Re: TWAIN_NATIVEXFER flag

Hi,

    What exactly this flag does and affects? There is nothing in documentation except that this invokes "native transfer". What is "native transfer"?

There are two very basic types of data transfers that every TWAIN Source must provide: Native and Buffered memory. A native data transfer is like the buffered memory transfer in that it deals with memory. The difference is that a native transfer allocates a whole block of contiguous memory and is limited to the DIB or PICT formats. This can be problematic if you have a limited amount of memory to work with, or the memory that you have is fragmented. The transfer will fail if a large enough block of memory is not available. The native transfer is set as the default and is the easiest to implement. If a different transfer is needed, capability negotiation will be required for the change. The transfer allocates a single block of memory to write the image to and then passes a pointer to the calling application. It is the application's responsibility to free that memory when done with it.
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The other transfer type defined by TWAIN, disk file, is not required to be provided by the Source. It allows for images to be written to disk. The application must first create an image file that the Source can handle and assure that it is accessible. The Source has a capability that specifies file formats which can be used. The application can then set the file format and the file name to be used. The application must be able to manage the file after it is created.
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Disk file transfers are ideal for large and very large images where native transfers are unrealistic. A disk file transfer is a little more difficult to implement than a native transfer but easier than a buffered memory transfer. This type of transfer is the slowest of the threes as well since it is dealing with the a hard disk as opposed to system memory.