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[master] e52e057 - Fix for issue #159.

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[master] e52e057 - Fix for issue #159. "ManoloFLTK" Nov 19, 2020  
 
commit e52e057cdff0b06a3d075740813ee098364c78c1
Author:     ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
AuthorDate: Thu Nov 19 15:59:33 2020 +0100
Commit:     ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
CommitDate: Thu Nov 19 15:59:48 2020 +0100

    Fix for issue #159.

 src/drivers/X11/Fl_X11_Screen_Driver.cxx           | 563 ++++++++++-----------
 src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx |  10 +
 2 files changed, 291 insertions(+), 282 deletions(-)

diff --git src/drivers/X11/Fl_X11_Screen_Driver.cxx src/drivers/X11/Fl_X11_Screen_Driver.cxx
index 9141047..7038b5a 100644
--- src/drivers/X11/Fl_X11_Screen_Driver.cxx
+++ src/drivers/X11/Fl_X11_Screen_Driver.cxx
@@ -839,31 +839,11 @@ Fl_RGB_Image *Fl_X11_Screen_Driver::read_win_rectangle(int X, int Y, int w, int
     }
   }
 
-  if (!image) return 0;
   if (s != 1) {
     w = ws;
     h = hs;
   }
 
-#ifdef DEBUG
-  printf("width            = %d\n", image->width);
-  printf("height           = %d\n", image->height);
-  printf("xoffset          = %d\n", image->xoffset);
-  printf("format           = %d\n", image->format);
-  printf("data             = %p\n", image->data);
-  printf("byte_order       = %d\n", image->byte_order);
-  printf("bitmap_unit      = %d\n", image->bitmap_unit);
-  printf("bitmap_bit_order = %d\n", image->bitmap_bit_order);
-  printf("bitmap_pad       = %d\n", image->bitmap_pad);
-  printf("depth            = %d\n", image->depth);
-  printf("bytes_per_line   = %d\n", image->bytes_per_line);
-  printf("bits_per_pixel   = %d\n", image->bits_per_pixel);
-  printf("red_mask         = %08x\n", image->red_mask);
-  printf("green_mask       = %08x\n", image->green_mask);
-  printf("blue_mask        = %08x\n", image->blue_mask);
-  printf("map_entries      = %d\n", fl_visual->visual->map_entries);
-#endif // DEBUG
-
   const int d = 3; // Depth of image
   uchar *p = NULL;
   // Allocate the image data array as needed...
@@ -871,299 +851,318 @@ Fl_RGB_Image *Fl_X11_Screen_Driver::read_win_rectangle(int X, int Y, int w, int
 
   // Initialize the default colors/alpha in the whole image...
   memset(p, 0, w * h * d);
-
-  // Check that we have valid mask/shift values...
-  if (!image->red_mask && image->bits_per_pixel > 12) {
-    // Greater than 12 bits must be TrueColor...
-    image->red_mask   = fl_visual->visual->red_mask;
-    image->green_mask = fl_visual->visual->green_mask;
-    image->blue_mask  = fl_visual->visual->blue_mask;
-
+  if (image) {
 #ifdef DEBUG
-    // Defined in Fl_Xlib_Graphics_Driver_color.cxx
-    extern uchar fl_redmask, fl_greenmask, fl_bluemask;
-    extern int fl_redshift, fl_greenshift, fl_blueshift;
-    puts("\n---- UPDATED ----");
-    printf("fl_redmask       = %08x\n", fl_redmask);
-    printf("fl_redshift      = %d\n", fl_redshift);
-    printf("fl_greenmask     = %08x\n", fl_greenmask);
-    printf("fl_greenshift    = %d\n", fl_greenshift);
-    printf("fl_bluemask      = %08x\n", fl_bluemask);
-    printf("fl_blueshift     = %d\n", fl_blueshift);
+    printf("width            = %d\n", image->width);
+    printf("height           = %d\n", image->height);
+    printf("xoffset          = %d\n", image->xoffset);
+    printf("format           = %d\n", image->format);
+    printf("data             = %p\n", image->data);
+    printf("byte_order       = %d\n", image->byte_order);
+    printf("bitmap_unit      = %d\n", image->bitmap_unit);
+    printf("bitmap_bit_order = %d\n", image->bitmap_bit_order);
+    printf("bitmap_pad       = %d\n", image->bitmap_pad);
+    printf("depth            = %d\n", image->depth);
+    printf("bytes_per_line   = %d\n", image->bytes_per_line);
+    printf("bits_per_pixel   = %d\n", image->bits_per_pixel);
     printf("red_mask         = %08x\n", image->red_mask);
     printf("green_mask       = %08x\n", image->green_mask);
     printf("blue_mask        = %08x\n", image->blue_mask);
+    printf("map_entries      = %d\n", fl_visual->visual->map_entries);
+#endif // DEBUG
+    
+    // Check that we have valid mask/shift values...
+    if (!image->red_mask && image->bits_per_pixel > 12) {
+      // Greater than 12 bits must be TrueColor...
+      image->red_mask   = fl_visual->visual->red_mask;
+      image->green_mask = fl_visual->visual->green_mask;
+      image->blue_mask  = fl_visual->visual->blue_mask;
+      
+#ifdef DEBUG
+      // Defined in Fl_Xlib_Graphics_Driver_color.cxx
+      extern uchar fl_redmask, fl_greenmask, fl_bluemask;
+      extern int fl_redshift, fl_greenshift, fl_blueshift;
+      puts("\n---- UPDATED ----");
+      printf("fl_redmask       = %08x\n", fl_redmask);
+      printf("fl_redshift      = %d\n", fl_redshift);
+      printf("fl_greenmask     = %08x\n", fl_greenmask);
+      printf("fl_greenshift    = %d\n", fl_greenshift);
+      printf("fl_bluemask      = %08x\n", fl_bluemask);
+      printf("fl_blueshift     = %d\n", fl_blueshift);
+      printf("red_mask         = %08x\n", image->red_mask);
+      printf("green_mask       = %08x\n", image->green_mask);
+      printf("blue_mask        = %08x\n", image->blue_mask);
 #endif // DEBUG
-  }
-
-  // Check if we have colormap image...
-  if (!image->red_mask) {
-    // Get the colormap entries for this window...
-    maxindex = fl_visual->visual->map_entries;
-
-    for (i = 0; i < maxindex; i ++) colors[i].pixel = i;
-
-    XQueryColors(fl_display, fl_colormap, colors, maxindex);
-
-    for (i = 0; i < maxindex; i ++) {
-      cvals[i][0] = colors[i].red >> 8;
-      cvals[i][1] = colors[i].green >> 8;
-      cvals[i][2] = colors[i].blue >> 8;
     }
-
-    // Read the pixels and output an RGB image...
-    for (y = 0; y < image->height; y ++) {
-      pixel = (unsigned char *)(image->data + y * image->bytes_per_line);
-      line  = p + y * w * d;
-
-      switch (image->bits_per_pixel) {
-        case 1 :
-          for (x = image->width, line_ptr = line, index_mask = 128;
-               x > 0;
-               x --, line_ptr += d) {
-            if (*pixel & index_mask) {
-              line_ptr[0] = cvals[1][0];
-              line_ptr[1] = cvals[1][1];
-              line_ptr[2] = cvals[1][2];
-            } else {
-              line_ptr[0] = cvals[0][0];
-              line_ptr[1] = cvals[0][1];
-              line_ptr[2] = cvals[0][2];
-            }
-
-            if (index_mask > 1) {
-              index_mask >>= 1;
-            } else {
-              index_mask = 128;
-              pixel ++;
-            }
-          }
-          break;
-
-        case 2 :
-          for (x = image->width, line_ptr = line, index_shift = 6;
-               x > 0;
-               x --, line_ptr += d) {
-            i = (*pixel >> index_shift) & 3;
-
-            line_ptr[0] = cvals[i][0];
-            line_ptr[1] = cvals[i][1];
-            line_ptr[2] = cvals[i][2];
-
-            if (index_shift > 0) {
-              index_shift -= 2;
-            } else {
-              index_shift = 6;
-              pixel ++;
-            }
-          }
-          break;
-
-        case 4 :
-          for (x = image->width, line_ptr = line, index_shift = 4;
-               x > 0;
-               x --, line_ptr += d) {
-            if (index_shift == 4) i = (*pixel >> 4) & 15;
-            else i = *pixel & 15;
-
-            line_ptr[0] = cvals[i][0];
-            line_ptr[1] = cvals[i][1];
-            line_ptr[2] = cvals[i][2];
-
-            if (index_shift > 0) {
-              index_shift = 0;
-            } else {
-              index_shift = 4;
-              pixel ++;
-            }
-          }
-          break;
-
-        case 8 :
-          for (x = image->width, line_ptr = line;
-               x > 0;
-               x --, line_ptr += d, pixel ++) {
-            line_ptr[0] = cvals[*pixel][0];
-            line_ptr[1] = cvals[*pixel][1];
-            line_ptr[2] = cvals[*pixel][2];
-          }
-          break;
-
-        case 12 :
-          for (x = image->width, line_ptr = line, index_shift = 0;
-               x > 0;
-               x --, line_ptr += d) {
-            if (index_shift == 0) {
-              i = ((pixel[0] << 4) | (pixel[1] >> 4)) & 4095;
-            } else {
-              i = ((pixel[1] << 8) | pixel[2]) & 4095;
-            }
-
-            line_ptr[0] = cvals[i][0];
-            line_ptr[1] = cvals[i][1];
-            line_ptr[2] = cvals[i][2];
-
-            if (index_shift == 0) {
-              index_shift = 4;
-            } else {
-              index_shift = 0;
-              pixel += 3;
-            }
-          }
-          break;
+    
+    // Check if we have colormap image...
+    if (!image->red_mask) {
+      // Get the colormap entries for this window...
+      maxindex = fl_visual->visual->map_entries;
+      
+      for (i = 0; i < maxindex; i ++) colors[i].pixel = i;
+      
+      XQueryColors(fl_display, fl_colormap, colors, maxindex);
+      
+      for (i = 0; i < maxindex; i ++) {
+        cvals[i][0] = colors[i].red >> 8;
+        cvals[i][1] = colors[i].green >> 8;
+        cvals[i][2] = colors[i].blue >> 8;
       }
-    }
-  } else {
-    // RGB(A) image, so figure out the shifts & masks...
-    red_mask  = image->red_mask;
-    red_shift = 0;
-
-    while ((red_mask & 1) == 0) {
-      red_mask >>= 1;
-      red_shift ++;
-    }
-
-    green_mask  = image->green_mask;
-    green_shift = 0;
-
-    while ((green_mask & 1) == 0) {
-      green_mask >>= 1;
-      green_shift ++;
-    }
-
-    blue_mask  = image->blue_mask;
-    blue_shift = 0;
-
-    while ((blue_mask & 1) == 0) {
-      blue_mask >>= 1;
-      blue_shift ++;
-    }
-
-    // Read the pixels and output an RGB image...
-    for (y = 0; y < image->height; y ++) {
-      pixel = (unsigned char *)(image->data + y * image->bytes_per_line);
-      line  = p + y * w * d;
-
-      switch (image->bits_per_pixel) {
-        case 8 :
-          for (x = image->width, line_ptr = line;
-               x > 0;
-               x --, line_ptr += d, pixel ++) {
-            i = *pixel;
-
-            line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
-            line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
-            line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
-          }
-          break;
-
-        case 12 :
-          for (x = image->width, line_ptr = line, index_shift = 0;
-               x > 0;
-               x --, line_ptr += d) {
-            if (index_shift == 0) {
-              i = ((pixel[0] << 4) | (pixel[1] >> 4)) & 4095;
-            } else {
-              i = ((pixel[1] << 8) | pixel[2]) & 4095;
-            }
-
-            line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
-            line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
-            line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
-
-            if (index_shift == 0) {
-              index_shift = 4;
-            } else {
-              index_shift = 0;
-              pixel += 3;
+      
+      // Read the pixels and output an RGB image...
+      for (y = 0; y < image->height; y ++) {
+        pixel = (unsigned char *)(image->data + y * image->bytes_per_line);
+        line  = p + y * w * d;
+        
+        switch (image->bits_per_pixel) {
+          case 1 :
+            for (x = image->width, line_ptr = line, index_mask = 128;
+                 x > 0;
+                 x --, line_ptr += d) {
+              if (*pixel & index_mask) {
+                line_ptr[0] = cvals[1][0];
+                line_ptr[1] = cvals[1][1];
+                line_ptr[2] = cvals[1][2];
+              } else {
+                line_ptr[0] = cvals[0][0];
+                line_ptr[1] = cvals[0][1];
+                line_ptr[2] = cvals[0][2];
+              }
+              
+              if (index_mask > 1) {
+                index_mask >>= 1;
+              } else {
+                index_mask = 128;
+                pixel ++;
+              }
             }
-          }
-          break;
-
-        case 16 :
-          if (image->byte_order == LSBFirst) {
-            // Little-endian...
-            for (x = image->width, line_ptr = line;
+            break;
+            
+          case 2 :
+            for (x = image->width, line_ptr = line, index_shift = 6;
                  x > 0;
-                 x --, line_ptr += d, pixel += 2) {
-              i = (pixel[1] << 8) | pixel[0];
-
-              line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
-              line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
-              line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+                 x --, line_ptr += d) {
+              i = (*pixel >> index_shift) & 3;
+              
+              line_ptr[0] = cvals[i][0];
+              line_ptr[1] = cvals[i][1];
+              line_ptr[2] = cvals[i][2];
+              
+              if (index_shift > 0) {
+                index_shift -= 2;
+              } else {
+                index_shift = 6;
+                pixel ++;
+              }
             }
-          } else {
-            // Big-endian...
-            for (x = image->width, line_ptr = line;
+            break;
+            
+          case 4 :
+            for (x = image->width, line_ptr = line, index_shift = 4;
                  x > 0;
-                 x --, line_ptr += d, pixel += 2) {
-              i = (pixel[0] << 8) | pixel[1];
-
-              line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
-              line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
-              line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+                 x --, line_ptr += d) {
+              if (index_shift == 4) i = (*pixel >> 4) & 15;
+              else i = *pixel & 15;
+              
+              line_ptr[0] = cvals[i][0];
+              line_ptr[1] = cvals[i][1];
+              line_ptr[2] = cvals[i][2];
+              
+              if (index_shift > 0) {
+                index_shift = 0;
+              } else {
+                index_shift = 4;
+                pixel ++;
+              }
             }
-          }
-          break;
-
-        case 24 :
-          if (image->byte_order == LSBFirst) {
-            // Little-endian...
+            break;
+            
+          case 8 :
             for (x = image->width, line_ptr = line;
                  x > 0;
-                 x --, line_ptr += d, pixel += 3) {
-              i = (((pixel[2] << 8) | pixel[1]) << 8) | pixel[0];
-
-              line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
-              line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
-              line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+                 x --, line_ptr += d, pixel ++) {
+              line_ptr[0] = cvals[*pixel][0];
+              line_ptr[1] = cvals[*pixel][1];
+              line_ptr[2] = cvals[*pixel][2];
             }
-          } else {
-            // Big-endian...
-            for (x = image->width, line_ptr = line;
+            break;
+            
+          case 12 :
+            for (x = image->width, line_ptr = line, index_shift = 0;
                  x > 0;
-                 x --, line_ptr += d, pixel += 3) {
-              i = (((pixel[0] << 8) | pixel[1]) << 8) | pixel[2];
-
-              line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
-              line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
-              line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+                 x --, line_ptr += d) {
+              if (index_shift == 0) {
+                i = ((pixel[0] << 4) | (pixel[1] >> 4)) & 4095;
+              } else {
+                i = ((pixel[1] << 8) | pixel[2]) & 4095;
+              }
+              
+              line_ptr[0] = cvals[i][0];
+              line_ptr[1] = cvals[i][1];
+              line_ptr[2] = cvals[i][2];
+              
+              if (index_shift == 0) {
+                index_shift = 4;
+              } else {
+                index_shift = 0;
+                pixel += 3;
+              }
             }
-          }
-          break;
-
-        case 32 :
-          if (image->byte_order == LSBFirst) {
-            // Little-endian...
+            break;
+        }
+      }
+    } else {
+      // RGB(A) image, so figure out the shifts & masks...
+      red_mask  = image->red_mask;
+      red_shift = 0;
+      
+      while ((red_mask & 1) == 0) {
+        red_mask >>= 1;
+        red_shift ++;
+      }
+      
+      green_mask  = image->green_mask;
+      green_shift = 0;
+      
+      while ((green_mask & 1) == 0) {
+        green_mask >>= 1;
+        green_shift ++;
+      }
+      
+      blue_mask  = image->blue_mask;
+      blue_shift = 0;
+      
+      while ((blue_mask & 1) == 0) {
+        blue_mask >>= 1;
+        blue_shift ++;
+      }
+      
+      // Read the pixels and output an RGB image...
+      for (y = 0; y < image->height; y ++) {
+        pixel = (unsigned char *)(image->data + y * image->bytes_per_line);
+        line  = p + y * w * d;
+        
+        switch (image->bits_per_pixel) {
+          case 8 :
             for (x = image->width, line_ptr = line;
                  x > 0;
-                 x --, line_ptr += d, pixel += 4) {
-              i = (((((pixel[3] << 8) | pixel[2]) << 8) | pixel[1]) << 8) | pixel[0];
-
+                 x --, line_ptr += d, pixel ++) {
+              i = *pixel;
+              
               line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
               line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
               line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
             }
-          } else {
-            // Big-endian...
-            for (x = image->width, line_ptr = line;
+            break;
+            
+          case 12 :
+            for (x = image->width, line_ptr = line, index_shift = 0;
                  x > 0;
-                 x --, line_ptr += d, pixel += 4) {
-              i = (((((pixel[0] << 8) | pixel[1]) << 8) | pixel[2]) << 8) | pixel[3];
-
+                 x --, line_ptr += d) {
+              if (index_shift == 0) {
+                i = ((pixel[0] << 4) | (pixel[1] >> 4)) & 4095;
+              } else {
+                i = ((pixel[1] << 8) | pixel[2]) & 4095;
+              }
+              
               line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
               line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
               line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              
+              if (index_shift == 0) {
+                index_shift = 4;
+              } else {
+                index_shift = 0;
+                pixel += 3;
+              }
+            }
+            break;
+            
+          case 16 :
+            if (image->byte_order == LSBFirst) {
+              // Little-endian...
+              for (x = image->width, line_ptr = line;
+                   x > 0;
+                   x --, line_ptr += d, pixel += 2) {
+                i = (pixel[1] << 8) | pixel[0];
+                
+                line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
+                line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
+                line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              }
+            } else {
+              // Big-endian...
+              for (x = image->width, line_ptr = line;
+                   x > 0;
+                   x --, line_ptr += d, pixel += 2) {
+                i = (pixel[0] << 8) | pixel[1];
+                
+                line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
+                line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
+                line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              }
+            }
+            break;
+            
+          case 24 :
+            if (image->byte_order == LSBFirst) {
+              // Little-endian...
+              for (x = image->width, line_ptr = line;
+                   x > 0;
+                   x --, line_ptr += d, pixel += 3) {
+                i = (((pixel[2] << 8) | pixel[1]) << 8) | pixel[0];
+                
+                line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
+                line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
+                line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              }
+            } else {
+              // Big-endian...
+              for (x = image->width, line_ptr = line;
+                   x > 0;
+                   x --, line_ptr += d, pixel += 3) {
+                i = (((pixel[0] << 8) | pixel[1]) << 8) | pixel[2];
+                
+                line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
+                line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
+                line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              }
+            }
+            break;
+            
+          case 32 :
+            if (image->byte_order == LSBFirst) {
+              // Little-endian...
+              for (x = image->width, line_ptr = line;
+                   x > 0;
+                   x --, line_ptr += d, pixel += 4) {
+                i = (((((pixel[3] << 8) | pixel[2]) << 8) | pixel[1]) << 8) | pixel[0];
+                
+                line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
+                line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
+                line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              }
+            } else {
+              // Big-endian...
+              for (x = image->width, line_ptr = line;
+                   x > 0;
+                   x --, line_ptr += d, pixel += 4) {
+                i = (((((pixel[0] << 8) | pixel[1]) << 8) | pixel[2]) << 8) | pixel[3];
+                
+                line_ptr[0] = 255 * ((i >> red_shift) & red_mask) / red_mask;
+                line_ptr[1] = 255 * ((i >> green_shift) & green_mask) / green_mask;
+                line_ptr[2] = 255 * ((i >> blue_shift) & blue_mask) / blue_mask;
+              }
             }
-          }
-          break;
+            break;
+        }
       }
     }
+    
+    // Destroy the X image we've read and return the RGB(A) image...
+    XDestroyImage(image);
   }
-
-  // Destroy the X image we've read and return the RGB(A) image...
-  XDestroyImage(image);
-
   Fl_RGB_Image *rgb = new Fl_RGB_Image(p, w, h, d);
   rgb->alloc_array = 1;
   return rgb;
diff --git src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx
index b20a84c..7815fbd 100644
--- src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx
+++ src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx
@@ -644,6 +644,16 @@ static void alpha_blend(Fl_RGB_Image *img, int X, int Y, int W, int H, int cx, i
   if (cy < 0) { H += cy; Y -= cy; cy = 0; }
   if (W + cx > img->data_w()) W = img->data_w() - cx;
   if (H + cy > img->data_h()) H = img->data_h() - cy;
+  // don't attempt to read outside the window/offscreen buffer limits
+  Window root_return;
+  int x_return, y_return;
+  unsigned int winW, winH;
+  unsigned int border_width_return;
+  unsigned int depth_return;
+  XGetGeometry(fl_display, fl_window, &root_return, &x_return, &y_return, &winW,
+               &winH, &border_width_return, &depth_return);
+  if (X+W > winW) W = winW-X;
+  if (Y+H > winH) H = winH-Y;
   if (W <= 0 || H <= 0) return;
   int ld = img->ld();
   if (ld == 0) ld = img->data_w() * img->d();
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