Re: exposing x_get_scale_factor into elisp level

[Alan Third <alan@idiocy.org>]

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On Thu, Mar 25, 2021 at 12:20:09AM +0300, Evgeny Zajcev wrote:
> ср, 24 мар. 2021 г. в 23:27, Alan Third <alan@idiocy.org>:
> 
> > On Mon, Mar 22, 2021 at 02:07:41AM +0300, Evgeny Zajcev wrote:
> > > HiDPI is very common nowadays.  Internally Emacs has decent support for
> > > HiDPI displays.  However elisp code, that generates non-svg images don't
> > > have any idea that logical pixel may differ from physical one, resulting
> > in
> > > generating images in low resolution on HiDPI displays.
> > >
> > > Emacs internally has a notion about HiDPI displays, such as
> > > `x_get_scale_factor`, maybe expose this function to elisp level, so
> > > packages may utilize it to generate images in highres?
> >
> > On NS platforms (macOS and GNUstep) and, I believe, native GTK the
> > scale factor is how much the TOOLKIT scales things up for display.
> >
> 
> The problem we hit on MacOS is that line height is reported in logical
> pixels, and we generate PNG images using that line height value.  This
> results in poor PNG quality since the physical pixel is x4 times larger.
> Now we use heavy heuristics to workaround this, it will be great to have
> some general approach as Emacs do internally (`x_get_scale_factor')

My point, which wasn't clear, is that x_get_scale_factor is not
general.

IMO exposing its output to lisp is likely to just cause confusion.

That said, I've attached an implementation that ignores X and possibly
won't work for Windows.

-- 
Alan Third

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From 96f96c5e9da6db91509afea4c1ac9b1ba04ddaae Mon Sep 17 00:00:00 2001
From: Alan Third <alan@idiocy.org>
Date: Wed, 24 Mar 2021 22:50:03 +0000
Subject: [PATCH] Implement frame-scale-factor

* src/frame.c (Fframe_scale_factor): New function.
(syms_of_frame): Add frame-scale-factor.
* src/frame.h: Add FRAME_SCALE_FACTOR.
* src/image.c: Move FRAME_SCALE_FACTOR to frame.h.
---
 src/frame.c | 12 ++++++++++++
 src/frame.h |  7 +++++++
 src/image.c |  8 --------
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/frame.c b/src/frame.c
index a62347c1fb..fd649ff0fa 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -3744,6 +3744,17 @@ DEFUN ("set-frame-window-state-change", Fset_frame_window_state_change,
   return (FRAME_WINDOW_STATE_CHANGE (f) = !NILP (arg)) ? Qt : Qnil;
 }
 
+DEFUN ("frame-scale-factor", Fframe_scale_factor, Sframe_scale_factor,
+       0, 1, 0,
+       doc: /* Return FRAMEs scale factor.
+The scale factor is the amount a logical pixel size must be multiplied
+to find the real number of pixels.  */)
+     (Lisp_Object frame)
+{
+  struct frame *f = decode_live_frame (frame);
+
+  return (make_float (FRAME_SCALE_FACTOR (f)));
+}
 \f
 /***********************************************************************
 				Frame Parameters
@@ -6457,6 +6468,7 @@ focus (where a frame immediately loses focus when it's left by the mouse
   defsubr (&Sframe_pointer_visible_p);
   defsubr (&Sframe_window_state_change);
   defsubr (&Sset_frame_window_state_change);
+  defsubr (&Sframe_scale_factor);
 
 #ifdef HAVE_WINDOW_SYSTEM
   defsubr (&Sx_get_resource);
diff --git a/src/frame.h b/src/frame.h
index 9ddcb4c681..9963112036 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -907,6 +907,13 @@ #define FRAME_HAS_MINIBUF_P(f)					\
   (WINDOWP (f->minibuffer_window)				\
    && XFRAME (XWINDOW (f->minibuffer_window)->frame) == f)
 
+/* Scale factor of frame F.  */
+#if defined HAVE_NS
+# define FRAME_SCALE_FACTOR(f) (FRAME_NS_P (f) ? ns_frame_scale_factor (f) : 1)
+#else
+# define FRAME_SCALE_FACTOR(f) 1;
+#endif
+
 /* Pixel width of frame F.  */
 #define FRAME_PIXEL_WIDTH(f) ((f)->pixel_width)
 
diff --git a/src/image.c b/src/image.c
index 3d504effa6..24089e5cb0 100644
--- a/src/image.c
+++ b/src/image.c
@@ -135,14 +135,6 @@ #define PIX_MASK_DRAW	1
 # define COLOR_TABLE_SUPPORT 1
 #endif
 
-#ifdef HAVE_RSVG
-#if defined HAVE_NS
-# define FRAME_SCALE_FACTOR(f) ns_frame_scale_factor (f)
-#else
-# define FRAME_SCALE_FACTOR(f) 1;
-#endif
-#endif
-
 static void image_disable_image (struct frame *, struct image *);
 static void image_edge_detection (struct frame *, struct image *, Lisp_Object,
                                   Lisp_Object);
-- 
2.29.2