tdf#141761 Enhance PrintDialog Preview for FormControls

The display quality of the Preview is pretty ugly when
FormControls are used. I made a deep-dive why this happens,
and in principle the reason is the Mteafile::Scale used
below. Since Metafile actions are integer, that floating point
scale leads to rounduing errors that make the lines painting
the FormControls disappear in the surrounding ClipRegions.
That Scale cannot be avoided since the Metafile contains it's
own SetMapMode commands which *will* be executed on ::Play,
so the ::Scale is the only possibility fr Metafile currently:
Giving a Size as parameter in ::Play will *not* work due to
the relativeMapMode that gets created will fail on
::SetMapMode actions in the Metafile - and FormControls DO
use ::SetMapMode(MapPixel).
This can only be solved better in the future using Primitives
which would allow any scale by embedding to a Transformation,
but that would be a bigger rework.
Until then, use this little 'trick' to improve qulatity.
It uses the fact to empirically having tested that the quality
gets really bad for FormControls starting by a scale factor
smaller than 0.2 - that makes the ClipRegion overlap start.
So - for now - try not to go below that.

Change-Id: I540de602634c6afa697b5659d69c34159c22075c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/114704
Reviewed-by: Armin Le Grand <Armin.Le.Grand@me.com>
Tested-by: Armin Le Grand <Armin.Le.Grand@me.com>
(cherry picked from commit 4722ad2cf3f2b91c217e3548f811f2972f2aa60c)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/114715
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>

1 files changed, 70 insertions, 18 deletions

diff --git a/vcl/source/window/printdlg.cxx b/vcl/source/window/printdlg.cxx
index d37c0ec068bc..8866abe43733 100644
--- a/vcl/source/window/printdlg.cxx
+++ b/vcl/source/window/printdlg.cxx
@@ -351,17 +351,14 @@ void PrintDialog::PrintPreviewWindow::preparePreviewBitmap()
         return;
     }
 
-    // create temporary VDev and render to it
+    // create temporary VDev with requested Size and DPI.
+    // CAUTION: DPI *is* important here - it DIFFRERS from 75x75, usually 600x600 is used
     ScopedVclPtrInstance<VirtualDevice> pPrerenderVDev(*Application::GetDefaultDevice());
     pPrerenderVDev->SetOutputSizePixel(aScaledSize, false);
     pPrerenderVDev->SetReferenceDevice( mnDPIX, mnDPIY );
-    pPrerenderVDev->EnableOutput();
-    pPrerenderVDev->SetBackground( Wallpaper(COL_WHITE) );
 
-    GDIMetaFile aMtf( maMtf );
-
-    Size aVDevSize( pPrerenderVDev->GetOutputSizePixel() );
-    const Size aLogicSize( pPrerenderVDev->PixelToLogic( aVDevSize, MapMode( MapUnit::Map100thMM ) ) );
+    // calculate needed Scale for Metafile (using Size and DPI from VDev)
+    Size aLogicSize( pPrerenderVDev->PixelToLogic( pPrerenderVDev->GetOutputSizePixel(), MapMode( MapUnit::Map100thMM ) ) );
     Size aOrigSize( maOrigSize );
     if( aOrigSize.Width() < 1 )
         aOrigSize.setWidth( aLogicSize.Width() );
@@ -369,30 +366,85 @@ void PrintDialog::PrintPreviewWindow::preparePreviewBitmap()
         aOrigSize.setHeight( aLogicSize.Height() );
     double fScale = double(aLogicSize.Width())/double(aOrigSize.Width());
 
+    // tdf#141761
+    // The display quality of the Preview is pretty ugly when
+    // FormControls are used. I made a deep-dive why this happens,
+    // and in principle the reason is the Mteafile::Scale used
+    // below. Since Metafile actions are integer, that floating point
+    // scale leads to rounduing errors that make the lines painting
+    // the FormControls disappear in the surrounding ClipRegions.
+    // That Scale cannot be avoided since the Metafile contains it's
+    // own SetMapMode commands which *will* be executed on ::Play,
+    // so the ::Scale is the only possibility fr Metafile currently:
+    // Giving a Size as parameter in ::Play will *not* work due to
+    // the relativeMapMode that gets created will fail on
+    // ::SetMapMode actions in the Metafile - and FormControls DO
+    // use ::SetMapMode(MapPixel).
+    // This can only be solved better in the future using Primitives
+    // which would allow any scale by embedding to a Transformation,
+    // but that would be a bigger rework.
+    // Until then, use this little 'trick' to improve qulatity.
+    // It uses the fact to empirically having tested that the quality
+    // gets really bad for FormControls starting by a scale factor
+    // smaller than 0.2 - that makes the ClipRegion overlap start.
+    // So - for now - try not to go below that.
+    static double fMinimumScale(0.2);
+    double fFactor(0.0);
+    if(fScale < fMinimumScale)
+    {
+        fFactor = fMinimumScale / fScale;
+        fScale = fMinimumScale;
+
+        double fWidth(aScaledSize.getWidth() * fFactor);
+        double fHeight(aScaledSize.getHeight() * fFactor);
+        const double fNewNeededPixels(fWidth * fHeight);
+
+        // to not risk using too big bitmaps and runninig into
+        // memory problems, still limit to a useful factor is
+        // necessary, also empirically estimated to
+        // avoid the quality from collapsing (using a direct
+        // in-between , ceil'd result)
+        static double fMaximumQualitySquare(1396221.0);
+
+        if(fNewNeededPixels > fMaximumQualitySquare)
+        {
+            const double fCorrection(fMaximumQualitySquare/fNewNeededPixels);
+            fWidth *= fCorrection;
+            fHeight *= fCorrection;
+            fScale *= fCorrection;
+        }
+
+        const Size aScaledSize2(basegfx::fround(fWidth), basegfx::fround(fHeight));
+        pPrerenderVDev->SetOutputSizePixel(aScaledSize2, false);
+        aLogicSize = pPrerenderVDev->PixelToLogic( aScaledSize2, MapMode( MapUnit::Map100thMM ) );
+    }
+
+    pPrerenderVDev->EnableOutput();
+    pPrerenderVDev->SetBackground( Wallpaper(COL_WHITE) );
     pPrerenderVDev->Erase();
-    pPrerenderVDev->Push();
     pPrerenderVDev->SetMapMode(MapMode(MapUnit::Map100thMM));
-    DrawModeFlags nOldDrawMode = pPrerenderVDev->GetDrawMode();
     if( mbGreyscale )
         pPrerenderVDev->SetDrawMode( pPrerenderVDev->GetDrawMode() |
                                 ( DrawModeFlags::GrayLine | DrawModeFlags::GrayFill | DrawModeFlags::GrayText |
                                   DrawModeFlags::GrayBitmap | DrawModeFlags::GrayGradient ) );
+
+    // Copy, Scale and Paint Metafile
+    GDIMetaFile aMtf( maMtf );
     aMtf.WindStart();
     aMtf.Scale( fScale, fScale );
     aMtf.WindStart();
-
-    const AntialiasingFlags nOriginalAA(pPrerenderVDev->GetAntialiasing());
-    pPrerenderVDev->SetAntialiasing(nOriginalAA | AntialiasingFlags::Enable);
     aMtf.Play( pPrerenderVDev.get(), Point( 0, 0 ), aLogicSize );
-    pPrerenderVDev->SetAntialiasing(nOriginalAA);
-
-    pPrerenderVDev->Pop();
 
     pPrerenderVDev->SetMapMode(MapMode(MapUnit::MapPixel));
+    maPreviewBitmap = pPrerenderVDev->GetBitmapEx(Point(0, 0), pPrerenderVDev->GetOutputSizePixel());
 
-    maPreviewBitmap = pPrerenderVDev->GetBitmapEx(Point(0, 0), aVDevSize);
-
-    pPrerenderVDev->SetDrawMode( nOldDrawMode );
+    if(0.0 != fFactor)
+    {
+        // Correct to needed size, BmpScaleFlag::Interpolate is acceptable,
+        // but BmpScaleFlag::BestQuality is just better. In case of time
+        // constraints, change to Interpolate would be possible
+        maPreviewBitmap.Scale(aScaledSize, BmpScaleFlag::BestQuality);
+    }
 }
 
 PrintDialog::ShowNupOrderWindow::ShowNupOrderWindow()