From fb4dc9a9cf91c455351567bd186cc18513636c7e Mon Sep 17 00:00:00 2001
From: Vlad Zagorodniy <vladzzag@gmail.com>
Date: Wed, 8 Aug 2018 12:46:21 +0300
Subject: [PATCH] [effects/sheet] Fix undesired perspective distortion

Summary:
If a modal window is near some of screen edges, it will be distorted
in undesired way when it's being animated.

In order to keep perspective distortions invariant, no matter where
the modal window is on the screen, we have to move that modal window
to the origin, scale it, rotate it, translate it, apply perspective projection,
and then move it back.

Test Plan:
* Opened Kate
* Opened "Open File" dialog (during the in animation, it was distorted as expected)
* Closed that dialog (during the out animation, it was distorted as expected)

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14687
---
 effects/sheet/sheet.cpp | 37 ++++++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/effects/sheet/sheet.cpp b/effects/sheet/sheet.cpp
index f40350bb1f..525e87da4f 100644
--- a/effects/sheet/sheet.cpp
+++ b/effects/sheet/sheet.cpp
@@ -85,14 +85,37 @@ void SheetEffect::prePaintWindow(EffectWindow* w, WindowPrePaintData& data, int
 void SheetEffect::paintWindow(EffectWindow* w, int mask, QRegion region, WindowPaintData& data)
 {
     InfoMap::const_iterator info = windows.constFind(w);
-    if (info != windows.constEnd()) {
-        const double progress = info->timeLine->currentValue();
-        QGraphicsRotation rot;
-        data.setRotationAxis(Qt::XAxis);
-        data.setRotationAngle(60.0 * (1.0 - progress));
-        data *= QVector3D(1.0, progress, progress);
-        data.translate(0.0, - (w->y() - info->parentY) * (1.0 - progress));
+    if (info == windows.constEnd()) {
+        effects->paintWindow(w, mask, region, data);
+        return;
     }
+
+    // Perspective projection distorts objects near edges of the viewport
+    // in undesired way. To fix this, the center of the window will be
+    // moved to the origin, after applying perspective projection, the
+    // center is moved back to its "original" projected position. Overall,
+    // this is how the window will be transformed:
+    //  [move to the origin] -> [scale] -> [rotate] -> [translate] ->
+    //    -> [perspective projection] -> [reverse "move to the origin"]
+    const QMatrix4x4 oldProjMatrix = data.screenProjectionMatrix();
+    const QRectF windowGeo = w->geometry();
+    const QVector3D invOffset = oldProjMatrix.map(QVector3D(windowGeo.center()));
+    QMatrix4x4 invOffsetMatrix;
+    invOffsetMatrix.translate(invOffset.x(), invOffset.y());
+    data.setProjectionMatrix(invOffsetMatrix * oldProjMatrix);
+
+    // Move the center of the window to the origin.
+    const QRectF screenGeo = effects->virtualScreenGeometry();
+    const QPointF offset = screenGeo.center() - windowGeo.center();
+    data.translate(offset.x(), offset.y());
+
+    const double progress = info->timeLine->currentValue();
+    QGraphicsRotation rot;
+    data.setRotationAxis(Qt::XAxis);
+    data.setRotationAngle(60.0 * (1.0 - progress));
+    data *= QVector3D(1.0, progress, progress);
+    data.translate(0.0, - (w->y() - info->parentY) * (1.0 - progress));
+
     effects->paintWindow(w, mask, region, data);
 }