Add a new `image-transform-smoothing' user option

* doc/lispref/display.texi (Image Descriptors): Document it.
* lisp/image.el (image-transform-smoothing): New user option.
(create-image): Use it.
(image--default-smoothing): New function.

doc/lispref/display.texi

@@ -5399,8 +5399,13 @@ is platform dependent, but should be equivalent to bilinear
 filtering.  Disabling smoothing will use the nearest neighbor
 algorithm.
 
-The default, if this property is not specified, is for down-scaling to
-apply smoothing, and for up-scaling to not apply smoothing.
+If this property is not specified, @code{create-image} will use the
+@code{image-transform-smoothing} user option to say whether scaling
+should be done or not.  This option can be @code{nil} (no smoothing),
+@code{t} (use smoothing) or a predicate function that's called with
+the image object as the only parameter, and should return either
+@code{nil} or @code{t}.  The default is for down-scaling to apply
+smoothing, and for large up-scaling to not apply smoothing.
 
 @item :index @var{frame}
 @xref{Multi-Frame Images}.

etc/NEWS

@@ -1483,6 +1483,12 @@ and nil to disable smoothing.
 The default behaviour of smoothing on down-scaling and not smoothing
 on up-scaling remains unchanged.
 
++++
+*** New user option 'image-transform-smoothing'.
+This controls whether to use smoothing or not for an image.  Values
+include nil (no smoothing), t (do smoothing) or a predicate function
+that's called with the image object and should return nil/t.
+
 ** EWW
 
 +++

lisp/image.el

@@ -141,6 +141,18 @@ based on the font pixel size."
                  (const :tag "Automatically compute" auto))
   :version "26.1")
 
+(defcustom image-transform-smoothing #'image--default-smoothing
+  "Whether to do smoothing when applying transforms to images.
+Common transforms are rescaling and rotation.
+
+Valid values are nil (no smoothing), t (smoothing) or a predicate
+function that is called with the image specification and should return
+either nil or non-nil."
+  :type '(choice (const :tag "Do smoothing" t)
+                 (const :tag "No smoothing" nil)
+                 function)
+  :version "28.1")
+
 (defcustom image-use-external-converter nil
   "If non-nil, `create-image' will use external converters for exotic formats.
 Emacs handles most of the common image formats (SVG, JPEG, PNG, GIF
@@ -485,11 +497,40 @@ Image file names that are not absolute are searched for in the
             type 'png
             data-p t)))
   (when (image-type-available-p type)
-    (append (list 'image :type type (if data-p :data :file) file-or-data)
-            (and (not (plist-get props :scale))
-                 (list :scale
-                       (image-compute-scaling-factor image-scaling-factor)))
-	    props)))
+    (let ((image
+           (append (list 'image :type type (if data-p :data :file)
+                         file-or-data)
+                   (and (not (plist-get props :scale))
+                        ;; Add default scaling.
+                        (list :scale
+                              (image-compute-scaling-factor
+                               image-scaling-factor)))
+	           props)))
+      ;; Add default smoothing.
+      (unless (plist-member props :transform-smoothing)
+        (setq image (nconc image
+                           (list :transform-smoothing
+                                 (pcase image-transform-smoothing
+                                   ('t t)
+                                   ('nil nil)
+                                   (func (funcall func image)))))))
+      image)))
+
+(defun image--default-smoothing (image)
+  "Say whether IMAGE should be smoothed when transformed."
+  (let* ((props (nthcdr 5 image))
+         (scaling (plist-get props :scale))
+         (rotation (plist-get props :rotation)))
+    (cond
+     ;; We always smooth when scaling down and small upwards scaling.
+     ((and scaling (< scaling 2))
+      t)
+     ;; Smooth when doing non-90-degree rotation
+     ((and rotation
+           (or (not (zerop (mod rotation 1)))
+               (not (zerop (% (truncate rotation) 90)))))
+      t)
+     (t nil))))
 
 (defun image--set-property (image property value)
   "Set PROPERTY in IMAGE to VALUE.