— windows type: dib

In the C world a DIB is a handle to a piece of memory containing the bits that represent information about the image and the pixels of the image. The handle is a machine-word sized piece of data which may be thought of as a 32 bit integer. The handle may be null (i.e. zero), indicating that there is no block of memory describing the DIB. The null value is usually returned by C functions that are supposed to create a DIB but failed, for some reason like the memory could not be allocated or a file could not be opened.

In the Scheme world a DIB is a structure containing information about the bitmap (specifically the integer that represents the handle). We also include #f in the dib windows type to mirror the null handle error value.

          (define dib-result
            (lambda (handle)
              (if (= handle 0)
                  #f
                  (make-dib handle))))
          
          (define dib-arg
            (lambda (dib)
              (if dib
                  (cell-contents (dib-handle dib))
                  0)))
          
          (define-windows-type dib
            (lambda (thing) (or (dib? thing) (eq? thing #f)))
            dib-arg
            dib-result)