2 Understanding the painting process
4 This file tries to describe the state of the metrics/painting
5 mechanism, and identify the improvements that could be made. The first
6 sections can be read alone, although the context for them is really
7 given in the following ones.
9 Please keep this file up to date as the code evolves!!!
18 These questions are consequences of the description made in the
19 following section. Some actions are proposed.
23 This flag only has an effect in the current BufferView, but I think it
24 is useful in other views too. Doing this will require some work on the
25 update pipeline, though.
28 ** Buffer::change issues
30 When calling Buffer::changed outside of bv::processUpdateFlags,
31 how do we know that the update strategy is set correctly? It is
32 possible to reset the strategy at the end of bv::draw. What would be
33 a good value? NoScreenUpdate?
35 On a related note, what is the semantics of a call to
36 Buffer::changed(false)? What does the caller mean?
39 ** How to avoid redraw with FitCursor when the cursor is already OK?
41 In this case, we invoke Buffer::change(false) with drawing disabled
42 and NoScreenUpdate strategy.
44 In the draw phase, bv::checkCursorScrollOffset (the horizontal
45 scrolling machinery) will change the strategy to FullScreenUpdate if
46 the current row needs further scrolling.
48 When the update strategy it kept to NoScreenUpdate, there is currently
49 a no-draw full repaint, which should not be necessary. It would be
50 possible to avoid that if the call to checkCursorScrollOffset was done
51 in bv::processUpdateFlags instead of bv::draw.
53 The global idea would be to extend FitCursor to cover also horizontal
57 * TODO Clean-up of drawing code
59 ** Set Row::changed() in a finer way
63 When the height of the current paragraph changes, there is no need for
64 a full screen update (at top level, at least). Only the rows after the
65 current one need to have their position recomputed.
70 It should be possible to check whether the new row is the same as the
71 old one and keep its changed() status in this case. This would reduce
72 a lot the amount of stuff to redraw.
75 ** Put labels and friends in the Row as elements
77 It should not be necessary to access the Paragraph object to draw.
78 Adding the static elements to Row is a lot of work, but worth it IMO.
81 ** When a paragraph ends with a newline, compute correctly the height of the extra row.
84 ** Merging bv::updateMetrics and tm::metrics
86 While the full metrics computation tries hard to limit the number of
87 paragraphs that are rebroken, the version that is used for inner inset
88 does not try any such optimization. This can be very bad for very tall
89 insets. We should re-use the bv::updateMetrics logic:
90 + transfer all the logic of bv::updateMetrics to tm.
91 + Main InsetText should not be special.
93 The difficulty for a tall table cell for example, is that it may be
94 necessary to break the whole contents to know the width of the cell.
95 Also, the anchor is relative to the outer paragraph, which means that
96 for a very long inset it is necessary to rebreak until the contents
97 that needs to be shown (to compute the heights).
99 All in all, this is difficult to get right. This is less important now
100 that SinglePar updates work in nested insets.
103 * Description of current drawing mechanism
105 ** Three-stage drawing
107 There are three parts to drawing the work area:
109 + the metrics phase computes the size of insets and breaks the
110 paragraphs into rows. It stores the dimension of insets (both
111 normal and math) in bv::coordCache and the vertical position of the
112 top-level paragraphs.
114 + the nodraw drawing phase paints the screen (see below) with a null
115 painter. The only useful effect is to store the positions of
118 + an update() signal is sent. This in turn will trigger a paint
119 event, and the actual screen painting will happen then.
121 The machinery is controlled via bv::processUpdateFlags. This method is
122 called at the end of bv::mouseEventDispatch and in
123 GuiApplication::dispatch, via the updateCurrentView method. There are
124 also several calls in code related to dialogs. We should evaluate
125 whether this is correct.
127 Depending on the Update::flags passed to the method, it sets an update
128 strategy in (NoScreenUpdate, SingleParUpdate, FullScreenUpdate,
129 DecorationUpdate). It triggers a call to updateMetrics when either:
131 + Update::Force has been specified
132 + Update::FitCursor has been specified and there is a need to scroll
134 + Update::SinglePar has been specified and the current paragraph has
137 If a computation of metrics has taken place, Force is removed from the
138 flags and ForceDraw is added instead.
140 It is OK to call processUpdateFlags several times before an update. In
141 this case, the effects are cumulative. processUpdateFlags executes the
142 metrics-related actions, but defers the actual drawing to the next
145 The screen is drawn (with appropriate update strategy), except when
146 update flag is Update::None.
149 ** Metrics computation (and nodraw drawing phase)
151 This is done bv::updateMetrics. When the parameter 'force' of this
152 method is true, that first thing that is done is to clear the metrics
153 caches to start from a clean slate.
155 Then, starting upwards then downwards from the anchor paragraph
156 (anchor_pit_) and its vertical position (anchor_ypos_),
157 tm::updateMetrics every visible paragraph whose metrics is not know
158 (all of them when force==true) is recomputed using tm::redoParagraph.
160 tm::redoParagraph will call Inset::metrics for each inset. In the case
161 of text insets, this will invoke recursively tm::metrics, which redoes
162 all the paragraphs of the inset. Then, a single big row is created in
163 tm::tokenizeParagraph, which is later broken in multiple rows by
166 If it turns out that the top or bottom margin are incorrect (paragraphs
167 are too high/low), tm::updateMetrics will be called again with fixed
168 values of anchor_ypos_ (this does not incur much extra work).
170 At the end of bv::updateMetrics, bv::updatePosCache is called. It triggers
171 a repaint of the document with a NullPainter (a painter that does
172 nothing). This has the effect of caching all insets positions.
174 This way of working means that scrolling can be achieved by just
175 updating anchor_ypos_ and calling bv::processUpdateFlags(Update::ForceDraw).
177 ** Drawing the work area.
179 This is done in bv::draw. This method is triggered by a paint event,
180 mainly called through Buffer::changed, which draws all the work areas
181 that show the given buffer.
183 Note that, When Buffer::changed is called outside of
184 bv::processUpdateFlags, it is not clear whether the update strategy
185 has been set to a reasonable value beforehand.
187 The action depends on the update strategy:
189 + NoScreenUpdate: repaint the whole screen with drawing disabled.
190 This is only needed to update the inset positions. I am not sure
191 when this is necessary, actually. This is triggered when:
192 - Update::FitCursor is set but the cursor is already visible. It is
193 not clear why something needs to be done in this case, since this
194 should be equivalent to Update::None.
195 - this is also set when update flag is Update::None, but this value
196 is AFAICS not acted on in the code (meaning that nothing happens
197 at all in this case).
199 + FullScreenUpdate: repaint the whole screen. This is set when:
200 - updateMetrics has been invoked.
201 - scroll value of current row has changed (although this should not
204 + DecorationUpdate: actually like FullScreenUpdate, although it is
205 intended to only update inset decorations. This triggers when:
206 - Update::Decoration is set (without Update::Force) as flag.
207 - An hovered inset is detected.
209 + SingleParUpdate: only tries to repaint current paragraph in a way
210 that is not yet very clear to me.
212 BufferView::draw can also be called with a null painter from
213 BufferView::updateMetrics().