Replace delta text labels with pre-rendered bitmaps to sidestep cross…

…-platform text rendering issues (#1496)

paparazzi/src/main/java/app/cash/paparazzi/internal/ImageUtils.kt

@@ -70,6 +70,33 @@ internal object ImageUtils {
     }
 
     if (error != null) {
+      val deltaWidth = max(goldenImageWidth, imageWidth)
+      if (deltaWidth > 80) {
+        /**
+         * AWT uses native text rendering under the hood, making it extremely difficult to get
+         * consistent cross-platform label text rendering, due to antialiasing, etc. This can
+         * result in false negatives when comparing delta images.
+         *
+         * As a workaround, we instead use text images pre-rendered on MacOSX 14 with the default
+         * font=Dialog, size=12 and composite them into the delta image here.
+         *
+         * We use that original font's ascent to offset the labels, which is determined by running
+         * the following on MacOSX 14:
+         *
+         * ```
+         * val z = BufferedImage(1, 1, TYPE_INT_ARGB)
+         * val MAC_OSX_FONT_DIALOG_SIZE_12_ASCENT = z.graphics.fontMetrics.ascent
+         * ```
+         */
+        val g = deltaImage.graphics
+        val yOffset = 20 - MAC_OSX_FONT_DIALOG_SIZE_12_ASCENT
+        val myClassLoader = ImageUtils::class.java.classLoader!!
+        val expectedLabel = ImageIO.read(myClassLoader.getResourceAsStream("expected_label.png"))
+        g.drawImage(expectedLabel, 10, yOffset, null)
+        val actualLabel = ImageIO.read(myClassLoader.getResourceAsStream("actual_label.png"))
+        g.drawImage(actualLabel, goldenImageWidth + deltaWidth + 10, yOffset, null)
+      }
+
       val deltaOutput = File(failureDir, "delta-$imageName")
       if (deltaOutput.exists()) {
         val deleted = deltaOutput.delete()
@@ -98,8 +125,7 @@ internal object ImageUtils {
   @Throws(IOException::class)
   fun compareImages(
     goldenImage: BufferedImage,
-    image: BufferedImage,
-    withText: Boolean = true
+    image: BufferedImage
   ): Pair<BufferedImage, Float> {
     var goldenImage = goldenImage
     if (goldenImage.type != TYPE_INT_ARGB) {
@@ -174,13 +200,6 @@ internal object ImageUtils {
     g.drawImage(goldenImage, 0, 0, null)
     g.drawImage(image, goldenImageWidth + deltaWidth, 0, null)
 
-    // Labels
-    if (withText && deltaWidth > 80) {
-      g.color = Color.RED
-      g.drawString("Expected", 10, 20)
-      g.drawString("Actual", goldenImageWidth + deltaWidth + 10, 20)
-    }
-
     g.dispose()
 
     // 3 different colors, 256 color levels
@@ -346,3 +365,5 @@ internal object ImageUtils {
     return relativePath.substring(relativePath.lastIndexOf(separatorChar) + 1)
   }
 }
+
+private const val MAC_OSX_FONT_DIALOG_SIZE_12_ASCENT: Int = 12

paparazzi/src/main/java/app/cash/paparazzi/internal/apng/ApngVerifier.kt

@@ -55,7 +55,7 @@ internal class ApngVerifier(
 
   fun verifyFrame(image: BufferedImage) {
     val (expectedFrame, actualFrame) = resizeMaxBounds(currentGoldenFrame ?: blankFrame, image)
-    val (deltaImage, percentDifferent) = ImageUtils.compareImages(expectedFrame, actualFrame, withErrorText)
+    val (deltaImage, percentDifferent) = ImageUtils.compareImages(expectedFrame, actualFrame)
     if (percentDifferent > maxPercentDifference) {
       if (deltaWriter == null) {
         deltaWriter = pngReader.initializeWriter()
@@ -79,7 +79,7 @@ internal class ApngVerifier(
       if (writer.frameCount % expectedDeltasPerFrame == 0) {
         currentGoldenFrame = pngReader.readNextFrame()
         val (expectedFrame, actualFrame) = resizeMaxBounds(currentGoldenFrame ?: blankFrame, image)
-        currentDelta = ImageUtils.compareImages(expectedFrame, actualFrame, withErrorText).first
+        currentDelta = ImageUtils.compareImages(expectedFrame, actualFrame).first
       }
     }
   }
@@ -88,7 +88,7 @@ internal class ApngVerifier(
     val deltaWriter = deltaWriter ?: return
     currentGoldenFrame?.let { lastFrame ->
       val (expectedFrame, actualFrame) = resizeMaxBounds(lastFrame, blankFrame)
-      val (currentDelta) = ImageUtils.compareImages(expectedFrame, actualFrame, withErrorText)
+      val (currentDelta) = ImageUtils.compareImages(expectedFrame, actualFrame)
 
       val times = expectedDeltasPerFrame - (deltaWriter.frameCount % expectedDeltasPerFrame)
       repeat(times) { deltaWriter.writeImage(currentDelta) }
@@ -98,8 +98,7 @@ internal class ApngVerifier(
     while (!pngReader.isFinished()) {
       val (deltaImage) = ImageUtils.compareImages(
         goldenImage = pngReader.readNextFrame()!!,
-        image = blankFrame,
-        withText = withErrorText
+        image = blankFrame
       )
       repeat(expectedDeltasPerFrame) { deltaWriter.writeImage(deltaImage) }
       invalidFrames++
@@ -143,8 +142,7 @@ internal class ApngVerifier(
         val nextFrame = readNextFrame() ?: blankFrame
         val (deltaImage) = ImageUtils.compareImages(
           goldenImage = nextFrame,
-          image = nextFrame,
-          withText = withErrorText
+          image = nextFrame
         )
 
         writeImage(deltaImage)