kth_channel_view fails when used with virtual views

[Shaun Marko <smarko@…>]

Given an image_view with a virtual_2d_locator that returns a planar_pixel_reference, kth_channel_view produces a kth_channel_deref_fn with an incorrect result_type which may result an invalid pointer dereference.

For example, consider a virtual_2d_locator instantiated with a dereference functor that returns a 'planar_pixel_reference<bits8&, rgb_t>'

An image_view instantiated with such a locator will have a reference type of 'planar_pixel_reference<bits8&,rgb_t>'. Now suppose this image_view is passed to kth_channel_view(). This results in the instantiation of a kth_channel_deref_fn with a result_type of 'pixel<bits8&, gray_layout_t>&'; however, the deref functor will initialize this result_type with a pixel<bits8,gray_layout_t>&'.

detail::kth_channel_deref_fn needs to ensure the channel type of its result_type is not a reference type.

[Shaun Marko <smarko@…>]

patch to kth_channel_deref_fn in image_view_factory.hpp

[Shaun Marko <smarko@…>]

Example program illustrating the issue.

[Shaun Marko <smarko@…>]

Please retract this bug report. The proposed fix makes the kth_channel_view read-only for virtual views with reference types of planar_pixel_reference.

[Mateusz Loskot]

Moved to ​https://github.com/boostorg/gil/issues/151