Re: Image transformation filter for upscaled images

[Alan Third <alan@idiocy.org>]

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On Tue, Mar 09, 2021 at 02:16:43PM +0100, Lars Ingebrigtsen wrote:
> Evgeny Zajcev <lg.zevlg@gmail.com> writes:
> 
> > I vote for "best" everywhere by default, making doc-view shine out-of-box
> 
> doc-view can call create-image with :filter 'best, so what's best for
> doc-view is pretty much irrelevant as to what the default should be.
> 
> The current default works pretty well for most images (just not on
> text), so I think it makes sense to not change the default.

Attached is a patch that should allow us to set smoothing using
":transform-smoothing t", or nil to disable it and use nearest
neighbour. Also sm in image mode lets you set it directly, and
doc-view will always use it.

I'm not sure about the term "smoothing", but it seemed better than
filter or similar. I dunno.

Anyway, it needs a news entry and documentation written, but I'll do
that later.

-- 
Alan Third

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From 62e13bfaa703bb1b19a3c170ca13a7cfe544c97b Mon Sep 17 00:00:00 2001
From: Alan Third <alan@idiocy.org>
Date: Tue, 9 Mar 2021 18:05:10 +0000
Subject: [PATCH] Enable selectable image smoothing (bug#38394)

* lisp/doc-view.el (doc-view-insert-image): Always use smoothing in
docview.
* lisp/image-mode.el (image-transform-smoothing): New variable.
(image-mode-map): Add smoothing binding.
(image-transform-properties): Apply smoothing when requested.
(image-transform-set-smoothing): New function.
(image-transform-reset): Reset smoothing.
* src/image.c (image_set_transform): Use new :transform-smoothing
attribute.
(syms_of_image): Add :transform-smoothing attribute.
---
 lisp/doc-view.el   |  2 ++
 lisp/image-mode.el | 20 ++++++++++++++++++--
 src/image.c        | 16 +++++++++++-----
 3 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/lisp/doc-view.el b/lisp/doc-view.el
index f6fcfae453..cef09009d9 100644
--- a/lisp/doc-view.el
+++ b/lisp/doc-view.el
@@ -1439,6 +1439,8 @@ doc-view-insert-image
 			    (apply #'create-image file doc-view--image-type nil args)
 			  (unless (member :width args)
 			    (setq args `(,@args :width ,doc-view-image-width)))
+                          (unless (member :transform-smoothing args)
+                            (setq args `(,@args :transform-smoothing t)))
 			  (apply #'create-image file doc-view--image-type nil args))))
 	     (slice (doc-view-current-slice))
 	     (img-width (and image (car (image-size image))))
diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index 7384abf3b2..ba3f104f1e 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -95,6 +95,9 @@ image-transform-scale
 (defvar-local image-transform-rotation 0.0
   "Rotation angle for the image in the current Image mode buffer.")
 
+(defvar-local image-transform-smoothing nil
+  "Whether to use transform smoothing.")
+
 (defvar image-transform-right-angle-fudge 0.0001
   "Snap distance to a multiple of a right angle.
 There's no deep theory behind the default value, it should just
@@ -457,6 +460,7 @@ image-mode-map
     (define-key map "sb" 'image-transform-fit-both)
     (define-key map "ss" 'image-transform-set-scale)
     (define-key map "sr" 'image-transform-set-rotation)
+    (define-key map "sm" 'image-transform-set-smoothing)
     (define-key map "so" 'image-transform-original)
     (define-key map "s0" 'image-transform-reset)
 
@@ -523,6 +527,8 @@ image-mode-map
 	 :help "Rotate the image"]
 	["Set Rotation..." image-transform-set-rotation
 	 :help "Set rotation angle of the image"]
+        ["Set Smoothing..." image-transform-set-smoothing
+        :help "Toggle smoothing"]
 	["Original Size" image-transform-original
 	 :help "Reset image to actual size"]
 	["Reset to Default Size" image-transform-reset
@@ -1474,7 +1480,10 @@ image-transform-properties
 	,@(when (cdr resized)
 	    (list :height (cdr resized)))
 	,@(unless (= 0.0 image-transform-rotation)
-	    (list :rotation image-transform-rotation))))))
+	    (list :rotation image-transform-rotation))
+        ,@(when image-transform-smoothing
+            (list :transform-smoothing
+                  (string= image-transform-smoothing "smooth")))))))
 
 (defun image-transform-set-scale (scale)
   "Prompt for a number, and resize the current image by that amount."
@@ -1507,6 +1516,12 @@ image-transform-set-rotation
   (setq image-transform-rotation (float (mod rotation 360)))
   (image-toggle-display-image))
 
+(defun image-transform-set-smoothing (smoothing)
+  (interactive (list (completing-read "Smoothing: "
+                                      '(none smooth) nil t)))
+  (setq image-transform-smoothing smoothing)
+  (image-toggle-display-image))
+
 (defun image-transform-original ()
   "Display the current image with the original (actual) size and rotation."
   (interactive)
@@ -1519,7 +1534,8 @@ image-transform-reset
   (interactive)
   (setq image-transform-resize image-auto-resize
 	image-transform-rotation 0.0
-	image-transform-scale 1)
+	image-transform-scale 1
+        image-transform-smoothing nil)
   (image-toggle-display-image))
 
 (provide 'image-mode)
diff --git a/src/image.c b/src/image.c
index 8137dbea8d..95ae573354 100644
--- a/src/image.c
+++ b/src/image.c
@@ -2230,7 +2230,12 @@ image_set_transform (struct frame *f, struct image *img)
      operations to use a blended filter, to avoid aliasing and the like.
 
      TODO: implement for Windows.  */
-  bool scale_down = (width < img->width) || (height < img->height);
+  bool smoothing;
+  Lisp_Object s = image_spec_value (img->spec, QCtransform_smoothing, NULL);
+  if (!s)
+    smoothing = (width < img->width) || (height < img->height);
+  else
+    smoothing = !NILP (s);
 # endif
 
   /* Perform scale transformation.  */
@@ -2344,13 +2349,13 @@ image_set_transform (struct frame *f, struct image *img)
   /* Under NS the transform is applied to the drawing surface at
      drawing time, so store it for later.  */
   ns_image_set_transform (img->pixmap, matrix);
-  ns_image_set_smoothing (img->pixmap, scale_down);
+  ns_image_set_smoothing (img->pixmap, smoothing);
 # elif defined USE_CAIRO
   cairo_matrix_t cr_matrix = {matrix[0][0], matrix[0][1], matrix[1][0],
 			      matrix[1][1], matrix[2][0], matrix[2][1]};
   cairo_pattern_t *pattern = cairo_pattern_create_rgb (0, 0, 0);
   cairo_pattern_set_matrix (pattern, &cr_matrix);
-  cairo_pattern_set_filter (pattern, scale_down
+  cairo_pattern_set_filter (pattern, smoothing
                             ? CAIRO_FILTER_BEST : CAIRO_FILTER_NEAREST);
   /* Dummy solid color pattern just to record pattern matrix.  */
   img->cr_data = pattern;
@@ -2369,13 +2374,13 @@ image_set_transform (struct frame *f, struct image *img)
              XDoubleToFixed (matrix[2][2])}}};
 
       XRenderSetPictureFilter (FRAME_X_DISPLAY (f), img->picture,
-                               scale_down ? FilterBest : FilterNearest, 0, 0);
+                               smoothing ? FilterBest : FilterNearest, 0, 0);
       XRenderSetPictureTransform (FRAME_X_DISPLAY (f), img->picture, &tmat);
 
       if (img->mask_picture)
         {
           XRenderSetPictureFilter (FRAME_X_DISPLAY (f), img->mask_picture,
-                                   scale_down ? FilterBest : FilterNearest, 0, 0);
+                                   smoothing ? FilterBest : FilterNearest, 0, 0);
           XRenderSetPictureTransform (FRAME_X_DISPLAY (f), img->mask_picture,
                                       &tmat);
         }
@@ -10693,6 +10698,7 @@ syms_of_image (void)
   DEFSYM (QCrotation, ":rotation");
   DEFSYM (QCmatrix, ":matrix");
   DEFSYM (QCscale, ":scale");
+  DEFSYM (QCtransform_smoothing, ":transform-smoothing");
   DEFSYM (QCcolor_adjustment, ":color-adjustment");
   DEFSYM (QCmask, ":mask");
 
-- 
2.29.2