RAW: Allow preview loading

This patch addresses reports of performance issues on large raw collections. Programs that generate previews must use the plugin correctly.

**Setting quality to 0 may return a different image than in the past** (Nothing changes for all other quality values): the plugin loads the embedded thumbnail and, in case of error, decodes the image with quality 1. When compiled with libRAW 0.21+, the plugin automatically select the largest preview from the ones in the file.

src/imageformats/raw.cpp

@@ -581,6 +581,66 @@ void setParams(QImageIOHandler *handler, LibRaw *rawProcessor)
     params.use_fuji_rotate = T_SR(quality) ? 0 : 1;
 }
 
+bool LoadTHUMB(QImageIOHandler *handler, QImage &img)
+{
+    std::unique_ptr<LibRaw> rawProcessor(new LibRaw);
+
+    // *** Open the stream
+    auto device = handler->device();
+#ifndef EXCLUDE_LibRaw_QIODevice
+    LibRaw_QIODevice stream(device);
+    if (rawProcessor->open_datastream(&stream) != LIBRAW_SUCCESS) {
+        return false;
+    }
+#else
+    auto ba = device->readAll();
+    if (rawProcessor->open_buffer(ba.data(), ba.size()) != LIBRAW_SUCCESS) {
+        return false;
+    }
+#endif
+
+#if (LIBRAW_VERSION < LIBRAW_MAKE_VERSION(0, 21, 0))
+    // *** Unpacking selected thumbnail
+    if (rawProcessor->unpack_thumb() != LIBRAW_SUCCESS) {
+        return false;
+    }
+#else
+    // *** Search for the bigger thumbnail
+    auto &&tlist = rawProcessor->imgdata.thumbs_list;
+    auto idx = 0;
+    for (auto n = 1; n < std::min(tlist.thumbcount, LIBRAW_THUMBNAIL_MAXCOUNT); ++n) {
+        if (tlist.thumblist[n].twidth > tlist.thumblist[idx].twidth)
+            idx = n;
+    }
+
+    // *** Unpacking selected thumbnail
+    if (rawProcessor->unpack_thumb_ex(idx) != LIBRAW_SUCCESS) {
+        return false;
+    }
+#endif
+
+    // *** Convert to QImage
+    pi_unique_ptr processedImage(rawProcessor->dcraw_make_mem_thumb(), LibRaw::dcraw_clear_mem);
+    if (processedImage == nullptr) {
+        return false;
+    }
+    auto ba = QByteArray(reinterpret_cast<const char *>(processedImage->data), qsizetype(processedImage->data_size));
+    if (ba.isEmpty()) {
+        return false;
+    }
+    if (processedImage->type == LIBRAW_IMAGE_BITMAP) {
+        // clang-format off
+        auto header = QString::fromUtf8("P%1\n%2 %3\n%4\n") // taken from KDcraw
+                          .arg(processedImage->colors == 3 ? QLatin1String("6") : QLatin1String("5"))
+                          .arg(processedImage->width)
+                          .arg(processedImage->height)
+                          .arg((1 << processedImage->bits)-1);
+        // clang-format on
+        ba.prepend(header.toLatin1());
+    }
+    return img.loadFromData(ba);
+}
+
 bool LoadRAW(QImageIOHandler *handler, QImage &img)
 {
     std::unique_ptr<LibRaw> rawProcessor(new LibRaw);
@@ -679,6 +739,11 @@ bool LoadRAW(QImageIOHandler *handler, QImage &img)
         if (params.output_color == 7) {
             img.setColorSpace(QColorSpace(QColorSpace::DisplayP3));
         }
+#if QT_VERSION >= QT_VERSION_CHECK(6, 8, 0)
+        if (params.output_color == 8) {
+            img.setColorSpace(QColorSpace(QColorSpace::Bt2020));
+        }
+#endif
     }
 
     // *** Set the metadata
@@ -756,7 +821,16 @@ bool RAWHandler::read(QImage *image)
     }
 
     QImage img;
-    if (!LoadRAW(this, img)) {
+    auto ok = false;
+    if (m_quality == 0) {
+        ok = LoadTHUMB(this, img);
+        if (!ok && !dev->isSequential())
+            dev->seek(m_startPos);
+    }
+    if (!ok) {
+        ok = LoadRAW(this, img);
+    }
+    if (!ok) {
         return false;
     }
 

src/imageformats/raw_p.h

@@ -60,7 +60,8 @@ private:
      * @note It is safe to set both W and A: W is used if camera white balance is found, otherwise A is used.
      *
      * When quality is a positive value, a value between 0 and 100 is expected. The values are interpreted as follows:
-     * - 00-09: I =  0, C = 1, B = 0, W = 1, A = 1, H = 1 (Linear, sRGB, 8-bits, Camera White, Auto White, Half-size)
+     * - 00   : Embedded preview, if fails same as Half-Size (01-09)
+     * - 01-09: I =  0, C = 1, B = 0, W = 1, A = 1, H = 1 (Linear, sRGB, 8-bits, Camera White, Auto White, Half-size)
      * - 10-19: I =  0, C = 1, B = 0, W = 1, A = 1, H = 0 (Linear, sRGB, 8-bits, Camera White, Auto White)
      * - 20-29: I =  3, C = 1, B = 0, W = 1, A = 1, H = 0 (AHD, sRGB, 8-bits, Camera White, Auto White)
      * - 30-39: I =  3, C = 1, B = 1, W = 1, A = 1, H = 0 (AHD, sRGB, 16-bits, Camera White, Auto White) [Default]
@@ -72,11 +73,12 @@ private:
      * - >= 90: I = 11, C = 4, B = 1, W = 1, A = 1, H = 0 (DHT, ProPhoto, 16-bits, Camera White, Auto White)
      *
      * When the quality is -1, default quality is used.
+     * @sa m_subType
      */
     qint32 m_quality;
 
-    /*!
+    /**
-     * \brief m_startPos
+     * @brief m_startPos
      * The initial device position to allow multi image load (cache value).
      */
     qint64 m_startPos;