EXR: RGB / YC subtype support

README.md

@@ -392,6 +392,21 @@ The plugin can set the following additional metadata:
 - `EXRLayerName`: A string containing the name of the EXR layer used to decode 
    the image.
 
+When writing, it is possible to specify compression ratio and quality. The 
+values ​​you can set depend on the version of the OpenEXR library you're using.
+Please check the [exr_p.h](./src/imageformats/exr_p.h) for more 
+info.
+
+When writing, it is also possible to specify a subtype:
+- `RGB` (default): preserves full-resolution color data (R, G, B) without 
+  subsampling or loss of chromatic detail.
+- `YC`: converts RGB data into luminance (Y) and 2x2 subsampled chroma 
+  (RY, BY), cutting raw color data by ~50% for significantly smaller file 
+  sizes.
+
+> [!note]
+> When writing grayscale images, the subtype makes no difference.
+
 ### The EPS plugin
 
 The plugin uses `Ghostscript` to convert the raster image. When reading it

src/imageformats/exr.cpp

@@ -200,12 +200,17 @@ void K_OStream::seekg(Imf::Int64 pos)
     m_dev->seek(pos);
 }
 
+#define EXR_SUBFORMAT_RGB QByteArray("RGB")
+
+#define EXR_SUBFORMAT_YC QByteArray("YC")
+
 EXRHandler::EXRHandler()
     : m_compressionRatio(-1)
     , m_quality(-1)
     , m_imageNumber(0)
     , m_imageCount(0)
     , m_startPos(-1)
+    , m_subType(EXR_SUBFORMAT_RGB)
 {
     // Set the number of threads to use (0 is allowed)
     Imf::setGlobalThreadCount(QThread::idealThreadCount() / 2);
@@ -688,6 +693,9 @@ bool EXRHandler::write(const QImage &image)
         // write the EXR
         K_OStream ostr(device());
         auto channelsType = image.hasAlphaChannel() ? Imf::RgbaChannels::WRITE_RGBA : Imf::RgbaChannels::WRITE_RGB;
+        if (m_subType == EXR_SUBFORMAT_YC) {
+            channelsType = channelsType == Imf::RgbaChannels::WRITE_RGBA ? Imf::RgbaChannels::WRITE_YCA : Imf::RgbaChannels::WRITE_YC;
+        }
         if (image.format() == QImage::Format_Mono ||
             image.format() == QImage::Format_MonoLSB ||
             image.format() == QImage::Format_Grayscale16 ||
@@ -743,6 +751,15 @@ void EXRHandler::setOption(ImageOption option, const QVariant &value)
             m_quality = q;
         }
     }
+    if (option == QImageIOHandler::SubType) {
+        auto subType = value.toByteArray();
+        auto list = EXRHandler::option(QImageIOHandler::SupportedSubTypes).value<QList<QByteArray>>();
+        if (list.contains(subType)) {
+            m_subType = subType;
+        } else {
+            m_subType = EXR_SUBFORMAT_RGB;
+        }
+    }
 }
 
 bool EXRHandler::supportsOption(ImageOption option) const
@@ -761,6 +778,12 @@ bool EXRHandler::supportsOption(ImageOption option) const
     if (option == QImageIOHandler::Quality) {
         return true;
     }
+    if (option == QImageIOHandler::SubType) {
+        return true;
+    }
+    if (option == QImageIOHandler::SupportedSubTypes) {
+        return true;
+    }
     return false;
 }
 
@@ -815,6 +838,14 @@ QVariant EXRHandler::option(ImageOption option) const
         v = QVariant(m_quality);
     }
 
+    if (option == QImageIOHandler::SupportedSubTypes) {
+        return QVariant::fromValue(QList<QByteArray>() << EXR_SUBFORMAT_RGB << EXR_SUBFORMAT_YC);
+    }
+
+    if (option == QImageIOHandler::SubType) {
+        return QVariant::fromValue(m_subType);
+    }
+
     return v;
 }
 

src/imageformats/exr_p.h

@@ -118,6 +118,14 @@ private:
      * The initial device position to allow multi image load (cache value).
      */
     qint64 m_startPos;
+
+    /*!
+     * \brief m_subType
+     * The EXR subtype to use when writing:
+     * - RGB: preserves full-resolution color data (R, G, B) without subsampling or loss of chromatic detail (default).
+     * - YC: converts RGB data into luminance (Y) and 2x2 subsampled chroma (RY, BY), cutting raw color data by ~50% for significantly smaller file sizes.
+     */
+    QByteArray m_subType;
 };
 
 class EXRPlugin : public QImageIOPlugin