fix black level computation for applying flat-field correction, taken from ART, thanks @agriggio

rtengine/rawimage.cc

@@ -84,6 +84,29 @@ eSensorType RawImage::getSensorType() const
  */
 void RawImage::get_colorsCoeff(float *pre_mul_, float *scale_mul_, float *cblack_, bool forceAutoWB)
 {
+    if (!pre_mul_ && !scale_mul_ && !forceAutoWB) {
+        // only black levels
+        if (isXtrans()) {
+            // for xtrans files dcraw stores black levels in cblack[6] .. cblack[41], but all are equal, so we just use cblack[6]
+            for (int c = 0; c < 4; c++) {
+                cblack_[c] = (float) this->get_cblack(6);
+            }
+        } else if ((this->get_cblack(4) + 1) / 2 == 1 && (this->get_cblack(5) + 1) / 2 == 1) {
+            for (int c = 0; c < 4; c++) {
+                cblack_[c] = this->get_cblack(c);
+            }
+
+            for (int c = 0; c < 4; c++) {
+                cblack_[FC(c / 2, c % 2)] = this->get_cblack(6 + c / 2 % this->get_cblack(4) * this->get_cblack(5) + c % 2 % this->get_cblack(5));
+            }
+        } else {
+            for (int c = 0; c < 4; c++) {
+                cblack_[c] = (float) this->get_cblack(c);
+            }
+        }
+        return;
+    }
+
     unsigned sum[8], c;
     unsigned W = this->get_width();
     unsigned H = this->get_height();

rtengine/rawimage.h

@@ -259,6 +259,10 @@ public:
     {
         return float_raw_image;
     }
+    void set_filters(unsigned f)
+    {
+        filters = f;
+    }
 
 public:
     // dcraw functions

rtengine/rawimagesource.cc

@@ -2400,10 +2400,11 @@ void RawImageSource::HLRecovery_Global(const ToneCurveParams &hrp)
  */
 void RawImageSource::copyOriginalPixels(const RAWParams &raw, RawImage *src, RawImage *riDark, RawImage *riFlatFile, array2D<float> &rawData)
 {
-    const float black[4] = {
+    const auto tmpfilters = ri->get_filters();
-                     static_cast<float>(ri->get_cblack(0)), static_cast<float>(ri->get_cblack(1)),
+    ri->set_filters(ri->prefilters); // we need 4 blacks for bayer processing
-                     static_cast<float>(ri->get_cblack(2)), static_cast<float>(ri->get_cblack(3))
+    float black[4];
-                     };
+    ri->get_colorsCoeff(nullptr, nullptr, black, false);
+    ri->set_filters(tmpfilters);
 
     if (ri->getSensorType() == ST_BAYER || ri->getSensorType() == ST_FUJI_XTRANS) {
         if (!rawData) {