Squashed 'test/' content from commit f2edc20

git-subtree-dir: test
git-subtree-split: f2edc20cba865a6ef67ca807c2ed6cee8e6c2836

sexp_effects/effects/trails.sexp

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+;; Trails effect - persistent motion trails
+(require-primitives "image" "blending")
+
+(define-effect trails
+  :params (
+    (persistence :type float :default 0.8 :range [0 0.99])
+  )
+  (let* ((buffer (state-get 'buffer nil))
+         (current frame))
+    (if (= buffer nil)
+        (begin
+          (state-set 'buffer (copy frame))
+          frame)
+        (let* ((faded (blending:blend-images buffer
+                                    (make-image (image:width frame) (image:height frame) (list 0 0 0))
+                                    (- 1 persistence)))
+               (result (blending:blend-mode faded current "lighten")))
+          (begin
+            (state-set 'buffer result)
+            result)))))