each_channel decorator

[Steven Silvester]

Tony,

How about a third option, computing on each channel in LAB space?

elif rgb_behavior == 'lab':
    lab = color.rgb2lab(image)
    c_new = [image_filter(c) for c in np.rollaxis(lab, -1)]
    out = np.rollaxis(np.array(c_new), 0, 3)
    out = color.lab2rgb(out)

I tried this with "edges", and it produced a pretty interesting image.


On Wednesday, October 17, 2012 8:48:58 PM UTC-5, Tony S Yu wrote:
>
>
>
> On Wed, Oct 17, 2012 at 4:37 PM, Stéfan van der Walt <ste... at sun.ac.za<javascript:>
> > wrote:
>
>> On Wed, Oct 17, 2012 at 7:39 AM, Schönberger Johannes
>> <hannessch... at gmail.com <javascript:>> wrote:
>> > Sounds good. Do you have a paper or any reliable source about this? 
>> Just interested in this...
>>
>> The question is, should we make this a decorator, or simply provide it
>> as utility functions?  I'm wondering, because now we already have two
>> different ways of handling color images: split them up, RGB, or
>> convert them to LAB and apply the filter to LAB.  How do we expose
>> both of these sensible alternatives to the user?
>>
>> We can have a *default*, so that all single-channel filters are done
>> on the luminance layer, but then combine that with utility functions.
>> E.g.
>>
>> @luminance_filter
>> def gray_filter(image, ...):
>>     ...
>>
>> gray_filter(color_image, params) --> convert to LAB, filter on L, convert 
>> back
>> filter_layers(color_image, gray_filter, params) -> filter each layer
>> separately and combine
>>
>> Alternatively, skip the decorators completely, and just provide two
>> utility functions: filter_layers and filter_luminance.
>>
>
>
> I'm not really sure how a utility function would work. Wouldn't you need 
> to provide at least 2 utility functions when converting to LAB:
>
>     image, ab = prepare_rgb(image)
>     # ... code to generate `filtered_image` from `image`
>     filtered_image = finalize_rgb(filtered_image, ab)
>
> Here, `ab` would be some dummy value if the input image is already gray. 
> This is actually a bit cryptic in order to prevent special-casing for RGB 
> images.  Otherwise, you'd have to add some conditionals as well. And don't 
> even know how the layer-by-layer approach would work as a utility function.
>
> Basically, that's all to say that I think a decorator makes a lot more 
> sense in this case. In fact, you can handle the different behaviors 
> (layer-by-layer vs lightness channel) by either introducing a parameter to 
> decorator or the wrapped filter (or both---the decorator parameter would 
> set the default, which you could override at runtime). Here's a mock up 
> adding a parameter to the filter:
>
>     https://gist.github.com/3909400
>
> The edge filter example is a bit strange: if you filter by lightness 
> (Oops, I called it "luminance" in the example), then you get weird results.
>
> -Tony
>
>  
>
>
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