[Development] Image and PreserveAspectCrop

[Albert Astals Cid]

Ping anyone?

On Mon, Jul 18, 2016 at 10:24 AM, Albert Astals Cid
<albert.astals at canonical.com> wrote:
> There is a problem when trying to optimally[*] show an Image with
> PreserveAspectCrop fillMode.
> [*]optimally => as best looking as possible while using as litte
> memory as possible
>
> You can see that problem in the screenshot at http://i.imgur.com/LSSlFEB.png
> that corresponds with the code at http://paste.ubuntu.com/19480453/
>
> As you can see when displaying a landscape (width > height) image in a
> square Image Item the optimal way
> is to set the source size height only, but if the image is portrait
> (height > width) then the optimal way
> is to set the source size width only.
>
> The requirement my program has is to have the best rendering quality
> and memory usage for Image Items using PreserveAspectCrop.
> Image sources are totally arbitrary, they can be from disk, from the
> internet or even from QQuickImageProviders
> (since we are plugin based and plugins can bring their own QQuickImageProvider)
>
> This can be fixed in several ways.
>
> Workaround A
> **********
> Changing the Image Item source size comparing the aspect ratio of the
> image file with the one Image Item.
>
> You can see an implementation of that workaround at
> http://bazaar.launchpad.net/~unity-team/unity8/trunk/view/head:/plugins/Dash/CroppedImageMinimumSourceSize.qml
>
> The problem with this workaround is that half of the times you end up
> loading the image a second time.
> This means extra CPU and potentially network usage.
>
>
>
> Workaround B
> **********
> Implementing your own image provider that does compare the aspect
> ratios before loading the image.
>
> You can see a partial implementation of this workaround at
> https://code.launchpad.net/~aacid/unity8/croppedImageMinimumSourceSizeProvider/+merge/300176
>
> There are two problems with this workaround:
>  * You end up implementing quite a bit of duplicated functionality
> from qquickpixmapcache.cpp
>  * For the chained image providers (i.e. the original source was an
> image provider url) you
>    still have to query the image provider twice half of the times
>
>
>
> Solution
> ********
> Implementing the change in QtQuick internals so that when
> PreserveAspectCrop fillMode is used
> together with a sourceSize that has both width and height it does
> return the optimal image
>
> You can see a work in progress implementation of this solution at
> https://codereview.qt-project.org/#/c/165299/
> And how the previews could would look at
> http://i.imgur.com/NRoXNzy.png (notice how the last column now is good
> in both cases)
>
> There are two issues with this solution:
>  * It's a small behaviour change (but in my opinion for the better)
>  * Needs new api for the QQuickImageProvider to be able to implement
> it, so we either need the proposed
>    QQuickImageProviderV2 or with a new "bool
> shouldPreserveAspectRatioCrop(url, requestSize)" getter in the
>    existing QQuickImageProvider API
>
>
>
> All in all I think the solution i propose for QtQuick is acceptable
> but i would like some agreeing that is fine adding new API before
> finishing the patch.