Read a GIF image's dimensions

[Giraffe69]

I need to be able to read an image's dimensions in VB6, but am struggling with the GIF format.

Its easy to do for a bitmap by directly reading the image header part of the file, and can do similar for a JPEG with the help of Intel's JPEG Library.

From code that writes GIF it appears that an image header:

Private Type GifImageDescriptor
     Left As Integer
     Top As Integer
     Width As Integer
     Height As Integer
     Format As Byte 'ImageFormat
End Type

is written to position 783 for an '87 GIF and 791 for an '89 GIF. The code I've written:

Private Function GetGIFDimensions(ByVal strGIFFile As String, ByRef intGIFWidth As Integer, ByRef intGIFHeight As Integer) As Boolean
    Dim intGFileNo As Integer, strGReadHead  As String * 5
    On Error GoTo errGetGIFDimensions
    intGFileNo = FreeFile
    Open strGIFFile For Binary Access Read Lock Read As #intGFileNo
        Get #intGFileNo, 1&, strGReadHead
        If strGReadHead = "GIF87" Then
            Get #intGFileNo, 787&, intGIFWidth
            Get #intGFileNo, 789&, intGIFHeight
            GetGIFDimensions = True
        ElseIf strGReadHead = "GIF89" Then
            Get #intGFileNo, 795&, intGIFWidth
            Get #intGFileNo, 797&, intGIFHeight
            GetGIFDimensions = True
        End If
    Close #intGFileNo
errGetGIFDimensions:
    On Error GoTo 0
    Reset
End Function

doesnt read the correct values.

I can do a workaround by using a control, loadpicture, autosize, scaleheight etc., but that's too slow when working with thousands of images.

Any advice would be really appreciated!

"We've all heard that a million monkeys banging on a million typewriters will eventually reproduce the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true." -- Robert Wilensky

 

[strongm]

Option Explicit

' Shows filename and dimensions of all gif files in named folder
Public Sub Example(Optional strFolder As String = "C:\")
Dim myShell As Shell32.Shell
Dim myFolderItems As FolderItems3
Dim myFile As ShellFolderItem ' This is the important declaration to get the extended properties later

Set myShell = New Shell32.Shell
Set myFolderItems = myShell.NameSpace(strFolder).Items
myFolderItems.Filter 64, "*.gif"

For Each myFile In myFolderItems
'Do your thing here. This example uses a message box
MsgBox myFile & vbCrLf & "Dimensions: " & myFile.ExtendedProperty("dimensions")
Next
End Sub

 

[Giraffe69]

Thanks strongm,
that looks brilliant, and so simple - I'm embarrassed!
sorry for being thick, but how do I reference Shell32?
thanks
G

"We've all heard that a million monkeys banging on a million typewriters will eventually reproduce the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true." -- Robert Wilensky

 

[strongm]

Sorry - that's the problem with putting an example together too quickly. What you need to dfo is add a reference to the Microsoft Shell Controls and Automation library

 

[Giraffe69]

works a treat! - thanks

"We've all heard that a million monkeys banging on a million typewriters will eventually reproduce the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true." -- Robert Wilensky