diff --git a/rtengine/rtthumbnail.cc b/rtengine/rtthumbnail.cc
index fa5abb8a4..9279e8829 100644
--- a/rtengine/rtthumbnail.cc
+++ b/rtengine/rtthumbnail.cc
@@ -112,6 +112,11 @@ void scale_colors (rtengine::RawImage *ri, float scale_mul[4], float cblack[4],
     } else if (ri->isXtrans()) {
         const int height = ri->get_iheight();
         const int width = ri->get_iwidth();
+        const bool isFloat = ri->isFloat();
+        const int top_margin = ri->get_topmargin();
+        const int left_margin = ri->get_leftmargin();
+        const int raw_width = ri->get_rawwidth();
+        const float * const float_raw_image = ri->get_FloatRawImage();
 
 #ifdef _OPENMP
         #pragma omp parallel for if(multiThread)
@@ -127,7 +132,12 @@ void scale_colors (rtengine::RawImage *ri, float scale_mul[4], float cblack[4],
             for (; col < width - 5; col += 6) {
                 for (int i = 0; i < 6; ++i) {
                     const unsigned ccol = c[i];
-                    float val = image[row * width + col + i][ccol];
+                    float val;
+                    if (isFloat) {
+                        val = float_raw_image[(row + top_margin) * raw_width + col + i + left_margin];
+                    } else {
+                        val = image[row * width + col + i][ccol];
+                    }
                     val -= cblack[ccol];
                     val *= scale_mul[ccol];
                     image[row * width + col + i][ccol] = rtengine::CLIP (val);
@@ -136,7 +146,12 @@ void scale_colors (rtengine::RawImage *ri, float scale_mul[4], float cblack[4],
 
             for (; col < width; ++col) { // remaining columns
                 const unsigned ccol = ri->XTRANSFC (row, col);
-                float val = image[row * width + col][ccol];
+                float val;
+                if (isFloat) {
+                    val = float_raw_image[(row + top_margin) * raw_width + col + left_margin];
+                } else {
+                    val = image[row * width + col][ccol];
+                }
                 val -= cblack[ccol];
                 val *= scale_mul[ccol];
                 image[row * width + col][ccol] = rtengine::CLIP (val);