Use % to determine if GIF frame is transparent.

Fixes #214

third_party/gif_encoder/src/main/java/com/bumptech/glide/gifencoder/AnimatedGifEncoder.java

@@ -4,6 +4,7 @@ package com.bumptech.glide.gifencoder;
 import android.graphics.Bitmap;
 import android.graphics.Canvas;
 import android.graphics.Color;
+import android.util.Log;
 
 import java.io.BufferedOutputStream;
 import java.io.FileOutputStream;
@@ -35,6 +36,10 @@ import java.io.OutputStream;
  */
 
 public class AnimatedGifEncoder {
+    private static final String TAG = "AnimatedGifEncoder";
+
+    // The minimum % of an images pixels that must be transparent for us to set a transparent index automatically.
+    private static final double MIN_TRANSPARENT_PERCENTAGE = 4d;
 
     private int width; // image size
 
@@ -384,14 +389,23 @@ public class AnimatedGifEncoder {
 
         int pixelsIndex = 0;
         hasTransparentPixels = false;
+        int totalTransparentPixels = 0;
         for (final int pixel : pixelsInt) {
             if (pixel == Color.TRANSPARENT) {
-                hasTransparentPixels = true;
+                totalTransparentPixels++;
             }
             pixels[pixelsIndex++] = (byte) (pixel & 0xFF);
             pixels[pixelsIndex++] = (byte) ((pixel >> 8) & 0xFF);
             pixels[pixelsIndex++] = (byte) ((pixel >> 16) & 0xFF);
         }
+
+        double transparentPercentage = 100 * totalTransparentPixels / (double) pixelsInt.length;
+        // Assume images with greater where more than n% of the pixels are transparent actually have transparency.
+        // See issue #214.
+        hasTransparentPixels = transparentPercentage > MIN_TRANSPARENT_PERCENTAGE;
+        if (Log.isLoggable(TAG, Log.DEBUG)) {
+            Log.d(TAG, "got pixels for frame with " + transparentPercentage + "% transparent pixels");
+        }
     }
 
     /**