reduce GUI lag/stuttering by eliminating unnecessary work #18132
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Don't call dt_dev_get_pointer_zoom_pos (which calls dt_dev_distort_transform, which can take 100+ms when a liquify instance is active with large areas being distorted) on mouse activity in the darkroom center image unless actually needed. Avoid calling distort transform from the darkroom view expose() function unless invoking a module post-expose callback which needs the transformed mouse pointer position. Reduce the number of distort_transform calls generated by dt_view_paint_surface (called by expose) from two to one. The overall result is much smoother mouse movement and scrolling when a module with a slow distort_transform() function is enabled.
This reduces overhead and thus lag/stuttering when moving the mouse across the center image in darkroom (especially for the liquify module, which needs to compute the distortion map *every* time). It also fixes a subtle UI bug -- when modules have a drawn mask and the shapes display is enabled, but the module itself is disabled, the mouse still interacts with the (invisible) drawn shapes. This leads to surprises such as a scroll or drag resizing/moving a shape rather than zooming/panning the image.
ralfbrown
added
scope: UI
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Whenever a widget is refreshed in darkroom view, the entire window is redrawn and a lot of other activity is kicked off, including running the coordinate transform chain (which gets very slow if there are any active instances of liquify) multiple times. This PR reworks things (short of the complete rearchitecting of expose() which is actually needed) to reduce the number of calls for transforms and the amount of work done in mouse-motion callbacks.
The latter also fixes a subtle UI bug -- when modules have a drawn mask and the shapes display is enabled, but the module itself is disabled, the mouse still interacts with the (invisible) drawn shapes. This leads to surprises such as a scroll or drag resizing/moving a shape rather
than zooming/panning the image. It may be worth cherry-picking 648a74b for 5.0.1.