Backwards compatibility fix: No default colormap for Mono QImages

This is a partial revert of a4e2f2e687.

That fix tried to avoid the risk of a crash in pixel() by ensuring
Mono QImages created with external data also got a default color
table. However, that broke usable behavior in existing code that was
painting in Mono QImages using color0/color1.

This commit reverts to the old behavior, and instead expands on the
checking in pixel() so that lacking color table is handled gracefully
for all indexed formats.

Task-number: QTBUG-54827
Change-Id: I9164198bed9d20c4b12cdba40a31c141bef3128d
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/image/qimage.cpp

@@ -828,17 +828,6 @@ QImageData *QImageData::create(uchar *data, int width, int height,  int bpl, QIm
     d->cleanupFunction = cleanupFunction;
     d->cleanupInfo = cleanupInfo;
 
-    switch (format) {
-    case QImage::Format_Mono:
-    case QImage::Format_MonoLSB:
-        d->colortable.resize(2);
-        d->colortable[0] = QColor(Qt::black).rgba();
-        d->colortable[1] = QColor(Qt::white).rgba();
-        break;
-    default:
-        break;
-    }
-
     return d;
 }
 
@@ -2237,21 +2226,30 @@ QRgb QImage::pixel(int x, int y) const
     }
 
     const uchar *s = d->data + y * d->bytes_per_line;
-    switch(d->format) {
+
+    int index = -1;
+    switch (d->format) {
     case Format_Mono:
-        return d->colortable.at((*(s + (x >> 3)) >> (~x & 7)) & 1);
+        index = (*(s + (x >> 3)) >> (~x & 7)) & 1;
+        break;
     case Format_MonoLSB:
-        return d->colortable.at((*(s + (x >> 3)) >> (x & 7)) & 1);
+        index = (*(s + (x >> 3)) >> (x & 7)) & 1;
+        break;
     case Format_Indexed8:
-        {
-            int index = (int)s[x];
-            if (index < d->colortable.size()) {
-                return d->colortable.at(index);
-            } else {
-                qWarning("QImage::pixel: color table index %d out of range.", index);
-                return 0;
-            }
+        index = s[x];
+        break;
+    default:
+        break;
+    }
+    if (index >= 0) {    // Indexed format
+        if (index >= d->colortable.size()) {
+            qWarning("QImage::pixel: color table index %d out of range.", index);
+            return 0;
         }
+        return d->colortable.at(index);
+    }
+
+    switch (d->format) {
     case Format_RGB32:
         return 0xff000000 | reinterpret_cast<const QRgb *>(s)[x];
     case Format_ARGB32: // Keep old behaviour.

tests/auto/gui/image/qimage/tst_qimage.cpp

@@ -99,6 +99,8 @@ private slots:
     void setPixel_data();
     void setPixel();
 
+    void defaultColorTable_data();
+    void defaultColorTable();
     void setColorCount();
     void setColor();
 
@@ -1450,6 +1452,38 @@ void tst_QImage::convertToFormatPreserveText()
     QCOMPARE(imgResult2.textKeys(), listResult);
 }
 
+void tst_QImage::defaultColorTable_data()
+{
+    QTest::addColumn<QImage::Format>("format");
+    QTest::addColumn<int>("createdDataCount");
+    QTest::addColumn<int>("externalDataCount");
+
+    // For historical reasons, internally created mono images get a default colormap.
+    // Externally created and Indexed8 images do not.
+    QTest::newRow("Mono") << QImage::Format_Mono << 2 << 0;
+    QTest::newRow("MonoLSB") << QImage::Format_MonoLSB << 2 << 0;
+    QTest::newRow("Indexed8") << QImage::Format_Indexed8 << 0 << 0;
+    QTest::newRow("ARGB32_PM") << QImage::Format_A2BGR30_Premultiplied << 0 << 0;
+}
+
+void tst_QImage::defaultColorTable()
+{
+    QFETCH(QImage::Format, format);
+    QFETCH(int, createdDataCount);
+    QFETCH(int, externalDataCount);
+
+    QImage img1(1, 1, format);
+    QCOMPARE(img1.colorCount(), createdDataCount);
+    QCOMPARE(img1.colorTable().size(), createdDataCount);
+
+    quint32 buf;
+    QImage img2(reinterpret_cast<uchar *>(&buf), 1, 1, format);
+    QCOMPARE(img2.colorCount(), externalDataCount);
+
+    QImage nullImg(0, 0, format);
+    QCOMPARE(nullImg.colorCount(), 0);
+}
+
 void tst_QImage::setColorCount()
 {
     QImage img(0, 0, QImage::Format_Indexed8);
@@ -3127,8 +3161,8 @@ void tst_QImage::pixel()
         QImage monolsb(&a, 1, 1, QImage::Format_MonoLSB);
         QImage indexed(&a, 1, 1, QImage::Format_Indexed8);
 
-        QCOMPARE(QColor(mono.pixel(0, 0)), QColor(Qt::black));
-        QCOMPARE(QColor(monolsb.pixel(0, 0)), QColor(Qt::black));
+        mono.pixel(0, 0); // Don't crash
+        monolsb.pixel(0, 0); // Don't crash
         indexed.pixel(0, 0); // Don't crash
     }
 }