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

[Fabio Niephaus]

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?

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