[unity8 shell] Surface snapshot is mirrored

[Gerry Boland]

In order to correctly display the snapshot of a surface, the snapshot needs to be mirrored (i.e. snapshot is upside-down). Here is the code I'm using:

    QImage image;

    app->session()->take_snapshot(
        [&](mir::shell::Snapshot const& snapshot)
        {
            image = QImage( (const uchar*)snapshot.pixels, // since we mirror, no need to offset starting position
                            snapshot.size.width.as_int(),
                            snapshot.size.height.as_int() - yOffset,
                            QImage::Format_ARGB32_Premultiplied).mirrored();
        });

You mentioned you were doing this mirroring in Mir too. Removing it could speed up snapshots a tiny bit more.

Adding bug so it won't be lost.

[kevin gunn (kgunn72)]

alf - is this still a bug ? raising priority just in case.

[Alexandros Frantzis (afrantzis)]

This is not a bug, it's a performance enhancement. Mir gets the pixel rows in bottom-up order from the texture, and has to reverse them to provide the usual top-down view of pixel data. Unity8, however, needs to upload the data to a texture so it needs to reverse the row order again. If Mir exports the data in texture row order, we save two reversals.

Marking this as medium priority, until we get evidence that this creates visible problems on some platform.