Re: "Fix" sag scaling for hidpi

[Alan Third <alan@idiocy.org>]

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On Wed, Feb 10, 2021 at 01:00:52PM -0500, Yuan Fu wrote:
> I agrees that we should keep the logical size, i.e., keep the size
> comparing against text. If we expose correct physical size, packages
> that generate bitmaps for display can generate crisp bitmaps with
> correct pixel size.
> 
> IIUC, a high-res image with :scale 0.5 should work across high and
> low-res displays. So ideally any package that wants to generate
> crisp bitmap can get the physical size and pixel-ratio from Emacs,
> generate the image and set :scale to 1/pixel-ratio. And this image
> works across different displays.

Yes, until the users drags the frame from a scale factor 1 screen to a
scale factor 2 screen, then it's blurry again. But I really don't
think there is a satisfactory solution to that.

> > We'll probably have to do more fiddling with SVGs though, since they
> > can define sizes in real-world units, like cm or inches, so the dpi
> > has to match the physical pixels, and if we move to a different screen
> > and regenerate the SVG the DPI will be different but the scale likely
> > won't be recalculated and the image will change size.
> 
> To simplify things, maybe we can assume DPI is 96. I.e., assume 1
> inch = 96 logical pixels = 96 * pixel-ratio physical pixels.
> 
> Obviously, if we can get DPI information from all terminals, then we
> could use that information. But from your previous message it
> doesn’t seem easy.

After messing about a bit today, I think the DPI thing is a red
herring. If we calculate the logical size of the SVG, then just
multiply it by the scale factor, then when drawing scale it down by
the scale factor everything is the right size and we can forget about
modifying the dpi.

> For SVGs I think we should automatically handle the pixel ratio and
> dpi so the image is always crisp and lisp doesn’t need to do
> anything (don’t need to add the :scale attribute or anything).
> 
> For bitmap images I think we display them in their physical size and
> let lisp alter the size by the :scale attribute.
> 
> For :width and :height attributes, I think they should be in logical
> pixels.Because the ratio of logical pixels and other text in a
> buffer doesn’t change when you drag a frame across different
> displays. So if the user set an image to have certain :width and
> drag the frame to a different display, the image doesn’t change its
> size comparing to everything else in the buffer.

Attached is a first go at this. I can't work out image.el and
image-mode.el, so I'll either need a lot more time or someone more
familiar with them can have a look.

The basic problem is that image-size returns physical pixels, and we
want to calculate logical pixels. I tried a few things and failed
miserably. I tried changing image-size to return logical pixels, but
that just seemed to make everything worse, and I don't think it's a
good idea to lie about the image size.

Anyway, SVGs will display nicely on macOS with the attached patch, and
frame-scale-factor returns a useful value. I didn't write the code for
GTK yet, but I guess it's probably not too hard.

I guess we also want to change the ghostscript code to do the same
thing as the SVG code too?
-- 
Alan Third

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From 6bff2bf4b22d4b018a8b13aef02b25185754f167 Mon Sep 17 00:00:00 2001
From: Alan Third <alan@idiocy.org>
Date: Wed, 10 Feb 2021 22:12:16 +0000
Subject: [PATCH] Scale SVGs to match frame's scale factor

---
 src/image.c  | 39 +++++++++++++++++++++++++++++++++------
 src/nsterm.h |  1 +
 src/nsterm.m | 11 +++++++++++
 3 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/src/image.c b/src/image.c
index a124cf91ba..f8cd0d140e 100644
--- a/src/image.c
+++ b/src/image.c
@@ -135,6 +135,12 @@ #define PIX_MASK_DRAW	1
 # define COLOR_TABLE_SUPPORT 1
 #endif
 
+#ifdef HAVE_NS
+# define FRAME_SCALE_FACTOR(f) ns_frame_scale_factor (f)
+#else
+# define FRAME_SCALE_FACTOR(f) 1;
+#endif
+
 static void image_disable_image (struct frame *, struct image *);
 static void image_edge_detection (struct frame *, struct image *, Lisp_Object,
                                   Lisp_Object);
@@ -2207,8 +2213,8 @@ image_set_transform (struct frame *f, struct image *img)
   /* SVGs are pre-scaled to the correct size.  */
   if (EQ (image_spec_value (img->spec, QCtype, NULL), Qsvg))
     {
-      width = img->width;
-      height = img->height;
+      width = img->width / FRAME_SCALE_FACTOR (f);
+      height = img->height / FRAME_SCALE_FACTOR (f);
     }
   else
 #endif
@@ -2238,9 +2244,9 @@ image_set_transform (struct frame *f, struct image *img)
 		  : img->height / (double) height),
 # elif defined HAVE_NTGUI || defined HAVE_NS
 	[0][0] = (!IEEE_FLOATING_POINT && img->width == 0 ? DBL_MAX
-		  : width / (double) img->width),
+		  : width / (double) (img->width)),
 	[1][1] = (!IEEE_FLOATING_POINT && img->height == 0 ? DBL_MAX
-		  : height / (double) img->height),
+		  : height / (double) (img->height)),
 # else
 	[0][0] = 1, [1][1] = 1,
 # endif
@@ -10008,6 +10014,9 @@ svg_load_image (struct frame *f, struct image *img, char *contents,
   compute_image_size (viewbox_width, viewbox_height, img->spec,
                       &width, &height);
 
+  width *= FRAME_SCALE_FACTOR (f);
+  height *= FRAME_SCALE_FACTOR (f);
+
   if (! check_image_size (f, width, height))
     {
       image_size_error ();
@@ -10499,9 +10508,8 @@ DEFUN ("lookup-image", Flookup_image, Slookup_image, 1, 1, 0,
 
 #endif /* GLYPH_DEBUG */
 
-
 /***********************************************************************
-			    Initialization
+			    Image Scaling
  ***********************************************************************/
 
 DEFUN ("image-transforms-p", Fimage_transforms_p, Simage_transforms_p, 0, 1, 0,
@@ -10538,6 +10546,24 @@ DEFUN ("image-transforms-p", Fimage_transforms_p, Simage_transforms_p, 0, 1, 0,
   return Qnil;
 }
 
+DEFUN ("frame-scale-factor", Fframe_scale_factor, Sframe_scale_factor, 0, 1, 0,
+       doc: /* Return the image scale factor for FRAME.  */)
+       (Lisp_Object frame)
+{
+  struct frame *f = decode_live_frame (frame);
+  if (FRAME_WINDOW_P (f))
+    {
+      return make_float (FRAME_SCALE_FACTOR (f));
+    }
+
+  return make_float (1);
+}
+
+
+/***********************************************************************
+			    Initialization
+ ***********************************************************************/
+
 DEFUN ("init-image-library", Finit_image_library, Sinit_image_library, 1, 1, 0,
        doc: /* Initialize image library implementing image type TYPE.
 Return t if TYPE is a supported image type.
@@ -10820,6 +10846,7 @@ syms_of_image (void)
 #endif
 
   defsubr (&Simage_transforms_p);
+  defsubr (&Sframe_scale_factor);
 
   DEFVAR_BOOL ("cross-disabled-images", cross_disabled_images,
     doc: /* Non-nil means always draw a cross over disabled images.
diff --git a/src/nsterm.h b/src/nsterm.h
index eae1d0725e..017c2394ef 100644
--- a/src/nsterm.h
+++ b/src/nsterm.h
@@ -1252,6 +1252,7 @@ #define NSAPP_DATA2_RUNFILEDIALOG 11
 extern void ns_init_events (struct input_event *);
 extern void ns_finish_events (void);
 
+extern double ns_frame_scale_factor (struct frame *);
 
 #ifdef NS_IMPL_GNUSTEP
 extern char gnustep_base_version[];  /* version tracking */
diff --git a/src/nsterm.m b/src/nsterm.m
index 8f2b61a165..c15355c84d 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -857,6 +857,17 @@ Free a pool and temporary objects it refers to (callable from C)
 }
 
 
+double
+ns_frame_scale_factor (struct frame *f)
+{
+#ifdef NS_IMPL_COCOA
+  return [[FRAME_NS_VIEW (f) window] backingScaleFactor];
+#else
+  return 1;
+#endif
+}
+
+
 /* ==========================================================================
 
     Focus (clipping) and screen update
-- 
2.29.2