Problems with ActiveX: WIA flatbed scanner
lazarus / 34-Windows

----------------------------------------------------------------------
BigChimp · 2014-04-04 10:28

Hi guys,

ActiveX newbie (in Lazarus at least). Using FPC 2.7.1 as well as
2.6.4, have tried to slap together support for scanning from WIA
scanners... and failed.

Project attached, including converted WIA 2 (Vista+) headers.

Code runs into a SIGSEGV on the line indicated:
Code: [Select]procedure TLocalWIAScanner.Scan;
// Adapted from
// http://stackoverflow.com/questions/721948/delphi-twain-issue-help
var
  i:integer;
  Scanner: Device;
  Picture: IItem;
  Image: OleVariant;
  AImage: IImageFile;
  ReturnString: string;
begin
  try
    // List of devices is a 1 based array
    showmessage('number of devices:
'+inttostr(FDevMgr.DeviceInfos.Count));
    for i:=1 to FDevMgr.DeviceInfos.Count do
    begin
      // sigsegv happens in line
below===============================>>>>>>
      ReturnString:=utf8encode(FDevMgr.DeviceInfos[POleVariant(i)].Pro
perties[POleVariant('Name')].get_Value);

I've also tried defining an OLEVariant with value Name in advance and
passing the address to the Properties call but no change.

All suggestions welcome - thanks,
BigChimp

----------------------------------------------------------------------
marcov · 2014-04-04 16:11

I find the polevariant(i) funny. That is typecasting an integer to a
pointer.

Declare an olevariant for i, assign i to it on every iteration, and
then pass that (or the pointer) in the crashing line.

----------------------------------------------------------------------
BigChimp · 2014-04-04 16:21

Well, I found it funny as well  (after some forum searching)
I have also tried this
Code: [Select]var
  i:integer;
  Scanner: Device;
  Picture: IItem;
  Image: OleVariant;
  InVar: OLEVariant;
  AImage: IImageFile;
  ReturnString: string;
begin
  try
    // List of devices is a 1 based array
    showmessage('number of devices:
'+inttostr(FDevMgr.DeviceInfos.Count));
    for i:=1 to FDevMgr.DeviceInfos.Count do
    begin
      InVar:=i;
      // sigsegv happens in line
below===============================>>>>>>
      ReturnString:=utf8encode(FDevMgr.DeviceInfos[@InVar].Properties[
POleVariant('Name')].get_Value);
      showmessage('Device: '+ReturnString);
with a similar error (see attachment)

Perhaps I should also declare a separate OLEVariant for 'Name' and
assign that to pass its address to the Properties call?

----------------------------------------------------------------------
marcov · 2014-04-04 16:45

Quote from: BigChimp on April 04, 2014, 06:21:21 pmPerhaps I should
also declare a separate OLEVariant for 'Name' and assign that to pass
its address to the Properties call?

Yes. Maybe there are shortcuts to do it inline, but in doubt, do it by
the book.

----------------------------------------------------------------------
BigChimp · 2014-04-04 16:54

Thanks.

Ah, discovering the book as we go along

This works better - corrected Type and Name capitalization:
Code: [Select]var
  i:integer;
  Scanner: Device;
  Picture: IItem;
  Image: OleVariant;
  InVar: OLEVariant;
  StringVar: OleVariant;
  OutVar: OLEVariant;
  AImage: IImageFile;
  ReturnString: string;
begin
  try
    // List of devices is a 1 based array
    showmessage('number of devices:
'+inttostr(FDevMgr.DeviceInfos.Count));

    for i:=1 to FDevMgr.DeviceInfos.Count do
    begin
      InVar:=i;
      // Only check scanners (ignore cameras etc)
      StringVar:='Type';
     
OutVar:=FDevMgr.DeviceInfos[@InVar].Properties[@StringVar].get_Value;
//Const ScannerDeviceType = $0000000000000001;
      if OutVar=ScannerDeviceType then
      begin
        StringVar:='Name';
        ReturnString:=utf8encode(FDevMgr.DeviceInfos[@InVar].Propertie
s[@StringVar].get_Value);
        showmessage('Device: '+ReturnString);
      end
      else
      begin
        showmessage('found a device but it is not a scanner');
       
OutVar:=FDevMgr.DeviceInfos[@InVar].Properties[@StringVar].get_Value;
        showmessage('got device type '+utf8encode(OutVar));
      end;
    end;
However,  if OutVar=ScannerDeviceType then goes wrong - first device
is a scanner but not detected as such: OutVar contains 65537 when
evaluated as VINTEGER in the debugger - what is the "by the book"
conversion to compare to a constant? Convert it to word or something?
Sounds a bit arbitrary...

----------------------------------------------------------------------
marcov · 2014-04-04 17:06

Quote from: BigChimp on April 04, 2014, 06:54:13 pm //Const
ScannerDeviceType = $0000000000000001;
 
However,  if OutVar=ScannerDeviceType then goes wrong - first device
is a scanner but not detected as such: OutVar contains 65537 when
evaluated as VINTEGER in the debugger - what is the "by the book"
conversion to compare to a constant? Convert it to word or something?
Sounds a bit arbitrary...

65537 mod (2**16)=1. IOW it sounds like a 16-bits value is interpreted
as a 32-bit value leading to an extra bit? Or maybe only the lower
bits of the value are the scanner type?

----------------------------------------------------------------------
BigChimp · 2014-04-05 10:22

Thanks for the help so far!

Well, the importer defined the const as longword (4 byte) value...
MSDN doesn't mention the type is used for anything else so I wouldn't
expect to need to only consider the lower bits...

http://msdn.microsoft.com/en-
us/library/windows/desktop/ms630792%28v=vs.85%29.aspx

Forcing the result to be seen as a word value did work
Code: [Select]      // Apparently need to force result to 4 bytes to
compare to constant:
      if word(OutVar)=ScannerDeviceType then
... and now am able to get the scanner name as well using widestring
instead of just string:
Code: [Select]procedure TLocalWIAScanner.Scan;
// Adapted from
// http://stackoverflow.com/questions/721948/delphi-twain-issue-help
var
  i:integer;
  Scanner: Device;
  Picture: IItem;
  Image: OleVariant;
  InVar: OLEVariant;
  StringVar: OleVariant;
  OutVar: OLEVariant;
  AImage: IImageFile;
  ReturnString: widestring;
begin
  try
    // List of devices is a 1 based array
    showmessage('number of devices:
'+inttostr(FDevMgr.DeviceInfos.Count));

    for i:=1 to FDevMgr.DeviceInfos.Count do
    begin
      InVar:=i;
      // Only check scanners (ignore cameras etc)
      StringVar:='Type';
     
OutVar:=FDevMgr.DeviceInfos[@InVar].Properties[@StringVar].get_Value;
      // Apparently need to force result to 4 bytes to compare to
constant:
      if word(OutVar)=ScannerDeviceType then
      begin
        StringVar:='Name';
        if FDevMgr.DeviceInfos[@InVar].Properties.Exists(StringVar)
then
        begin
          ReturnString:=FDevMgr.DeviceInfos[@InVar].Properties[@String
Var].get_Value;
          showmessage('Device: '+utf8encode(ReturnString));
        end
        else
        begin
          ShowMessage('Name property does not exist. Aborting.');
          exit;
        end;
      end
      else
      begin
        showmessage('found a device but it is not a scanner');
       
OutVar:=FDevMgr.DeviceInfos[@InVar].Properties[@StringVar].get_Value;
        showmessage('got device type '+utf8encode(OutVar));
      end;
    end;
    //=========< code works until here - will need to fix stuff below
>==============
    // This takes the first device. Todo: Figure out first scanner
(adapt code above)
    Scanner:=FDevMgr.DeviceInfos.Item[POleVariant(1)].Connect;
    // to do: figure out which command scans
    //reference: wia item property constants - grayscale deskew etc
    //http://msdn.microsoft.com/en-us/library/ms630196%28v=VS.85%29
    Picture :=
Scanner.ExecuteCommand(Scanner.Commands.Item[1].CommandID);
    { todo: add scan selection dialog:
    lDialog.ShowAcquireImage(WIA_TLB.ScannerDeviceType,WIA_TLB.Graysca
leIntent,WIA_TLB.MinimizeSize,
           jpegFormat,false,false,false);
    }
    //Transfer as JPG if scanner supports it. todo: change to bmp or
tiff
    Image := Picture.Transfer(Picture.Formats.Item[1]);
    { todo: image may need to be converted to bmp/tiff
    http://msdn.microsoft.com/en-
us/library/ms630826%28v=VS.85%29.aspx#SharedSample002
    }
    //Save the image
    {todo: replace with in memory manipulation
    IImageFile has a property FileData with provides access to the
binary image data, via IVector.BinaryData
    via vector binarydata - an array of bytes
    http://msdn.microsoft.com/en-
us/library/windows/desktop/ms630518%28v=vs.85%29.aspx
    }
    AImage := IImageFile(Image);
    AImage.SaveFile('wiaimage.' + AImage.FileExtension);
  except
    //for now
    //to do: debug
    on E: Exception do
    begin
      writeln(stderr,'Error scanning locally: '+E.Message);
    end;
  end;
end;

Will report when I hit more problems or have got everything working