Detecting the camera that scanned image belongs to

Question

I'm using Leadtools 20 API to control the scanner and scan some documents. I have 2 questions.
1 - When using L_TwainAcquire with a callback function that receives the images from the scanner and keeps returning SUCCESS to get the next image. Is there anyway inside the callback function to determine if the image is from the front camera or the back camera?
2 - Is there a way to force the scanner to use only the back camera for scanning?

Thank you
Sam

Answer 1

If you mean the 2 sides of a scanned document when using a duplex scanner, the answer is as follows:

1. To know which side of the paper is currently being processed in the bitmap callback, the following code can be used inside the callback function:

L_INT EXT_CALLBACK LTwainBitmapCallback(HTWAINSESSION hSession, pBITMAPHANDLE pBitmap, L_VOID * pUserData)
{
   L_INT nRet = SUCCESS;
   TW_EXTIMAGEINFO* pExtImg = NULL;
   int nExtraInfoAftreZero = 0;  // we have only one info, so no extra needed
   pExtImg = (TW_EXTIMAGEINFO *)malloc(sizeof TW_EXTIMAGEINFO  + nExtraInfoAftreZero * sizeof TW_INFO);
   if (pExtImg)  
   { 
      pExtImg->NumInfos = 1;
      pExtImg->Info[0].InfoID = TWEI_PAGESIDE;
      pExtImg->Info[0].ItemType = TWTY_UINT16;
      nRet = L_TwainGetExtendedImageInfo (hSession, pExtImg); 
      if (nRet == SUCCESS) 
      { 
         // Do processing to returned values
         if(pExtImg->Info[0].Item == TWCS_TOP)
            OutputDebugString("Front of sheet.\n");
         else if (pExtImg->Info[0].Item == TWCS_BOTTOM)
            OutputDebugString("Rear of sheet.\n");
         else
            OutputDebugString("Unexpected!\n");
         nRet = L_TwainFreeExtendedImageInfoStructure (&pExtImg); 
         if(nRet != SUCCESS) 
            return nRet; 
      }
      free(pExtImg);
   }
   return SUCCESS;
}

2. Forcing a specific side to scan can be done using a combination of CAP_CAMERASIDE and CAP_CAMERAENABLED capabilities, as explained in the Twain Spec.

Quoting from that document:

To enable bottom only scanning, set CAP_CAMERASIDE to TWCS_BOTTOM and set CAP_CAMERAENABLED to TRUE, then set CAP_CAMERASIDE to TWCS_TOP and set CAP_CAMERAENABLED to FALSE.

If you mean you have a computer with 2 Twain camera sources (back and front), the answer becomes as follows:

1. To know which Twain source (e.g. camera) triggered the Twain Bitmap Callback when L_TwainAcquire() was called, you can use the L_TwainGetSources() function with the LTWAIN_SOURCE_ENUMERATE_DEFAULT flag to get the currently-selected Twain device. This can be done inside the Bitmap Callback as shown in the following code:

// Source Info Callback
L_INT EXT_CALLBACK TwainSourceInfoCallbackCurrent(HTWAINSESSION hSession, pLTWAINSOURCEINFO pSourceInfo, L_VOID * pUserData)
{
   strcpy((L_CHAR *)pUserData, pSourceInfo->pszTwnSourceName);
   return SUCCESS;
}

// Twain Bitmap Callback
L_INT EXT_CALLBACK LTwainBitmapCallback(HTWAINSESSION hSession, pBITMAPHANDLE pBitmap, L_VOID * pUserData)
{
   char szSourceName[1024];
   L_TwainGetSources(hSession, TwainSourceInfoCallbackCurrent, sizeof LTWAINSOURCEINFO, LTWAIN_SOURCE_ENUMERATE_DEFAULT, szSourceName);
   // Now szSourceName contains name of Twain Source.
   return SUCCESS;
}

2. Yes, you can force scanning from a specific device (scanner, camera, etc.) by sending its name to the L_TwainSelectSource() function before scanning, as follows:

LTWAINSOURCE TwainSource;
TwainSource.uStructSize = sizeof LTWAINSOURCE;
TwainSource.pszTwainSourceName = pszRearCameraTwainSourceName;
L_TwainSelectSource(twainSession, &TwainSource);
// Now L_TwainAcquire() will capture from Rear Camera

To find the name of all devices, you can use this code:

L_TwainGetSources(twainSession, TwainSourceInfoCallback, sizeof LTWAINSOURCEINFO, LTWAIN_SOURCE_ENUMERATE_ALL, NULL);

The Source Info Callback will be triggered once for each Twain source. It can be implemented like this:

L_INT EXT_CALLBACK TwainSourceInfoCallback(HTWAINSESSION hSession, pLTWAINSOURCEINFO pSourceInfo, L_VOID * pUserData)
{
   OutputDebugString(pSourceInfo->pszTwnSourceName); // You can save the names of Twain Sources into global variables if you like
   OutputDebugString("\n");
   return SUCCESS;
}