From ae2ef9dbf060ba101e32bae2b5edfed979630e30 Mon Sep 17 00:00:00 2001 From: Paul Wicking Date: Thu, 10 Jun 2021 15:41:25 +0200 Subject: Doc: Improve QTransform documentation MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit * 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 Reviewed-by: Topi Reiniö Reviewed-by: Andy Nichols --- src/gui/painting/qtransform.cpp | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) (limited to 'src/gui/painting') diff --git a/src/gui/painting/qtransform.cpp b/src/gui/painting/qtransform.cpp index 8f2dc21a21..7d057732c7 100644 --- a/src/gui/painting/qtransform.cpp +++ b/src/gui/painting/qtransform.cpp @@ -213,6 +213,7 @@ static void nanWarning(const char *func) transformation is achieved by setting both the projection factors and the scaling factors. + \section2 Combining Transforms Here's the combined transformations example using basic matrix operations: @@ -223,6 +224,26 @@ static void nanWarning(const char *func) \snippet transform/main.cpp 2 \endtable + The combined transform first scales each operand, then rotates it, and + finally translates it, just as in the order in which the product of its + factors is written. This means the point to which the transforms are + applied is implicitly multiplied on the left with the transform + to its right. + + \section2 Relation to Matrix Notation + The matrix notation in QTransform is the transpose of a commonly-taught + convention which represents transforms and points as matrices and vectors. + That convention multiplies its matrix on the left and column vector to the + right. In other words, when several transforms are applied to a point, the + right-most matrix acts directly on the vector first. Then the next matrix + to the left acts on the result of the first operation - and so on. As a + result, that convention multiplies the matrices that make up a composite + transform in the reverse of the order in QTransform, as you can see in + \l {Combining Transforms}. Transposing the matrices, and combining them to + the right of a row vector that represents the point, lets the matrices of + transforms appear, in their product, in the order in which we think of the + transforms being applied to the point. + \sa QPainter, {Coordinate System}, {painting/affine}{Affine Transformations Example}, {Transformations Example} */ -- cgit v1.2.3