Fix effective interpolation detection

It used a malformed quad for the source, so Qt always said that there
was no way to transform it to the destination, leading to the raw
interpolation being used every time. Now it detects trivial transforms
properly and returns to nearest-neighbor interpolation as it should to
avoid pointless blur.

ChangeLog

@@ -4,6 +4,7 @@ Unreleased Version 2.2.2-pre
  * Fix: Disregard hidden layers when layer picking in frame view.
  * Feature: Add a search bar to the key frame properties dialog.
  * Fix: Reset locale to "C" after Qt messes it up on startup. Thanks Meru for reporting.
+ * Fix: Don't use bilinear interpolation in transforms unnecessarily.
 
 2024-11-06 Version 2.2.2-beta.4
  * Fix: Solve rendering glitches with selection outlines that happen on some systems. Thanks xxxx for reporting.

src/libclient/canvas/transformmodel.cpp

@@ -230,8 +230,7 @@ int TransformModel::getEffectiveInterpolation(int interpolation) const
 	if(interpolation != DP_MSG_TRANSFORM_REGION_MODE_NEAREST &&
 	   m_dstQuadValid) {
 		QTransform t;
-		if(QTransform::quadToQuad(
-			   QPolygonF(QRectF(m_srcBounds)), m_dstQuad.polygon(), t)) {
+		if(QTransform::quadToQuad(srcPolygon(), m_dstQuad.polygon(), t)) {
 			t.setMatrix(
 				t.m11(), t.m12(), t.m13(), t.m21(), t.m22(), t.m23(), 0.0, 0.0,
 				t.m33());
@@ -779,6 +778,22 @@ bool TransformModel::containsNullMessages(const QVector<net::Message> &msgs)
 	return false;
 }
 
+QPolygonF TransformModel::srcPolygon() const
+{
+	// QRectF::toPolygon returns an excess point, so we can't use that here.
+	// Also, using width and height is correct! Don't use bottom or right.
+	qreal x = m_srcBounds.x();
+	qreal y = m_srcBounds.y();
+	qreal w = m_srcBounds.width();
+	qreal h = m_srcBounds.height();
+	return QPolygonF({
+		QPointF(x, y),
+		QPointF(x + w, y),
+		QPointF(x + w, y + h),
+		QPointF(x, y + h),
+	});
+}
+
 bool TransformModel::isRightAngleRotationOrReflection(const QTransform &t) const
 {
 	QPoint scales[] = {

src/libclient/canvas/transformmodel.h

@@ -118,6 +118,7 @@ class TransformModel : public QObject {
 	QImage convertMaskToMono() const;
 	static bool containsNullMessages(const QVector<net::Message> &msgs);
 
+	QPolygonF srcPolygon() const;
 	bool isRightAngleRotationOrReflection(const QTransform &t) const;
 
 	static bool isQuadValid(const TransformQuad &quad);