Doc: Improve QTransform documentation

* Explain QTransform's model of vector/matrix operations.
* Improve snippets used to illuminate QTransform's behavior.

Fixes: QTBUG-83869
Pick-to: 6.2 6.1 5.15
Change-Id: I84c3b1a221c139ee992f82c3ee4aebadeef8ee63
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

2 files changed, 10 insertions, 12 deletions

diff --git a/src/gui/doc/snippets/code/src_gui_painting_qtransform.cpp b/src/gui/doc/snippets/code/src_gui_painting_qtransform.cpp
index 23d55febfe..97b9ee8af9 100644
--- a/src/gui/doc/snippets/code/src_gui_painting_qtransform.cpp
+++ b/src/gui/doc/snippets/code/src_gui_painting_qtransform.cpp
@@ -54,7 +54,7 @@ namespace src_gui_painting_qtransform {
 //! [0]
 x' = m11*x + m21*y + dx
 y' = m22*y + m12*x + dy
-if (is not affine) {
+if (!isAffine()) {
     w' = m13*x + m23*y + m33
     x' /= w'
     y' /= w'
@@ -65,7 +65,7 @@ if (is not affine) {
 //! [1]
 x' = m11*x + m21*y + dx
 y' = m22*y + m12*x + dy
-if (is not affine) {
+if (!isAffine()) {
     w' = m13*x + m23*y + m33
     x' /= w'
     y' /= w'
@@ -76,7 +76,7 @@ if (is not affine) {
 //! [2]
 x' = m11*x + m21*y + dx
 y' = m22*y + m12*x + dy
-if (is not affine) {
+if (!isAffine()) {
     w' = m13*x + m23*y + m33
     x' /= w'
     y' /= w'
@@ -87,7 +87,7 @@ if (is not affine) {
 //! [3]
 x' = m11*x + m21*y + dx
 y' = m22*y + m12*x + dy
-if (is not affine) {
+if (!isAffine()) {
     w' = m13*x + m23*y + m33
     x' /= w'
     y' /= w'
diff --git a/src/gui/doc/snippets/transform/main.cpp b/src/gui/doc/snippets/transform/main.cpp
index f3b31b0f83..d15b059020 100644
--- a/src/gui/doc/snippets/transform/main.cpp
+++ b/src/gui/doc/snippets/transform/main.cpp
@@ -48,6 +48,7 @@
 **
 ****************************************************************************/
 #include <QApplication>
+#include <QMath>
 #include <QPainter>
 #include <QVBoxLayout>
 #include <QWidget>
@@ -105,18 +106,15 @@ class BasicOperations : public QWidget
 //! [2]
 void BasicOperations::paintEvent(QPaintEvent *)
 {
-    double pi = 3.14;
-
-    double a    = pi/180 * 45.0;
+    const double a = qDegreesToRadians(45.0);
     double sina = sin(a);
     double cosa = cos(a);
 
-    QTransform translationTransform(1, 0, 0, 1, 50.0, 50.0);
-    QTransform rotationTransform(cosa, sina, -sina, cosa, 0, 0);
-    QTransform scalingTransform(0.5, 0, 0, 1.0, 0, 0);
+    QTransform scale(0.5, 0, 0, 1.0, 0, 0);
+    QTransform rotate(cosa, sina, -sina, cosa, 0, 0);
+    QTransform translate(1, 0, 0, 1, 50.0, 50.0);
 
-    QTransform transform;
-    transform = scalingTransform * rotationTransform * translationTransform;
+    QTransform transform = scale * rotate * translate;
 
     QPainter painter(this);
     painter.setPen(QPen(Qt::blue, 1, Qt::DashLine));