Fix extrapolation in the backward 4-point interpolator

krita/image/kis_four_point_interpolator_backward.h

@@ -75,15 +75,18 @@ public:
 
         m_qB_varY = y * m_d.x();
         m_qC_varY = y * m_a.x();
+        m_py = y;
     }
 
     inline QPointF getValue() const {
+        static const qreal eps = 1e-10;
+
         qreal qB = m_qB_const + m_qB_varX + m_qB_varY;
         qreal qC = m_qC_varX + m_qC_varY;
 
         qreal nu = 0.0;
 
-        if (qAbs(m_qA) < 1e-10) {
+        if (qAbs(m_qA) < eps) {
             nu = -qC / qB;
         } else {
             qreal D = pow2(qB) - 4 * m_qA * qC;
@@ -104,7 +107,15 @@ public:
             }
         }
 
-        qreal mu = (m_px - nu * m_c.x()) / (m_a.x() + nu * m_d.x());
+        qreal xBasedDenominator = m_a.x() + nu * m_d.x();
+
+        qreal mu;
+
+        if (qAbs(xBasedDenominator) > eps) {
+            mu = (m_px - nu * m_c.x()) / xBasedDenominator;
+        } else {
+            mu = (m_py - nu * m_c.y()) / (m_a.y() + nu * m_d.y());
+        }
 
         return m_srcBase + QPointF(mu * m_xCoeff, nu * m_yCoeff);
     }
@@ -123,6 +134,7 @@ private:
     qreal m_qC_varY; // quadratic equation C coeff, Y-dep part
     qreal m_qD_div; // inverted divisor of the quadratic equation solution
     qreal m_px; // saved relative X coordinate
+    qreal m_py; // saved relative Y coordinate
 
     QPointF m_srcBase;
     QPointF m_dstBase;

krita/image/tests/kis_warp_transform_worker_test.cpp

@@ -285,4 +285,42 @@ void KisWarpTransformWorkerTest::testGridSize()
     QCOMPARE(GridIterationTools::calcGridDimension(0, 300, 8), 39);
 }
 
+void KisWarpTransformWorkerTest::testBackwardInterpolatorExtrapolation()
+{
+    QPolygonF src;
+
+    src << QPointF(0, 0);
+    src << QPointF(100, 0);
+    src << QPointF(100, 100);
+    src << QPointF(0, 100);
+
+    QPolygonF dst(src);
+    std::rotate(dst.begin(), dst.begin() + 1, dst.end());
+    KisFourPointInterpolatorBackward interp(src, dst);
+
+    // standard checks
+    QCOMPARE(interp.map(QPointF(0,0)), QPointF(0,100));
+    QCOMPARE(interp.map(QPointF(100,0)), QPointF(0,0));
+    QCOMPARE(interp.map(QPointF(100,100)), QPointF(100,0));
+    QCOMPARE(interp.map(QPointF(0,100)), QPointF(100,100));
+
+    // extrapolate!
+    QCOMPARE(interp.map(QPointF(-10,0)), QPointF(0,110));
+    QCOMPARE(interp.map(QPointF(0,-10)), QPointF(-10,100));
+    QCOMPARE(interp.map(QPointF(-10,-10)), QPointF(-10,110));
+
+    QCOMPARE(interp.map(QPointF(110,0)), QPointF(0,-10));
+    QCOMPARE(interp.map(QPointF(100,-10)), QPointF(-10,0));
+    QCOMPARE(interp.map(QPointF(110,-10)), QPointF(-10,-10));
+
+    QCOMPARE(interp.map(QPointF(110,100)), QPointF(100, -10));
+    QCOMPARE(interp.map(QPointF(100,110)), QPointF(110, 0));
+    QCOMPARE(interp.map(QPointF(110,110)), QPointF(110,-10));
+
+    QCOMPARE(interp.map(QPointF(-10,100)), QPointF(100, 110));
+    QCOMPARE(interp.map(QPointF(0,110)), QPointF(110, 100));
+    QCOMPARE(interp.map(QPointF(-10,110)), QPointF(110,110));
+}
+
+
 QTEST_KDEMAIN(KisWarpTransformWorkerTest, GUI)

krita/image/tests/kis_warp_transform_worker_test.h

@@ -33,6 +33,7 @@ private slots:
     void testBackwardInterpolatorXYShear();
     void testBackwardInterpolatorRoundTrip();
     void testGridSize();
+    void testBackwardInterpolatorExtrapolation();
 };
 
 #endif /* __KIS_WARP_TRANSFORM_WORKER_TEST_H */