Re: Brightness filter in CSS Filters spec is wrong?

On Jan 26, 2012, at 9:25 AM, Rik Cabanier wrote:

> Hi Chris,
> 
> I got a reply from the Photoshop team:
> Photoshop has two algorithms for brightness / contrast, depending on whether you turn on the "legacy" checkbox. The legacy approach is the old naïve version of just sliding the histogram right and left or spreading it out linearly, per channel.
> 
>  
> In legacy mode, 
> 
> Brightness n adds or subtracts n to / from each channel, clamping at 0 or 255
> 
> Contrast spreads or compresses the histogram linearly for each channel. Contrast +100 makes all channel values 0 or 255. Contrast –100 makes all channel values 127. Contrast –100 and Brightness +100 makes all channels 227.
> 
>  
> These operations can be useful for steps in analysis of images, or for manipulating masks or in some cases for manipulating graphics, but they tend to be horrible for photographic images, and so the first advice in every Photoshop book for adjusting brightness and contrast on photographs used to be "never use brightness and contrast". 
> 
>  
> The non-legacy B&C adjustments copied Lightroom (camera raw)'s original process version. They're curve-based and produce much better results on photographic images. 
> 
> 
> 
> 
> 
> 
> It sounds like 'legacy' mode in Photoshop matches the filters spec but it doesn't produce good results.
> I think we know have 3 options:
> - use the current spec. Output is not very good + Brighten doesn't go to white
> - use the flash approach. Output is not very good but brighten goes to white.
> - get an improved algorithm, possibly based on what existing editing software does.

Well, legacy mode sounds like my proposed change to the brightness filter. The current brightness filter multiplies the amount by rgb. My proposal adds it, which sounds like what the legacy mode of photoshop does. That is what I've implemented in WebKit now and it gives you the results you expect. I'm not sure which of these you're talking about (1 or 2)? 

I'd be concerned about developing a new algorithm. I understand that editing software has more advanced, curve based approaches. But I'd be concerned going down that path because we could easily get ratholed. What curves do we use? How do we accelerate it? We removed gamma from the spec for similar reasons. Hardware accelerating non-linear transforms is difficult. It seems reasonable to let more advanced editing be done offline and keep the CSS filters simple.


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~Chris
cmarrin@apple.com