使用MITab切图

    一般的切图都是基于GIS平台，这种方式编程简单，但效率不高。通过MITab在内存中切图，可极大的提高效率。实现切图的步骤：

1、空间对象的读取。

2、空间对象的分块。

3、空间对象在GDI中的映射。

4、空间对象在GDI中的绘制。

    我们重温下使用MITab读取MapInfo的TAB文件代码：

'/************************************************************************/
'/*                              ReadFile                              */
'/************************************************************************/
Sub ReadFile(ByVal pszFilename As String)
    Dim dataset As Long = mitab_c_open(pszFilename)

    If (dataset = 0) Then
      sTmp = Space$(255)
      i1 = mitab_c_getlasterrormsg_vb(sTmp, 255)
      MsgBox ("mitab_c_open: " & pszFilename & " failed." & vbCrLf & Left$(sTmp, i1))
      Exit Sub
    End If

    feature_id As Long = mitab_c_next_feature_id(dataset, -1)
    Do While feature_id <> -1
'/* -------------------------------------------------------------------- */
'/*      Read next feature object                                        */
'/* -------------------------------------------------------------------- */
      feature = mitab_c_read_feature(dataset, feature_id)
      If (feature = 0) Then
        sTmp = Space$(255)
        i1 = mitab_c_getlasterrormsg_vb(sTmp, 255)
        MsgBox ("Failed to read feature " & Str$(feature_id) & vbCrLf & Left$(sTmp, i1))
        Exit Sub
      End If

      feature_type = mitab_c_get_type(feature)
      num_parts = mitab_c_get_parts(feature)
'/* -------------------------------------------------------------------- */
'/*      ... and coordinates.                                            */
'/*      In real applications, we would probably want to handle each     */
'/*      object type differently but we won't do it here.                */
'/* -------------------------------------------------------------------- */
      For partno = 0 To num_parts - 1
        num_points = mitab_c_get_vertex_count(feature, partno)
        For pointno = 0 To num_points - 1
          dX = mitab_c_get_vertex_x(feature, partno, pointno)
          dY = mitab_c_get_vertex_y(feature, partno, pointno)
        Next
      Next

      mitab_c_destroy_feature (feature)
      feature_id = mitab_c_next_feature_id(dataset, feature_id)
    Loop

    mitab_c_close (dataset)

End Sub

    地图的分块可以参考谷歌地图的金字塔模型的地图瓦片技术。

    映射根据地图所在块的大小和图像尺寸进行换算，如：(X坐标 - 空间对象所在块左上角X坐标) / 块大小 * 图像大小, (空间对象所在块左上角Y坐标 - Y坐标) / 块大小 * 图像大小)。其中空间坐标系的Y坐标向上为正，而图像坐标系的Y坐标向下为正。

    绘制代码：

'区域
Sub DrawPolygon(ByVal feature As Long, ByVal partno As Long, ByVal block As Rectangle, ByVal g As Graphics)
    Dim num_points As Long = mitab_c_get_vertex_count(feature, partno)
    Dim ps(0 To num_points - 1) As Drawing.PointF
    For pointno As Long = 0 To num_points - 1
          dX = mitab_c_get_vertex_x(feature, partno, pointno)
          dY = mitab_c_get_vertex_y(feature, partno, pointno)
          ps(i) = New PointF((dX - block.Left) / block.Width * 256, (block.Top - dY) / block.Height * 256)
    Next
    Dim fc As Color = Color.FromArgb(mitab_c_get_brush_fgcolor(feature))
    fc = Color.FromArgb(fc.R, fc.G, fc.B)
    Dim bc As Color = Color.FromArgb(mitab_c_get_brush_bgcolor(feature))
    bc = Color.FromArgb(bc.R, bc.G, bc.B)
    g.FillPolygon(New Drawing.SolidBrush(fc), ps)
    Dim c As Color = Color.FromArgb(mitab_c_get_pen_color(feature))
    c = Color.FromArgb(c.R, c.G, c.B)
    g.DrawPolygon(New Drawing.Pen(c, mitab_c_get_pen_width(feature)), ps)
End Sub

'线
Sub DrawLine(ByVal feature As Long, ByVal partno As Long, ByVal block As Rectangle, ByVal g As Graphics)
    Dim num_points As Long = mitab_c_get_vertex_count(feature, partno)
    Dim ps(0 To num_points - 1) As Drawing.PointF
    For pointno As Long = 0 To num_points - 1
          dX = mitab_c_get_vertex_x(feature, partno, pointno)
          dY = mitab_c_get_vertex_y(feature, partno, pointno)
          ps(i) = New PointF((dX - block.Left) / block.Width * 256, (block.Top - dY) / block.Height * 256)
    Next
    Dim c As Color = Color.FromArgb(mitab_c_get_pen_color(feature))
    c = Color.FromArgb(c.R, c.G, c.B)
    g.DrawLines(New Drawing.Pen(c, mitab_c_get_pen_width(feature)), ps)
End Sub