JPEG-XL: fix import/export of HDR alpha channels

CCBUG: 460380
(cherry picked from commit e6e7b467)

plugins/impex/jxl/JPEGXLExport.cpp

@@ -86,11 +86,11 @@ inline QByteArray writeLayer(const int width,
         for (int x = 0; x < width; x++) {
             CSTrait::normalisedChannelsValue(it->rawDataConst(), pixelValues);
             if (!convertToRec2020 && !isLinear) {
-                for (int i = 0; i < 4; i++) {
+                for (int i = 0; i < channels; i++) {
                     src[i] = static_cast<double>(dst[i]);
                 }
                 profile->linearizeFloatValue(pixelValuesLinear);
-                for (int i = 0; i < 4; i++) {
+                for (int i = 0; i < channels; i++) {
                     dst[i] = static_cast<float>(src[i]);
                 }
             }
@@ -100,7 +100,12 @@ inline QByteArray writeLayer(const int width,
             }
 
             for (int ch = 0; ch < channels; ch++) {
-                dst[ch] = applyCurveAsNeeded<conversionPolicy>(dst[ch]);
+                if (ch == CSTrait::alpha_pos) {
+                    dst[ch] = applyCurveAsNeeded<ConversionPolicy::KeepTheSame>(
+                        dst[ch]);
+                } else {
+                    dst[ch] = applyCurveAsNeeded<conversionPolicy>(dst[ch]);
+                }
             }
 
             if (swap) {

plugins/impex/jxl/JPEGXLImport.cpp

@@ -104,14 +104,21 @@ inline void imageOutCallback(void *that, size_t x, size_t y, size_t numPixels, c
         const double *lCoef = data->lCoef.constData();
         QVector<float> pixelValues(static_cast<int>(cs->channelCount()));
         float *tmp = pixelValues.data();
+        const quint32 alphaPos = cs->alphaPos();
 
         for (size_t i = 0; i < numPixels; i++) {
             for (size_t i = 0; i < channels; i++) {
-                tmp[i] = 0;
+                tmp[i] = 1.0;
             }
 
             for (size_t ch = 0; ch < channels; ch++) {
-                tmp[ch] = value<policy, channelsType>(src, ch);
+                if (ch == alphaPos) {
+                    tmp[ch] = value<policy, channelsType>(src, ch);
+                } else {
+                    tmp[ch] =
+                        value<LinearizePolicy::KeepTheSame, channelsType>(src,
+                                                                          ch);
+                }
             }
 
             if (swap) {