From d91c1422a9d0b96172fb13da5a179688b590d125 Mon Sep 17 00:00:00 2001
From: aavit <eirik.aavitsland@digia.com>
Date: Wed, 7 Nov 2012 15:19:20 +0100
Subject: [PATCH] Fix GIF image decoding: do not zero transparent pixels

For the special transparent color index, the decoder would skip
writing anything out (thus leaving the pixels at 0 rgba value).
Although correct for later frames, for the initial frame this would
loose the color information for such pixels (which one otherwise
could have made visible e.g. by converting then image to an
alpha-less image format).

Change-Id: I316cefce8f21797feedebfbf98296ad84eaa4b99
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
(cherry picked from qt/c309d424f45dc0e7b62fbbbabf20dbfe355f48a7)
Reviewed-by: aavit <eirik.aavitsland@digia.com>
---
 src/gui/image/qgifhandler.cpp                          |  15 ++++++---------
 tests/auto/gui/image/qimagereader/images/trans.gif     | Bin 0 -> 3234 bytes
 tests/auto/gui/image/qimagereader/tst_qimagereader.cpp |   9 +++++++++
 3 files changed, 15 insertions(+), 9 deletions(-)
 create mode 100644 tests/auto/gui/image/qimagereader/images/trans.gif

diff --git a/src/gui/image/qgifhandler.cpp b/src/gui/image/qgifhandler.cpp
index cedb27c..b60d1a1 100644
--- a/src/gui/image/qgifhandler.cpp
+++ b/src/gui/image/qgifhandler.cpp
@@ -488,7 +488,7 @@ int QGIFFormat::decode(QImage *image, const uchar *buffer, int length,
                 } else {
                     if (needfirst) {
                         firstcode=oldcode=code;
-                        if (!out_of_bounds && image->height() > y && firstcode!=trans_index)
+                        if (!out_of_bounds && image->height() > y && ((frame == 0) || (firstcode != trans_index)))
                             ((QRgb*)FAST_SCAN_LINE(bits, bpl, y))[x] = color(firstcode);
                         x++;
                         if (x>=swidth) out_of_bounds = true;
@@ -540,17 +540,13 @@ int QGIFFormat::decode(QImage *image, const uchar *buffer, int length,
                         }
                         oldcode=incode;
                         const int h = image->height();
-                        const QRgb *map = lcmap ? localcmap : globalcmap;
                         QRgb *line = 0;
                         if (!out_of_bounds && h > y)
                             line = (QRgb*)FAST_SCAN_LINE(bits, bpl, y);
                         while (sp>stack) {
                             const uchar index = *(--sp);
-                            if (!out_of_bounds && h > y && index!=trans_index) {
-                                if (index > ncols)
-                                    line[x] = Q_TRANSPARENT;
-                                else
-                                    line[x] = map ? map[index] : 0;
+                            if (!out_of_bounds && h > y && ((frame == 0) || (index != trans_index))) {
+                                line[x] = color(index);
                             }
                             x++;
                             if (x>=swidth) out_of_bounds = true;
@@ -1031,11 +1027,12 @@ void QGIFFormat::nextY(unsigned char *bits, int bpl)
 
 inline QRgb QGIFFormat::color(uchar index) const
 {
-    if (index == trans_index || index > ncols)
+    if (index > ncols)
         return Q_TRANSPARENT;
 
     QRgb *map = lcmap ? localcmap : globalcmap;
-    return map ? map[index] : 0;
+    QRgb col = map ? map[index] : 0;
+    return index == trans_index ? col & Q_TRANSPARENT : col;
 }
 
 //-------------------------------------------------------------------------
diff --git a/tests/auto/gui/image/qimagereader/images/trans.gif b/tests/auto/gui/image/qimagereader/images/trans.gif
new file mode 100644
index 0000000000000000000000000000000000000000..e26398af5354d04432ae7c4235f3b6b0194eb1c8
GIT binary patch
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literal 0
HcmV?d00001

diff --git a/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp b/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp
index 8965cbd..57aaeb9 100644
--- a/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp
+++ b/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp
@@ -756,6 +756,15 @@ void tst_QImageReader::gifHandlerBugs()
         QVERIFY(io.canRead());
         QCOMPARE(io.loopCount(), -1);
     }
+
+    // Check that pixels with the transparent color are transparent but not zeroed
+    {
+        QImageReader io(prefix + "trans.gif");
+        QVERIFY(io.canRead());
+        QImage im = io.read();
+        QCOMPARE(im.pixel(0,0), qRgba(0x3f, 0xff, 0x7f, 0x00));
+        QCOMPARE(im.pixel(10,10), qRgba(0x3f, 0xff, 0x7f, 0x00));
+    }
 }
 
 void tst_QImageReader::animatedGif()
-- 
2.7.4