[Interest] QImage detect full transparent

[Xavier Bigand]

I think that I forgot to tell that it can be faster to do a pseudo random
search than a pure straight forward brut-force lookup in the alpha channel
buffer.
Depending on type of pictures you will analyse, it might be faster to have
an algorithm that do something like the quick search algorithm.
For exemple just consider the alpha channel as one dimensional array, and
put a pivot point at the half of the array, each pivot point should be
tested and considered as a end and start of 2 new chunks of array. Then
continue to add pivot points at the half of chunks,...
You may also consider to multi-thread a such algorithm with a map reduce,
take a look to https://doc.qt.io/qt-5.10/qtconcurrent-index.html.


2018-02-25 21:27 GMT+01:00 Xavier Bigand <flamaros.xavier at gmail.com>:

> A complete scan will always be relatively slow, if you can you should
> consider to do a cache or something similar.
> Depending of your constraints the cache should be created while
> packaging/building your application or when the user will request the info
> the first time.
>
> The cacheKey() won't work in my opinion because their should be have no
> guaranty that rgb components are at 0 too if the alpha channel is at 0, so
> their is many different images (with different keys) that can be fully
> transparent.
>
> 2018-02-25 20:23 GMT+01:00 maitai <maitai at virtual-winds.org>:
>
>> Hello,
>> Seems quite basic, but I need a **fast** way to know if a QImage is just
>> full transparent. I know the size in advance (256x256). If it were a
>> QPixmap I would use cacheKey() and compare with the cacheKey of a full
>> transparent image. For a QImage I can compare it with a preset transparent
>> image, or extract the alpha channel, or etc of course. But I need a really
>> fast way.
>>
>> Any idea?
>>
>> Thanks
>> Philippe.
>> _______________________________________________
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>> Interest at qt-project.org
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>>
>
>
>
> --
> Xavier
>



-- 
Xavier
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