filling shapes with patterns in CImg

Question

I want to be able to draw shapes and flood-fill them with various fill patterns (diagonal lines, stipple dots, etc).

The CImg library includes functions for drawing various shapes with an arbitrary line pattern for the outline. But I don't see anything about fill pattern.

I think this can probably be done using a bitwise or mathematical operator to mask the pattern onto a solid image, but I'd like to see the specific code for doing it.

Answer 1

Yes, polygon fill patterns can be achieved by first drawing the polygon in a solid color and then using the &= operator with a pre-loaded black-and-white pattern image.

// preload several pattern images (black and white versions of the desired fill patterns)
CImg<unsigned char> *fillPatternImages[ NumFillPatterns ] = { NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL };
fillPatternImages[ 0 ] = new CImg<unsigned char>( "256x256_bw_dotted_fill.png" );
... etc. for all patterns you want to use

// create an empty image
CImg<unsigned char> image( 256, 256, 1, 4, 0 );

// draw the polygon (or in the case of my code, any number of polygons) on the image in a solid color
if ( nShapeType == SHPT_POLYGON && fillPattern != FILL_PATTERN_NONE )
{
  for( int i = 0 ; i < nShapeCount ; i++ )
  {
    SHPObject *psShape;
    psShape = SHPReadObject( hSHP, panHits[ i ] );

    for ( int part = 0 ; part < psShape->nParts ; part++ )
    {
      int numPoints;
      if ( part < ( psShape->nParts - 1 ) )
      {
        numPoints = psShape->panPartStart[ part + 1 ] - psShape->panPartStart[ part ];
      }
      else
      {
        numPoints = psShape->nVertices - psShape->panPartStart[ part ];
      }
      CImg<int> pointImage( numPoints, 2, 1, 1, 0 );
      int s = psShape->panPartStart[ part ];
      for ( int p = 0 ; p < numPoints ; p++ )
      {
        int screenX;
        int screenY;
        GetTileXYFromMercatorLonLat( (float)psShape->padfX[ s + p ], (float)psShape->padfY[ s + p ], x, y, z, &screenX, &screenY );
          pointImage( p, 0 ) = screenX;
          pointImage( p, 1 ) = screenY;
      }
      image.draw_polygon( pointImage, fillColor );
    }

    SHPDestroyObject( psShape );
  }
}

// to achieve a non-solid pattern, & the image with a pre-loaded pattern image
if ( fillPattern > -1 )
{
  image &= *fillPatternImages[ fillPattern ];
}