Revision as of 08:28, 24 February 2012

Drawing masked bitmaps using a monochrome (1 bit per pixel) bitmap as a mask may produce incorrect output on S60 3rd Edition, Feature Pack 2 devices.

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How to reproduce

Create or load a bitmap (CFbsBitmap) with the same colour depth as the screen device (screen display mode, typically EColor16MU) and an accompanying monochrome bitmap (display mode EGray2) with the identical size.

Draw the bitmap using a masked blit as follows:

 void CMyContainerControl::Draw( const TRect& aRect ) const
   {
   CWindowGc& gc = SystemGc();
   TRect rect( Rect() );
   gc.BitBltMasked(rect.iTl, iBitmapEColor16MU, rect, iMaskEGray2, EFalse);
   }

On some devices, such as the Nokia N85, the monochrome mask does not work correctly but displays vertical stripes instead.

Solution

This problem does not occur when using masks with the colour depth identical to the parent bitmap. However, this consumes significantly more RAM memory compared to using 1-bpp monochrome bitmaps.

However, inverting the monochrome mask seems to produce correct results:

   gc.BitBltMasked(rect.iTl, iBitmapEColor16MU, rect, iMaskEGray2, '''ETrue''');

As a workaround for monochrome bitmaps, each mask could be inverted and drawn with the 5th parameter of BitBltMasked() (aInvertMask) set to ETrue.