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192 lines
6.9 KiB
Org Mode
192 lines
6.9 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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This flag only has an effect in the current BufferView, but I think it
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is useful in other views too. Doing this will require some work on the
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update pipeline, though.
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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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** Set Row::changed() in a finer way
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*** singleParUpdate
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When the height of the current paragraph changes, there is no need for
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a full screen update. Only the rows after the current one need to have
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their position recomputed.
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This is also true when scrolling (how to do that?)
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*** redoParagraph
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It should be possible to check whether the new row is the same as the
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old one and keep its changed() status in this case. This would reduce
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a lot the amount of stuff to redraw.
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** Put labels and friends in the Row as elements
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It should not be necessary to access the Paragraph object to draw.
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Adding the static elements to Row is a lot of work, but worth it IMO.
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** do not add the vertical margin of main text to first/last row
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Would make code cleaner. Probably no so difficult.
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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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+ Update::SinglePar has been specified and the current paragraph has
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not changed height.
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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 processUpateFlags several times before an update. In
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this case, the effects are cumulative. processUpdateFlags executes 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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