fixed rendering of transparent pdfs; white overlay for very dark pdf

art/open_pdf_white.svg

@@ -0,0 +1,55 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   version="1.1"
+   width="24"
+   height="24"
+   viewBox="0 0 24 24"
+   id="svg4"
+   sodipodi:docname="open_pdf_white.svg"
+   inkscape:version="0.92.4 5da689c313, 2019-01-14">
+  <metadata
+     id="metadata10">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title />
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs8" />
+  <sodipodi:namedview
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="3840"
+     inkscape:window-height="1600"
+     id="namedview6"
+     showgrid="false"
+     inkscape:zoom="9.8333333"
+     inkscape:cx="-23.084746"
+     inkscape:cy="11.084746"
+     inkscape:window-x="1920"
+     inkscape:window-y="0"
+     inkscape:window-maximized="0"
+     inkscape:current-layer="svg4" />
+  <path
+     d="M12,10.5H13V13.5H12V10.5M7,11.5H8V10.5H7V11.5M20,6V18A2,2 0 0,1 18,20H6A2,2 0 0,1 4,18V6A2,2 0 0,1 6,4H18A2,2 0 0,1 20,6M9.5,10.5A1.5,1.5 0 0,0 8,9H5.5V15H7V13H8A1.5,1.5 0 0,0 9.5,11.5V10.5M14.5,10.5A1.5,1.5 0 0,0 13,9H10.5V15H13A1.5,1.5 0 0,0 14.5,13.5V10.5M18.5,9H15.5V15H17V13H18.5V11.5H17V10.5H18.5V9Z"
+     id="path2"
+     style="fill:#ffffff;fill-opacity:0.69803923" />
+</svg>

art/render.rb

@@ -24,6 +24,7 @@ images = {
     'play_video_black.svg' => ['play_video_black', 128],
     'play_gif_black.svg' => ['play_gif_black', 128],
     'open_pdf_black.svg' => ['open_pdf_black', 128],
+    'open_pdf_white.svg' => ['open_pdf_white', 128],
 	'conversations_mono.svg' => ['conversations/ic_notification', 24],
     'quicksy_mono.svg' => ['quicksy/ic_notification', 24],
 	'ic_send_text_offline.svg' => ['ic_send_text_offline', 36],

src/main/java/eu/siacs/conversations/persistance/FileBackend.java

@@ -825,7 +825,7 @@ public class FileBackend {
                 } else if (mime.startsWith("video/")) {
                     thumbnail = getVideoPreview(file, size);
                 } else {
-                    Bitmap fullsize = getFullsizeImagePreview(file, size);
+                    Bitmap fullsize = getFullSizeImagePreview(file, size);
                     if (fullsize == null) {
                         throw new FileNotFoundException();
                     }
@@ -844,7 +844,7 @@ public class FileBackend {
         return thumbnail;
     }
 
-    private Bitmap getFullsizeImagePreview(File file, int size) {
+    private Bitmap getFullSizeImagePreview(File file, int size) {
         BitmapFactory.Options options = new BitmapFactory.Options();
         options.inSampleSize = calcSampleSize(file, size);
         try {
@@ -886,6 +886,21 @@ public class FileBackend {
         return record < 0;
     }
 
+    private boolean paintOverlayBlackPdf(final Bitmap bitmap) {
+        final int h = bitmap.getHeight();
+        final int w = bitmap.getWidth();
+        int white = 0;
+        for (int y = 0; y < h; ++y) {
+            for (int x = 0; x < w; ++x) {
+                int pixel = bitmap.getPixel(x, y);
+                if ((Color.red(pixel) * 0.299 + Color.green(pixel) * 0.587 + Color.blue(pixel) * 0.114) > 186) {
+                    white++;
+                }
+            }
+        }
+        return white > (h * w * 0.4f);
+    }
+
     private Bitmap cropCenterSquareVideo(Uri uri, int size) {
         MediaMetadataRetriever metadataRetriever = new MediaMetadataRetriever();
         Bitmap frame;
@@ -925,8 +940,9 @@ public class FileBackend {
             final PdfRenderer.Page page = pdfRenderer.openPage(0);
             Dimensions dimensions = scalePdfDimensions(new Dimensions(page.getHeight(), page.getWidth()));
             final Bitmap rendered = Bitmap.createBitmap(dimensions.width, dimensions.height, Bitmap.Config.ARGB_8888);
+            rendered.eraseColor(0xffffffff);
             page.render(rendered, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY);
-            drawOverlay(rendered, R.drawable.open_pdf_black, 0.75f);
+            drawOverlay(rendered, paintOverlayBlackPdf(rendered) ? R.drawable.open_pdf_black : R.drawable.open_pdf_white, 0.75f);
             return rendered;
         } catch (IOException e) {
             Log.d(Config.LOGTAG, "unable to render PDF document preview", e);