrast test there is an option to set
>>> the "pixel radius" to 1, 2, or 3.
>>> > Anyone know which value to use under which circumstances? I have pages
>>> that fail with radius 1 but pass with radius 2.
>>> > Is this bad or not?
>>> > A
>>> >
>>> > >>> > >>> > >>> > >>>
>>> >>> >>> >>> >>>
>>> >> >> >> >>
>
>
> --
> Charles McCathie Nevile - web standards - CTO Office, Yandex
> <EMAIL REMOVED> - - - Find more at http://yandex.com
>
> > > > >