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598f7e4a45
* remove optional arguments to the helpers that use a FontInfo * add a textwidth argument to the text() methods that are used by rowpainter. Now textwidth is only computed if a null value was passed to the text() method. This means that in the use case of rowpainter, no textwidth needs to be computed.
252 lines
8.9 KiB
Org Mode
252 lines
8.9 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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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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* Clean-up of drawing code
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The goal is to make painting with drawing disable fast enough that it
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can be used after every metrics computation. Then we can separate real
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drawing from metrics.
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** DONE RowPainter
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Inset position is set in paintInset, paintOnlyInsets, and paintText.
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This should be done only once in paintInset
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** DONE TextMetrics::drawParagraph
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We can really simplify the code when drawing is disabled only
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paintInset needs to be called. + do right at the start when drawing is already disabled
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+ do it in the loop for rows that are not visible on screen.
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The only thing we want to do here is to set inset positions (for
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text). The other insets still use the painter with drawing disabled.
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** DONE Painter::text
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We cannot remove (or make private) the version that uses a
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FontInfo because it is used by PainterInfo::draw. Document this and
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remove unused arguments rtl and double spacing. This would become a specialized helper.
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Proposed solution: keep the existing function, but private and without
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optional arguments.
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Avoid to return (and thus compute) the width of strings? + used by InsetSpecialChar (fixable)
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+ used by textDecoration() in text(): more difficult to fix
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Idea: add a version of text where wordspacing and textwidth (giving
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the width of strings) are required parameters and remove optional
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version.
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==> more versions, no optional parameters.
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** make Inset::display() more useful
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Extending the DisplayType enum would allow to remove special cases
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from the code.
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The enumeration could be like
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: Inline = 0
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: BreakBefore = 1 // break row before this inset
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: BreakAfter = 2 // break row after this inset
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: CanBreakAfter = 4 // optionally break row after this inset
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: AlignLeft = 8
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: AlignRight = 16
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: NoBoundary = 32 // do not allow cursor to go at the end of the row
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: //before display inset
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: Display = BreakBefore|BreakAfter
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A display equation would be Display, other could be Display|AlignLeft
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BreakAfter can be used by Newline or separator insets
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CanBreakAfter can be used by the optional hyphen InsetSpecialChar.
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** Set inset position during metrics phase
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In order to do that, a no-paint drawing will be initiated after every
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redoParagraph. This code path will need to be made as fast as possible.
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Effect: avoid depending on actual drawing having taken place. In turn,
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it will allow to do drawing on paint events, like any reasonable
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application would do.
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** Cleanup after complete metrics
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Then the following can be done: + remove hack in InsetMathNest::drawSelection
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+ remove painting when not inside in drawParagraph + remove Cursor::inCoordCache?
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** Use Row for MathData
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It may not be so difficult. Implement x2pos and pos2x from
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the TM:cursorX and TM::getPosNearX, and use them for both text and
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math.
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Will the strings display OK if drawing string-wise?
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Then it would be possible to streamline drawing with disabled painter.
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** Paint directly to screen
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Instead of using an intermediary pixmap. I have no idea of how
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difficult it will prove.
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One benefit will be that subpixel aliasing will work again (#9972)
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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:
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+ transfer all the logic of bv::updateMetrics to tm. + Main InsetText should not be special.
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The difficuly 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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** Two stage 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. + 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 + 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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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 + all visible paragraphs
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+ paragraph above the screen (up to one page) + 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). + 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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