add canvas comparsion to asserts.equals

res/modules/internal/asserts.lua

@@ -1,8 +1,77 @@
 local this = {}
+local app = __vc_app
 
-function this.equals(expected, fact)
+local function diff_canvas(expected, fact)
+    this.equals(expected.width, fact.width, "image width")
+    this.equals(expected.width, fact.width, "image height")
+
+    local diff = Canvas({expected.width, expected.height})
+
+    local mismatches = 0
+
+    for y = 0, expected.height - 1 do
+        for x = 0, expected.width - 1 do
+            local ca = expected:at(x, y)
+            local cb = fact:at(x, y)
+
+            if ca == cb then
+                diff:set(x, y, ca)
+            else
+                diff:set(x, y, 0xff0000ff)
+                mismatches = mismatches + 1
+            end
+        end
+    end
+
+    return diff, mismatches
+end
+
+local TEST_ARTIFACTS_PATH = "export:test-results"
+
+local function get_test_artifact_path(name)
+    local path = TEST_ARTIFACTS_PATH
+    if not file.isdir(path) then
+        file.mkdirs(path)
+    end
+    return file.join(path, name)
+end
+
+function this.equals(expected, fact, prefix)
+    local mt = getmetatable(expected)
+    if mt == Canvas then
+        local diff, mismatches = diff_canvas(expected, fact)
+        if mismatches > 0 then
+            local test_name = app.script or random.uuid()
+            local path_diff = get_test_artifact_path(
+                string.format("diff_%s.png", test_name)
+            )
+            local path_fact = get_test_artifact_path(
+                string.format("fact_%s.png", test_name)
+            )
+            local path_expected = get_test_artifact_path(
+                string.format("expected_%s.png", test_name)
+            )
+            file.write_bytes(path_diff, diff:encode('png'))
+            file.write_bytes(path_fact, fact:encode('png'))
+            file.write_bytes(path_expected, expected:encode('png'))
+            local total_pixels = expected.width * expected.height
+            assert(mismatches == 0, string.format(
+                "%s pixels of %s (%s%%) are different, diff saved as %s",
+                mismatches,
+                total_pixels,
+                math.round(mismatches / total_pixels * 100),
+                string.escape(path_diff))
+            )
+        end
+        return
+    end
+    if prefix then
+        prefix = prefix .. " "
+    else
+        prefix = ""
+    end
     assert(fact == expected, string.format(
-        "(fact == expected) assertion failed\n  Expected: %s\n  Fact:     %s",
+        prefix .. "(fact == expected) assertion failed\n  Expected: %s\n  Fact:     %s",
         expected, fact
     ))
 end