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26eb5092fb
We already have a CoordCache of insets dimensions. It is not necessary to store the same information in two places. Give a name to CoordCache tables types to improve code readability. Remove ParagraphMetrics::singleWidth, which is not used anymore.
175 lines
6.4 KiB
Org Mode
175 lines
6.4 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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* Proposals
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** set inset position during metrics phase
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This implies to set inset positions relative to outer inset during
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metrics phase and then in a second loop to descend into insets and
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update positions correctly.
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Effect: avoid going through the painter machinery when it is not necessary.
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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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** Metrics outside of visible area
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Currently metrics are computed for current visible paet of text, the
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page above and the page below. It should be possible to compute hidden
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rows ony on demand, although it might be a bit slow.
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There was a proposal to always compute _all_ rows, but this may become
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expensive for large files. This would though help scrolling.
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* Description of current drawing mechanism
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** Two stage drawing
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There are two 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 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: + 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. + 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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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 + 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: + 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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