[PATCH v9 1/5] libcamera: geometry: Clarify Rectangle's top-left corner

[Jacopo Mondi]

The libcamera::Rectangle class allows defining rectangles regardless of
the orientation of the reference system where a rectangle is used in.

This implies that, depending on the reference system in use, the
rectangle's top-left corner, as defined by libcamera, doesn't correspond
to the visual top-left position.

         ^
         |
         |      -------------------
         |      ^                 | h
         |      |                 |
        y|      o---->-------------
         |               w
          ------------------------------->
         (0,0)  x

         (0,0)  x
           ------------------------------>
          |              w
         y|     o---->-------------
          |     |                 | h
          |     v                 |
          |     -------------------
          |
          V

Clarify that a Rectangle's top-left corner corresponds to the point
with the smaller x and y coordinates and that the horizontal and
vertical dimensions are obtained by positive increments along the
corresponding axes.

Signed-off-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
---
 src/libcamera/geometry.cpp | 93 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)

diff --git a/src/libcamera/geometry.cpp b/src/libcamera/geometry.cpp
index 000151364c7f..6eb432e5d803 100644
--- a/src/libcamera/geometry.cpp
+++ b/src/libcamera/geometry.cpp
@@ -595,6 +595,88 @@ std::ostream &operator<<(std::ostream &out, const SizeRange &sr)
  * Rectangles are used to identify an area of an image. They are specified by
  * the coordinates of top-left corner and their horizontal and vertical size.
  *
+ * libcamera canonically defines a rectangle's 'top-left' corner as the point
+ * with the smaller 'x' and smaller 'y' coordinates. Depending on the reference
+ * system where the rectangle is used this might not correspond to the visual
+ * top-left corner of the rectangle.
+ *
+ * The rectangle's horizontal and vertical dimensions are obtained by positively
+ * increments along the corresponding axes by the given horizontal and vertical
+ * sizes.
+ *
+ * Examples:
+ *
+ * \verbatim
+
+  X = top-left corner
+  o = reference system origin point
+
+          ^
+          |
+          |      -----------------
+          |     ^                 |
+          |     |                 |
+          |     X--->-------------
+          |
+          o------------------------------->
+         (0,0)
+
+
+                                      (0,0)
+          <-------------------------------o
+                                          |
+                 -------------<---X       |
+                |                 |       |
+                |                 V       |
+                 -----------------        |
+                                          V
+
+
+         (0,0)
+          o------------------------------->
+          |     X--->-------------
+          |     |                 |
+          |     v                 |
+          |      -----------------
+          |
+          V
+
+                                          ^
+                                          |
+                 -----------------        |
+                |                 ^       |
+                |                 |       |
+                 -------------<---X       |
+          <-------------------------------o
+                                      (0,0)
+
+                         ^
+                         |
+                   ------|-----------
+                  ^      |           |
+                  |      |           |
+                  X--->--|-----------
+          ---------------o---------------->
+                         |(0,0)
+                         |
+                         |
+                         |
+                         |
+
+                         ^
+                         |
+                         |
+                         |
+                         |
+                         |
+          ---------------o---------------->
+                    (0,0)|   ------------
+                         |  ^            |
+                         |  |            |
+                         |  X--->--------
+                         |
+   \endverbatim
+ *
  * The measure unit of the rectangle coordinates and size, as well as the
  * reference point from which the Rectangle::x and Rectangle::y displacements
  * refers to, are defined by the context were rectangle is used.
@@ -611,6 +693,8 @@ std::ostream &operator<<(std::ostream &out, const SizeRange &sr)
  * \param[in] x The horizontal coordinate of the top-left corner
  * \param[in] y The vertical coordinate of the top-left corner
  * \param[in] size The size
+ *
+ * The rectangle's top-left corner is the point with the smaller x and y values.
  */
 
 /**
@@ -620,6 +704,8 @@ std::ostream &operator<<(std::ostream &out, const SizeRange &sr)
  * \param[in] y The vertical coordinate of the top-left corner
  * \param[in] width The width
  * \param[in] height The height
+ *
+ * The rectangle's top-left corner is the point with the smaller x and y values.
  */
 
 /**
@@ -632,11 +718,15 @@ std::ostream &operator<<(std::ostream &out, const SizeRange &sr)
 /**
  * \var Rectangle::x
  * \brief The horizontal coordinate of the rectangle's top-left corner
+ *
+ * The rectangle's top-left corner is the point with the smaller x and y values.
  */
 
 /**
  * \var Rectangle::y
  * \brief The vertical coordinate of the rectangle's top-left corner
+ *
+ * The rectangle's top-left corner is the point with the smaller x and y values.
  */
 
 /**
@@ -685,6 +775,9 @@ Point Rectangle::center() const
 /**
  * \fn Point Rectangle::topLeft() const
  * \brief Retrieve the coordinates of the top left corner of this Rectangle
+ *
+ * The rectangle's top-left corner is the point with the smaller x and y values.
+ *
  * \return The Rectangle's top left corner
  */
 
-- 
2.46.1