Limit `clear` to visible part of the framebuffer

[rfuest]

At the moment the DrawTarget::clear implementation clears the entire framebuffer, even for smaller display. The unnecessary writes makes clearing the display slower than necessary on these displays. I would suggest to change the implementation of clear or simply to remove it and rely on the default implementation, that uses fill_solid.

I'm not sure if it is ever necessary to clear the entire framebuffer, but if that is the case we could add another method clear_all to clear the entire framebuffer.

[almindor]

So this is conflicting. I think we should change it to fn clear(entire_framebuffer: bool) since clearing the whole thing makes sense for HW scroll displays for example.

[rfuest]

I think we should change it to fn clear(entire_framebuffer: bool)

That wouldn't work for the DrawTarget implementation.

[rfuest]

A solution, that would be compatible with the embedded-graphics drawing model, is using a second DrawTarget for the entire framebuffer:

let mut display = Builder::st7789_pico1(di).init(&mut delay, Some(rst))?;

// clear only the visible area
display.clear(Rgb565::BLACK)?;
// display is DrawTarget with 135x240 pixel

// clear the entire framebuffer
display.framebuffer().clear(Rgb565::BLACK)?
// display.framebuffer() returns a DrawTarget with 240x320 pixel
// display.framebuffer() could also be used to draw anything else outside the visible area

[almindor]

That'd work and should probably be good enough for the hw scroll edge cases. The change will need to be documented as it's breaking expectations.

[almindor]

@rfuest how do you feel about this issue? I'm trying to collect things that need to be done before the next release. I'm on the fence on this one.

[rfuest]

I accidentally implemented this change in https://github.com/almindor/mipidsi/commit/5f455eb8171153d4ccb9080905782ee17ae0439f, otherwise the display wasn't cleared correctly. I would prefer to leave it this way because it can improve performance in the most common use case, when scrolling isn't used.

[almindor]

Closing as done