expo: Support object alignment

Add support for left, right and centred alignment for text, in the
horizontal dimension.

Also support top, bottom and centred in the vertical dimension, for the
text object as a whole.

Alignment is not yet implemented for images. It has no meaning for
menus. A textline object uses a text object internally, so alignment
is supported there.

Provide some documentation to explain how objects are positioned.

Signed-off-by: Simon Glass <[email protected]>

1 files changed, 31 insertions, 0 deletions

diff --git a/doc/develop/expo.rst b/doc/develop/expo.rst
index 8f63ccbe3ef..d6fc487e030 100644
--- a/doc/develop/expo.rst
+++ b/doc/develop/expo.rst
@@ -107,6 +107,37 @@ refer to objects which have been created. So a menu item is just a collection
 of IDs of text and image objects. When adding a menu item you must create these
 objects first, then create the menu item, passing in the relevant IDs.
 
+Position and alignment
+~~~~~~~~~~~~~~~~~~~~~~
+
+Objects are typically positioned automatically, when scene_arrange() is called.
+However it is possible to position objects manually. The scene_obj_set_pos()
+sets the coordinates of the top left of the object.
+
+All objects have a bounding box. Typically this is calculated by looking at the
+object contents, in `scene_calc_arrange()`. The calculated dimensions of each
+object are stored in the object's `dims` field.
+
+It is possible to adjust the size of an object with `scene_obj_set_size()` or
+even set the bounding box, with `scene_obj_set_bbox()`. The `SCENEOF_SIZE_VALID`
+flag tracks whether the width/height should be maintained when the position
+changes.
+
+If the bounding box is larger than the object needs, the object can be aligned
+to different edges within the box. Objects can be left- or right-aligned,
+or centred. For text objects this applies to each line of text. Normally objects
+are drawn starting at the top of their bounding box, but they can be aligned
+vertically to the bottom, or centred vertically within the box.
+
+Where the width of a text object's bounding box is smaller than the space needed
+to show the next, the text is word-wrapped onto multiple lines, assuming there
+is enough vertical space. Newline characters in the next cause a new line to be
+started. The measurement information is created by the Truetype console driver
+and stored in an alist in `struct scene_txt_generic`.
+
+When the object is drawn the `ofs` field indicates the x and y offset to use,
+from the top left of the bounding box. These values are affected by alignment.
+
 Creating an expo
 ----------------