workaround for compiler (?) bug with clang from Xcode 4.6.3

fix-bitmap-import-debug-msg-v12.diff

=== modified file 'src/extension/internal/image-resolution.cpp'
--- src/extension/internal/image-resolution.cpp	2014-10-01 07:24:30 +0000
+++ src/extension/internal/image-resolution.cpp	2015-08-18 13:33:27 +0000
@@ -118,17 +118,35 @@
     png_read_info(png_ptr, info_ptr);
 
     png_uint_32 res_x, res_y;
+#ifdef PNG_INCH_CONVERSIONS_SUPPORTED
+    g_message("readpng - PNG_INCH_CONVERSIONS_SUPPORTED");
+    res_x = png_get_x_pixels_per_inch(png_ptr, info_ptr);
+    res_y = png_get_y_pixels_per_inch(png_ptr, info_ptr);
+    if (res_x != 0 && res_y != 0) {
+        ok_ = true;
+        x_ = res_x * 1.0;  // FIXME: implicit conversion of png_uint_32 to double ok?
+        y_ = res_y * 1.0;  // FIXME: implicit conversion of png_uint_32 to double ok?
+    }
+#else
+    g_message("readpng - PNG_RESOLUTION_METER");
     int unit_type;
-    png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type);
-
-    png_destroy_read_struct(&png_ptr, &info_ptr, 0);
-    fclose(fp);
-
+    png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type);  // FIXME: fails to return expected values with clang (based on LLVM 3.2svn) from Xcode 4.6.3 (OS X 10.7.5)
     if (unit_type == PNG_RESOLUTION_METER) {
         ok_ = true;
         x_ = res_x * 2.54 / 100;
         y_ = res_y * 2.54 / 100;
     }
+#endif
+
+    png_destroy_read_struct(&png_ptr, &info_ptr, 0);
+    fclose(fp);
+
+    if (ok_) {
+        g_message("readpng - xdpi: %f", x_);
+        g_message("readpng - ydpi: %f", y_);
+    } else {
+        g_message("readpng - FAILED");
+    }
 }
 #else
 
@@ -354,6 +372,8 @@
         return;
     }
 
+    g_message("readmagick - image.[xy]Resolution");
+
     std::string const type = image.magick();
     x_ = image.xResolution();
     y_ = image.yResolution();
@@ -367,6 +387,13 @@
     if (x_ != 0 && y_ != 0) {
         ok_ = true;
     }
+
+    if (ok_) {
+        g_message("readmagick - xdpi: %f", x_);
+        g_message("readmagick - ydpi: %f", y_);
+    } else {
+        g_message("readmagick - FAILED");
+    }
 }
 
 #else

Summary

diff above allows to work around failure in Inkscape 0.91 and trunk to import certain kinds of PNG images / paste from clipboard correctly (so far only known to affect builds on OS X 10.7.5 after upgrading Xcode from 4.3.2 to 4.6.3).

Known trigger

affected PNG images have Units: Undefined (in the output of ImageMagick's identify -verbose command).

Known limitation

The current patch is only in effect with more recent versions of libpng which support PNG_INCH_CONVERSIONS (png_get_x_pixels_per_inch(), png_get_y_pixels_per_inch()).

Possibly cause

Underlying issue seems most likely a compiler bug (clang from Xcode 4.6.3), triggered e.g. with -02, -O1, but apparently not with debug build -O0 -g. Might be related to libpng's typedef png_uint_32 (likely specific to OS/platform).

Affected compiler (Xcode 4.6.3, OS X 10.7.5):
$ clang --version Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn) Target: x86_64-apple-darwin11.4.2 Thread model: posix

Observed symptoms

with current code (0.91, 0.91+devel):

• incorrect res_x (--> computed width is incorrect)
• erroneous (huge) res_y (--> computed height is close to zero)
• incorrect unit_type (1 (same as PNG_RESOLUTION_METER) despite unspecified units)

Example image (PNG)