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GDAL VB6 Bindings Tutorial

# Introduction

A partial set of Visual Basic 6 bindings have been build for GDAL. Internally these bindings use Declare based calls into the GDAL DLL C API but a set of shadow classes are also provided to provide object oriented access to GDAL services in VB6 similar to those provided in C++.

Note that the VB6 bindings are nowhere near comprehensive, nor are they documented. However, in combination with the corresponding C++ class documentation, and the following docs, it should be possible to use GDAL to accomplish a variety of operations. It is not believed that the VB6 bindings will be of any utility with earlier version of VB nor with VB.Net.

The classes for which access has been implemented includes GDALDriver, GDALDataset, GDALRasterBand, GDALColorTable, OGRSpatialReference and OGRCoordinateTransformation.

A mailing list specifically on VB6 GDAL topics has been setup at http://groups.yahoo.com/group/gdal-vb6-appdev .

# Using GDAL VB6 Classes

To use VB6 GDAL bindings it is necessary to ensure that GDAL has been built with appropriate C entry points exported using the "stdcall" calling convention. This is the current default, but was not as recently as GDAL 1.2.6. So ensure you get a version more recent than 1.2.6.

Then add the GDAL VB6 class and module files to your VB6 project. These come from the gdal/vb6 directory and include the following key files:

- GDAL.bas - The main user visible module.
- GDALCore.bas - This module is for internal use.
- GDALDriver.cls - The GDALDriver class.
- GDALDataset.cls - The GDALDataset class.
- GDALRasterBand.cls - The GDALRasterBand class.
- GDALColorTable.cls - The GDALColorTable class.
- OGRSpatialReference.cls - The OGRSpatialReference class.
- OGRCoordinateTransformation.cls - The OGRCoordinateTransformation class.

You may need to edit GDALCore.bas, and change occurrences of gdal12.dll to match what your GDAL DLL is called. You can include a full path to the DLL if it can't be guaranteed to be in the current working directory of the application (or the windows system32 directory).

You should also be able to load the "test" project from the gdal\vb6\test directory. The test project has test menu items roughly corresponding to the tasks in the following tutorial topics.

# Tutorial - Read Dataset

This brief tutorial will demonstrate open a GDAL file, and fetching out some information, about the dataset, and the individual bands. The results are printed to the default from in the following example for simplicity.

Before opening the file we need to register the GDAL format drivers. Normally we will just register all the drivers with GDALAllRegister().

Call GDAL.AllRegister()

Then we need to try and open the dataset. The GDAL.OpenDS() function returns a GDALDataset object, so we dimension an appropriate object for this. GDAL.OpenDS() is the VB6 equivalent of the GDALDataset::GDALOpen() function.

Dim ds As GDALDataset
Set ds = GDAL.OpenDS( "utm.tif", GDAL.GA_ReadOnly )

Then we need to check if the open succeeded, and if not report an error.

If not ds.IsValid() Then
Call MsgBox( "Open failed: " & GDAL.GetLastErrorMsg() )
Exit Sub
End If

If things succeeded, we query width of the image in pixels (XSize), Height of the image in pixels (YSize) and number of bands (BandCount) from the dataset properties.

Print "Size: " & ds.XSize & "x" & ds.YSize & "x" & ds.BandCount

Next we read metadata from the dataset using the VB6 equivalent of the GDALMajorObject::GetMetadata() method, and report it to the user. Metadata is returned as an array of strings of "name=value" items. Array indices start at zero in the returned array. The domain argument should normally be vbNullString though in specialized circumstances other domains might apply.

Dim MD As Variant
MD = ds.GetMetadata(vbNullString)
If (UBound(MD) > 0) Then
Print "Metadata:"
For i = 1 To UBound(MD)
Print " " & MD(i)
Next i
End If

Parsing the "name=value" strings from GetMetadata() can be a bit of a bother, so if we were looking for specific values we could use GetMetadataItem() and provide a specific item we want to extract. This would extract just the value if it is found, or an empty string otherwise. The GetMetadataItem() is an analog of the C++ GDALMajorObject::GetMetadataItem() method.

Dim MDValue As String
MDValue = ds.GetMetadataItem( "TIFF_DATETIME", vbNullString )
if MDValue <> "" Then
Print "Creation Date: " & MDValue
End If

The GDALDataset::GetGeoTransform() method is used to get fetch the affine transformation used to relate pixel/line locations on the image to georeferenced locations in the current coordinate system. In the most common case (image is not rotated or sheared) you can just report the origin (upper left corner) and pixel size from these values. The method returns 0 on success or an error class if it fails, so we only use the return result (placed into the Geotransform array) on success.

Dim Geotransform(6) As Double
If ds.GetGeoTransform( Geotransform ) = 0 Then
If Geotransform(2) = 0 and Geotransform(4) = 0 Then
Print "Origin: " & Geotransform(0) & "," & Geotransform(3)
Print "Pixel Size: " & Geotransform(1) & "x" & (-1 * Geotransform(5))
End If
End If

The coordinate system can be fetched using the GDALDataset::GetProjectionRef() analog, GDALDataset.GetProjection(). The returned string is in OpenGIS Well Known Text format. A later example will show how to use an OGRSpatialReference object to reformat the WKT into more readable format and make other use of it.

Dim WKT As String
WKT = ds.GetProjection()
If Len(WKT) > 0 Then
Print "Projection: " & WKT
End If

GDALDataset objects have one or more raster bands associated with them. GDALRasterBand objects can have metadata (accessed the same as on the GDALDataset) as well as an array of pixel values, and various specialized metadata items like data type, color interpretation, offset/scale. Here we report a few of the items.

First we loop over all the bands, fetching a band object for each band and report the band number, and block size.

For i = 1 To ds.BandCount
Dim band As GDALRasterBand
Set band = ds.GetRasterBand(i)
Print "Band " & i & " BlockSize: " & band.BlockXSize & "x" & band.BlockYSize

The GDALRasterBand has a DataType property which has the value returned by the C++ method GDALRasterBand::GetRasterDataType(). The returned value is an integer, but may be compared to the predefined constants GDAL.GDT_Byte, GDAL.GDT_UInt16, GDAL.GDT_Int16, GDAL.GDT_UInt32, GDAL.GDT_Int32, GDAL.GDT_Float32, GDAL.GDT_Float64, GDAL.GDT_CInt16, GDAL.GDT_CInt32, GDAL.GDT_CFloat32 and GDAL.GDT_CFloat64. In this case we use the GDAL.GetDataTypeName() method to convert the data type into a name we can show the user.

Print " DataType=" & GDAL.GetDataTypeName(band.DataType) _

We also report the offset, scale, minimum and maximum for the band.

Print " Offset=" & band.GetOffset() & " Scale=" & band.GetScale() _
& " Min=" & band.GetMinimum() & " Max=" & band.GetMaximum()

GDALRasterBands can also have GDALColorTable objects associated with them. They are read with the GDALRasterBand::GetColorTable() analog in VB6. Individual RGBA entries should be read into a 4 Integer array.

Dim ct As GDALColorTable
Set ct = band.GetColorTable()
If ct.IsValid() Then
Dim CEntry(4) As Integer
Print " Has Color Table, " & ct.EntryCount & " entries"
For iColor = 0 To ct.EntryCount - 1
Call ct.GetColorEntryAsRGB(iColor, CEntry)
Print " " & iColor & ": " & CEntry(0) & "," & CEntry(1) & "," & CEntry(2) & "," & CEntry(3)
Next iColor
End If

But of course, the most important contents of a GDAL file is the raster pixel values themselves. The C++ GDALRasterBand::RasterIO() method is provided in a somewhat simplified form. A predimensioned 1D or 2D array of type Byte, Int, Long, Float or Double is passed to the RasterIO() method along with the band and window to be read. Internally the "buffer size" and datatype is extracted from the dimensions of the passed in buffer.

This example dimensions the RawData array to be the size of one scanline of data (XSize x 1) and reads the first whole scanline of data from the file, but only prints out the second and tenth values (since the buffer indexes are zero based).

Dim err As Long
Dim RawData() As Double
ReDim RawData(ds.XSize) As Double
err = band.RasterIO(GDAL.GF_Read, 0, 0, ds.XSize, 1, RawData)
if err = 0 Then
Print " Data: " & RawData(1) & " " & RawData(9)
End If

Finally, when done accessing a GDALDataset we can explicitly close it using the CloseDS() method, or just let it fall out of scope in which case it will be closed automatically.

Call ds.CloseDS()

# Tutorial - Creating Files

Next we address creating a new file from an existing file. To create a new file, you have to select a GDALDriver to do the creating. The GDALDriver is essentially an object representing a file format. We fetch it with the GetDriverByName() call from the GDAL module using the driver name.

Dim Drv As GDALDriver
Call GDAL.AllRegister
Drv = GDALCore.GetDriverByName( "GTiff" )
If Not Drv.IsValid() Then
Call MsgBox( "GTiff driver not found " )
Exit Sub
End If

You could get a list of registered drivers, and identify which support creation something like this:

drvCount = GDAL.GetDriverCount
For drvIndex = 0 To drvCount - 1
Set Drv = GDAL.GetDriver(drvIndex)
If Drv.GetMetadataItem(GDAL.DCAP_CREATE, "") = "YES" _
Or Drv.GetMetadataItem(GDAL.DCAP_CREATECOPY, "") = "YES" Then
xMsg = " (Read/Write)"
Else
xMsg = " (ReadOnly)"
End If
Print Drv.GetShortName() & ": " & Drv.GetMetadataItem(GDAL.DMD_LONGNAME, "") & xMsg
Next drvIndex

Once we have the driver object, the simplest way of creating a new file is to use CreateCopy(). This tries to create a copy of the input file in the new format. A complete segment (without any error checking) would look like the following. The CreateCopy() method corresponds to the C++ method GDALDriver::CreateCopy(). The VB6 implementation does not support the use of progress callbacks.

Dim Drv As GDALDriver
Dim SrcDS As GDALDataset, DstDS As GDALDataset
Call GDAL.AllRegister
Set Drv = GDALCore.GetDriverByName( "GTiff" )
Set SrcDS = GDAL.Open( "in.tif", GDAL.GA_ReadOnly )
Set DstDS = Drv.CreateCopy( "out.tif", SrcDS, True, Nothing )

This is nice and simple, but sometimes we need to create a file with more detailed control. So, next we show how to create a file and then copy pieces of data to it "manually". The GDALDriver::Create() analog is Create().

Set DstDS = Drv.Create("out.tif", SrcDS.XSize, SrcDS.YSize, _
SrcDS.BandCount, GDAL.GDT_Byte, Nothing)

In some cases we may want to provide some creation options, which is demonstrated here. Creation options (like metadata set through the SetMetadata() method) are arrays of Strings.

Dim CreateOptions(1) As String
CreateOptions(1) = "PHOTOMETRIC=MINISWHITE"
Set DstDS = Drv.Create("out.tif", SrcDS.XSize, SrcDS.YSize, _
SrcDS.BandCount, GDAL.GDT_Byte, CreateOptions)

When copying the GeoTransform, we take care to check that reading the geotransform actually worked. Most methods which return CPLErr in C++ also return it in VB6. A return value of 0 will indicate success, and non-zero is failure.

Dim err As Long
Dim gt(6) As Double
err = SrcDS.GetGeoTransform(gt)
If err = 0 Then
Call DstDS.SetGeoTransform(gt)
End If

Copy the projection. Even if GetProjection() fails we get an empty string which is safe enough to set on the target. Similarly for metadata.

Call DstDS.SetProjection(SrcDS.GetProjection())
Call DstDS.SetMetadata(SrcDS.GetMetadata(""), "")

Next we loop, processing bands, and copy some common data items.

For iBand = 1 To SrcDS.BandCount
Dim SrcBand As GDALRasterBand, DstBand As GDALRasterBand
Set SrcBand = SrcDS.GetRasterBand(iBand)
Set DstBand = DstDS.GetRasterBand(iBand)
Call DstBand.SetMetadata(SrcBand.GetMetadata(""), "")
Call DstBand.SetOffset(SrcBand.GetOffset())
Call DstBand.SetScale(SrcBand.GetScale())
Dim NoDataValue As Double, Success As Long
NoDataValue = SrcBand.GetNoDataValue(Success)
If Success <> 0 Then
Call DstBand.SetNoDataValue(NoDataValue)
End If

Then, if one is available, we copy the palette.

Dim ct As GDALColorTable
Set ct = SrcBand.GetColorTable()
If ct.IsValid() Then
err = DstBand.SetColorTable(ct)
End If

Finally, the meat and potatoes. We copy the image data. We do this one scanline at a time so that we can support very large images without require large amounts of RAM. Here we use a Double buffer for the scanline, but if we knew in advance the type of the image, we could dimension a buffer of the appropriate type. The RasterIO() method internally knows how to convert pixel data types, so using Double ensures all data types (except for complex) are properly preserved, though at the cost of some extra data conversion internally.

Dim Scanline() As Double, iLine As Long
ReDim Scanline(SrcDS.XSize) As Double
' Copy band raster data.
For iLine = 0 To SrcDS.YSize - 1
Call SrcBand.RasterIO(GDAL.GF_Read, 0, iLine, SrcDS.XSize, 1, _
Scanline)
Call DstBand.RasterIO(GDAL.GF_Write, 0, iLine, SrcDS.XSize, 1, _
Scanline)
Next iLine

# Tutorial - Coordinate Systems and Reprojection

The GDAL VB6 bindings also include limited support for use of the OGRSpatialReference and OGRCoordinateTransformation classes. The OGRSpatialReference represents a coordinate system and can be used to parse, manipulate and form WKT strings, such as those returned by the GDALDataset.GetProjection() method. The OGRCoordinateTransformation class provides a way of reprojecting between two coordinate systems.

The following example shows how to report the corners of an image in georeferenced and geographic (lat/long) coordinates. First, we open the file, and read the geotransform.

Dim ds As GDALDataset
Call GDALCore.GDALAllRegister
Set ds = GDAL.OpenDS(FileDlg.Filename, GDAL.GA_ReadOnly)
If ds.IsValid() Then
Dim Geotransform(6) As Double
Call ds.GetGeoTransform(Geotransform)

Next, we fetch the coordinate system, and if it is non-empty we try to instantiate an OGRSpatialReference from it.

' report projection in pretty format.
Dim WKT As String
Dim srs As New OGRSpatialReference
Dim latlong_srs As OGRSpatialReference
Dim ct As New OGRCoordinateTransformation
WKT = ds.GetProjection()
If Len(WKT) > 0 Then
Print "Projection: "
Call srs.SetFromUserInput(WKT)

If the coordinate system is projected it will have a PROJECTION node. In that case we build a new coordinate system which is the corresponding geographic coordinate system. So for instance if the "srs" was UTM 11 WGS84 then it's corresponding geographic coordinate system would just be WGS84. Once we have these two coordinate systems, we build a transformer to convert between them.

If srs.GetAttrValue("PROJECTION", 0) <> "" Then
Set latlong_srs = srs.CloneGeogCS()
Set ct = GDAL.CreateCoordinateTransformation(srs, latlong_srs)
End If
End If

Next we call a helper function to report each corner, and the center. We pass in the name of the corner, the pixel/line location at the corner, and the geotransform and transformer object.

Call ReportCorner("Top Left ", 0, 0, _
Geotransform, ct)
Call ReportCorner("Top Right ", ds.XSize, 0, _
Geotransform, ct)
Call ReportCorner("Bottom Left ", 0, ds.YSize, _
Geotransform, ct)
Call ReportCorner("Bottom Right ", ds.XSize, ds.YSize, _
Geotransform, ct)
Call ReportCorner("Center ", ds.XSize / 2#, ds.YSize / 2#, _
Geotransform, ct)

The ReportCorner subroutine starts by computing the corresponding georeferenced x and y location using the pixel/line coordinates and the geotransform.

Private Sub ReportCorner(CornerName As String, pixel As Double, line As Double, _
gt() As Double, ct As OGRCoordinateTransformation)
Dim geox As Double, geoy As Double
geox = gt(0) + pixel * gt(1) + line * gt(2)
geoy = gt(3) + pixel * gt(4) + line * gt(5)

Next, if we have a transformer, we use it to compute a corresponding latitude and longitude.

Dim longitude As Double, latitude As Double, Z As Double
Dim latlong_valid As Boolean
latlong_valid = False
If ct.IsValid() Then
Z = 0
longitude = geox
latitude = geoy
latlong_valid = ct.TransformOne(longitude, latitude, Z)
End If

Then we report the corner location in georeferenced, and if we have it geographic coordinates.

If latlong_valid Then
Print CornerName & geox & "," & geoy & " " & longitude & "," & latitude
Else
Print CornerName & geox & "," & geoy
End If
End Sub

$Id: vb6_tutorial.dox 13528 2008-01-14 19:37:42Z warmerdam $

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