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This is a work in progress intended to start collective work towards improving the performance of our painting process. The intent is to make it a living document that is updated as code evolves.
160 lines
5.8 KiB
Plaintext
160 lines
5.8 KiB
Plaintext
# -*- org-mode -*-
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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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** Inset cache
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Why do we store inset dimensions both in bv::coordCache and in
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pm::insetDimension? The later one is bad when same
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buffer is shown in different views.
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I propose to get rid of pm::insetDimension.
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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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** Two phase drawing
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Why is it necessary to set the inset positions in the drawing phase?
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It seems to me that everything can be done in the metrics phase (at
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least for non-math insets).
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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 now 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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** Metrics outside of visible area
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Why is it required to compute metrics on page above and below the
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visible area? Couldn't this be done on demand? I suspect this could be
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made transparent by doing it in the proper metrics-fetching method.
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** What happens 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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which means that the paint machinery is invoked to update inset
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positions. Why is this necessary at all?
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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. How difficult would it be to re-use the bv::updateMetrics logic?
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* Two phase drawing
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There are two parts to drawing the work area:
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+ 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, and the size of normal
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insets in pm::insetDimension.
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+ the drawing phase draws the contents and caches the inset
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positions. Since the caching of positions is useful in itself,
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there is a provision for drawing "without" drawing when the only
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thing we want is to cache inset positions
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(Painter::setDrawingEnabled).
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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
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+ 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, has 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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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
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This is triggered by bv::updateMetrics, which calls tm::redoParagraph for
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+ all visible paragraphs
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+ paragraph above the screen (up to one page)
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+ paragraphs below the screen (up to one page again)
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The paragraphs outside of the screen are required to make PageUp/Down
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work.
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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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** Drawing the work area.
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This is done in bv::draw. This method is triggered mainly by
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Buffer::changed, which draws all the work areas 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:
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+ 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).
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+ 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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