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The goal of this commit is to ensure that a processUpdateFlags call that requires no redraw will not override a previous one that did require a redraw. To this end, the semantics of the flag argument is now different: its value is now OR'ed with a private update_flags_ variable. This variable is only reset after the buffer view has actually been redrawn. A new Update::ForceRedraw flag has been added. It requires a full redraw but no metrics computation. It is not used in the main code (yet), but avoids to compute metrics repeatedly in consecutive processUpdateFlags calls. The process is now as follows: - if flags is just None, return immediately, there is nothing to do. - the Force flag is honored (full metrics computation) and replaced with ForceDraw. - the FitCursor flag is honored and removed from the flags. - the SinglePar update is added if ForceDraw is not in flags and only the current par has been modified. The remaining flags are only then added to the BufferView update flags, and the update strategy is computed for the next paint event. Finally the dubious call to updateMacros in updateMetrics has been removed for performance reasons.
173 lines
6.5 KiB
Org Mode
173 lines
6.5 KiB
Org Mode
# -*- org -*-
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Understanding the painting process
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This file tries to describe the state of the metrics/painting
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mechanism, and identify the improvements that could be made. The first
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section can be read alone, although the context for them is really
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given in the following ones.
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Please keep this file up to date as the code evolves!!!
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Abbreviations:
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bv: BufferView
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pm: ParagraphMetrics
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tm::TextMetrics
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* Questions / Ideas
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These questions are consequences of the description made in the
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following section. Some actions are proposed.
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** SinglePar update
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The flag Update::SinglePar is set in many places but never acted on.
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Instead, metrics update is skipped whenever the recorded height of
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current paragraph did not change and Update::Force was not specified.
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Is it better to keep that (which incurs extra work) or to condition it
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on Update::SinglePar? If the first solution is kept, the flag
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SingleParUpdate shall be removed.
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Moreover, I fail to see (yet) where the 'single' part of the program
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is acted on.
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** Buffer::change issues
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When calling Buffer::changed outside of bv::processUpdateFlags,
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how do we know that the update strategy is set correctly? It is
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possible to reset the strategy at the end of bv::draw. What would be
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a good value? NoScreenUpdate?
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On a related note, what is the semantics of a call to
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Buffer::changed(false)? What does the caller mean?
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** How to avoid redraw with FitCursor when the cursor is already OK?
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In this case, we invoke Buffer::change(false) with drawing disabled
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and NoScreenUpdate strategy.
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In the draw phase, bv::checkCursorScrollOffset (the horizontal
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scrolling machinery) will change the strategy to FullScreenUpdate if
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the current row needs further scrolling.
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When the update strategy it kept to NoScreenUpdate, there is currently
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a no-draw full repaint, which should not be necessary. It would be
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possible to avoid that if the call to checkCursorScrollOffset was done
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in bv::processUpdateFlags instead of bv::draw.
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The global idea would be to extend FitCursor to cover also horizontal
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cursor.
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* Clean-up of drawing code
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** When a paragraph ends with a newline, compute correctly the height of the extra row.
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** Merging bv::updateMetrics and tm::metrics
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While the full metrics computation tries hard to limit the number of
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paragraphs that are rebroken, the version that is used for inner inset
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does not try any such optimization. This can be very bad for very tall
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insets. We should re-use the bv::updateMetrics logic: + transfer all the logic of bv::updateMetrics to tm.
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+ Main InsetText should not be special.
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The difficulty for a tall table cell for example, is that it may be
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necessary to break the whole contents to know the width of the cell.
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* Description of current drawing mechanism
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** Three-stage drawing
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There are three parts to drawing the work area: + the metrics phase computes the size of insets and breaks the
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paragraphs into rows. It stores the dimension of insets (both
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normal and math) in bv::coordCache.
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+ the nodraw drawing phase paints the screen (see below) with a null
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painter. The only useful effect is to store the inset positions. + an update() signal is sent. This in turn will trigger a paint
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event, and the actual screen painting will happen then.
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The machinery is controlled via bv::processUpdateFlags. This method is
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called at the end of bv::mouseEventDispatch and in
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GuiApplication::dispatch, via the updateCurrentView method. There are
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also several calls in code related to dialogs. We should evaluate
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whether this is correct.
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Depending on the Update::flags passed to the method, it sets an update
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strategy in (NoScreenUpdate, SingleParUpdate, FullScreenUpdate,
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DecorationUpdate). It triggers a recomputation of the metrics when either:
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+ Update::Force has been specified + Update::FitCursor has been specified and there is a need to scroll
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the display.
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+ the current paragraph, after rebreak, does not have the same height as in
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existing metrics. Note that the Update::SinglePar flag is *never*
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taken into account.
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If a computation of metrics has taken place, Force is removed from the
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flags and ForceDraw is added instead.
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It is OK to call processUptateFlags several times before an update. In
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this case, the effects are cumulative.processUpdateFlags execute the
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metrics-related actions, but defers the actual drawing to the next
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paint event.
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The screen is drawn (with appropriate update strategy), except when
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update flag is Update::None.
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** Metrics computation (and nodraw drawing phase)
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This is triggered by bv::updateMetrics, which calls tm::redoParagraph for
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all visible paragraphs. Some Paragraphs above or below the screen (needed
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for page up/down) and computed as needed.
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tm::redoParagraph will call Inset::metrics for each inset. In the case
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of text insets, this will invoke recursively tm::metrics, which redoes
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all the paragraphs of the inset.
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At the end of the function, bv::updatePosCache is called. It triggers
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a repaint of the document with a NullPainter (a painter that does
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nothing). This has the effect of caching all insets positions.
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** Drawing the work area.
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This is done in bv::draw. This method is triggered by a paint event,
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mainly called through Buffer::changed, which draws all the work areas
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that show the given buffer.
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Note that, When Buffer::changed is called outside of
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bv::processUpdateFlags, it is not clear whether the update strategy
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has been set to a reasonable value beforehand.
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The action depends on the update strategy: + NoScreenUpdate: repaint the whole screen with drawing disabled.
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This is only needed to update the inset positions. I am not sure
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when this is necessary, actually. This is triggered when:
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- Update::FitCursor is set but the cursor is already visible. It is
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not clear why something needs to be done in this case, since this
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should be equivalent to Update::None.
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- this is also set when update flag is Update::None, but this value
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is AFAICS not acted on in the code (meaning that nothing happens
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at all in this case).
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+ FullScreenUpdate: repaint the whole screen. This is set when:
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- updateMetrics has been invoked.
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- scroll value of current row has changed (although this should not
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be necessary). + DecorationUpdate: actually like FullScreenUpdate, although it is
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intended to only update inset decorations. This triggers when:
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- Update::Decoration is set (without Update::Force) as flag.
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- An hovered inset is detected.
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+ SingleParUpdate: only tries to repaint current paragraph in a way
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that is not yet very clear to me.
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BufferView::draw can also be called with a null painter from
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BufferView::updateMetrics().
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