VipsImagesink_screen

This operation renders in in the background, making pixels available on out as they are calculated. The notify_fn callback is run every time a new set of pixels are available. Calculated pixels are kept in a cache with tiles sized tile_width by tile_height pixels and with at most max_tiles tiles. If max_tiles is -1, the cache is of unlimited size (up to the maximum image * size). The mask image is a one-band uchar image and has 255 for pixels which are currently in cache and 0 for uncalculated pixels.

Renders with a positive priority are assumed to be large, gh-priority, foreground images. Although there can be many of these, only one is ever active, to avoid overcommitting threads.

Renders with a negative priority are assumed to be small, thumbnail images consisting of a single tile. Single tile images are effectively single-threaded, so all these renders are evaluated together.

Calls to vips_region_prepare() on out return immediately and hold whatever is currently in cache for that VipsRect (check mask to see which parts of the VipsRect are valid). Any pixels in the VipsRect which are not in cache are added to a queue, and the notify_fn callback will trigger when those pixels are ready.

The notify_fn callback is run from one of the background threads. In the callback you need to somehow send a message to the main thread that the pixels are ready. In a glib-based application, this is easily done with g_idle_add().

If notify_fn is NULL then vips_sink_screen() runs synchronously. vips_region_prepare() on out will always block until the pixels have been calculated.