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       im_render, im_render_fade, im_cache - make image in the background


       #include <vips/vips.h>

       int im_render_fade( IMAGE *in, IMAGE *out, IMAGE *mask,
         int width, int height, int max,
         int fps, int steps,
         int priority,
         void (*notify)( IMAGE *, Rect *, void * ), void *client );

       int im_render( IMAGE *in, IMAGE *out, IMAGE *mask,
         int width, int height, int max,
         void (*notify)( IMAGE *, Rect *, void * ), void *client );

       int im_cache( IMAGE *in, IMAGE *out,
         int width, int height, int max );


       im_render_fade(3)  behaves rather like im_copy(3) between images in and
       out, except that it keeps a cache of computed  pixels.  This  cache  is
       made  of  up  to  max  tiles (a value of -1 for max means any number of
       tiles), and each tile is of size width by  height  pixels.  Each  cache
       tile  is  made  with  a  call  to  im_prepare_thread(3),  so cache tile
       calculation will be accelerated on  multi-CPU  machines.  If  image  in
       represents  a  large  computation,  im_render_fade(3) can save a lot of

       If the notify parameter points  to  a  function,  then  tiles  are  not
       calculated  immediately,  but are added to a job list and calculated as
       CPU becomes available. When a tile  has  been  calculated,  the  notify
       function  is  passed  the image which was being cached, the area of the
       image which is  now  available,  and  a  client  pointer.   The  notify
       function  will  be  called  by a background thread, so you will usually
       need to implement some mechanism to  interrupt  your  main  thread  and

       The  mask  image  is a one band uchar image the same size as out, which
       has 255 for every pixels which is in the cache and 0  everywhere  else.
       You should not read pixels from mask after out has been closed. If mask
       is NULL, then no mask image is written.

       If steps is greater than zero, then pixels in mask are moved between  0
       and  255  as  tiles  age in that many steps. The fps parameter sets how
       many times per second the notify function is called to indicate that an
       updated tile is ready.

       If  priority  is zero, then the render will only run when the system is
       idle, and only a few renders will run at any time. Higher  values  will
       cause the render to happen more quickly.

       im_cache(3) is a convenience function for im_render_fade(3) that caches
       image pixels synchronously. If you ask for an area not  in  the  cache,
       execution blocks until the area has been calculated.

       im_render(3)  is  deprecated:  it  is  the  old interface to the render
       function, before the fps and steps parameters were added.


       The function returns 0 on  success,  and  non-zero  on  error,  setting




       J Cupitt, 2003

                                5 October 2003