Only byte-swap 16-bit PNGs on little-endian (#7792)

AdamWill · AdamWill · commit e96a2cac2fed · 2017-01-10T16:41:44.000-08:00
_png has some code that unconditionally byte-swaps 16-bit PNG data (which is, per the spec, stored in big-endian order). This isn't appropriate on a big-endian platform, though: this swap being done unconditionally breaks the handling of 16-bit PNGs on big-endian platforms (e.g. Fedora ppc64) (#7792). So, let's use some macros numpy kindly defines for us to decide whether to do this swap or not.
diff --git a/src/_png.cpp b/src/_png.cpp
@@ -532,10 +532,12 @@ static PyObject *_read_png(PyObject *filein, bool float_result)
         png_set_shift(png_ptr, sig_bit);
     }
 
+#if NPY_BYTE_ORDER == NPY_LITTLE_ENDIAN
     // Convert big endian to little
     if (bit_depth == 16) {
         png_set_swap(png_ptr);
     }
+#endif
 
     // Convert palletes to full RGB
     if (png_get_color_type(png_ptr, info_ptr) == PNG_COLOR_TYPE_PALETTE) {

Original file line number	Diff line number	Diff line change
`@@ -532,10 +532,12 @@ static PyObject _read_png(PyObject filein, bool float_result)`
`532`	`532`	`png_set_shift(png_ptr, sig_bit);`
`533`	`533`	`}`
`534`	`534`
	`535`	`+#if NPY_BYTE_ORDER == NPY_LITTLE_ENDIAN`
`535`	`536`	`// Convert big endian to little`
`536`	`537`	`if (bit_depth == 16) {`
`537`	`538`	`png_set_swap(png_ptr);`
`538`	`539`	`}`
	`540`	`+#endif`
`539`	`541`
`540`	`542`	`// Convert palletes to full RGB`
`541`	`543`	`if (png_get_color_type(png_ptr, info_ptr) == PNG_COLOR_TYPE_PALETTE) {`