[Vm-dev] Installing high dpi support and backwards compatibility of the VM

[Eliot Miranda]

On Fri, Oct 9, 2020 at 12:31 AM Fabio Niephaus <lists at fniephaus.com> wrote:

>
> Hi Eliot,
>
> What would be the consequences of moving the display creation logic
> from DisplayScreen class>>actualScreenSize to beDisplay? If beDisplay
> is called afterwards, it at least should be feasible, right?
>

I don't know.  There would need to be a design where the semantics of
beDisplay were "Make me the display but set my size to whatever you think
best".  But that's not how the interface is designed.  The interface is
designed as

1. tell Display how big the actual GUI's screen is
      => at this point the actual GUI's screen must be opened to find out
how big it is
2. Display adjusts itself accordingly
3. beDisplay simply records the state that was actually started in 1.

I don't know how to change this order without breaking
backward-compatibility, which is where we came in.  Also, a deBisplay
primitive which reached in and changed the inst vars in Display would be a
really bad thing (IMO).

So it seems to me that we're stuck with the actual GUI's screen being
opened in DisplayScreen class>>actualScreenSize <primitive: 106>


> Cheers,
> Fabio
>
> On Thu, Oct 8, 2020 at 7:59 PM Eliot Miranda <eliot.miranda at gmail.com>
> wrote:
> >
> >
> > Hi All,
> >
> >     ideally adding the high dpi support to the VM will not break
> backwards-compatibility.  But that implies that the VM is informed before
> it creates the display of whether to create a high dpi display or not.
> Off-list Tobias asked me where the VM sets up the display on Mac and I was
> surprised by the answer.
> >
> > I thought it would be as part of beDisplay.  But it's actually as a
> side-effect of DisplayScreen class>>actualScreenSize, primitive 106, which
> calls the ioScreenSize function.  It is this functions' responsibility to
> actually create the display, deriving the size from the savedWindowSize
> info in the image header (which can or could be overridden on the VM
> command line, and is when -headless is supplied).
> >
> > So any new primitive added to allow DisplayScreen to inform the VM of
> whether to use high dpi or not would have to be invoked before primitive
> 106.  So one way to implement this is to modify the chain of invocations
> leading up to primitive 106.  For this route I'd like to propose the
> following refactoring:
> >
> > DisplayScreen class>>actualScreenSize
> > <primitive: 106>
> > ^ 640 at 480
> >
> > becomes
> >
> > DisplayScreen class>>actualScreenSize
> > self primitiveUseHighDPI: self useHighDPI. "where this is a preference"
> > ^self primitiveScreenSize
> >
> > primitiveScreenSize
> > <primitive: 106>
> > ^ 640 at 480
> >
> >
> > Another route is to make the useHighDPI flag part of the image header
> state alongside the saved window size.  This would mean it was added to the
> flags accessed via vmParameterAt: 48.  There could be a command-line
> argument to override.
> >
> >
> > Finally I note that the beDisplay primitive simply stores the display
> object in the specialObjectsArray and assigns the interpreter variables
> that track the display, displayBits, displayWidth, displayHeight &
> displayDepth.  It then invokes ioNoteDisplayChangedwidthheightdepth, but
> *all* the implementations of this function are empty.  I propose that we
> should eliminate 5this call and its implementation. It is confusing to
> follow it and find it does nothing.  The argument could be that a platform
> might require it.  But if that's so we can always put it back.  We have an
> existence proof in all our platforms that this is unlikely.  Thoughts?
> > _,,,^..^,,,_
> > best, Eliot
>


-- 
_,,,^..^,,,_
best, Eliot
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