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## 3.6.2 Eval callbacks

These are callbacks which are invoked by VIPS during evaluation. They are called each time a region is filled with data (for PIO functions), or each time `im_writeline()` is called (for WIO functions). The callback has access to a large struct containing information about the progress of evaluation, useful for user-interfaces built on top of VIPS. See the manual page for `im_add_eval_callback()` for full details.