Add conversions from I;16 to F to retain 16 bit precision

Tests/test_image_convert.py

@@ -24,3 +24,27 @@ def test_default():
     assert_image(im, "RGB", im.size)
     im = im.convert()
     assert_image(im, "RGB", im.size)
+
+
+
+# ref https://github.com/python-imaging/Pillow/issues/274
+
+def _test_float_conversion(im):
+    orig = im.getpixel((5,5))
+    converted = im.convert('F').getpixel((5,5))
+    assert_equal(orig, converted)
+
+def test_8bit():
+    im = Image.open('Images/lena.jpg')
+    _test_float_conversion(im.convert('L'))
+
+def test_12bit():
+    im = Image.open('Tests/images/12bit.tif')
+    _test_float_conversion(im)
+
+def test_12bit_workaround():
+    im = Image.open('Tests/images/12bit.tif')
+    _test_float_conversion(im.convert('I'))
+    
+
+    

libImaging/Convert.c

@@ -498,6 +498,25 @@ I16B_I(UINT8* out_, const UINT8* in, int xsize)
         *out++ = in[1] + ((int) in[0] << 8);
 }
 
+static void
+I16L_F(UINT8* out_, const UINT8* in, int xsize)
+{
+    int x;
+    FLOAT32* out = (FLOAT32*) out_;
+    for (x = 0; x < xsize; x++, in += 2)
+        *out++ = (FLOAT32) (in[0] + ((int) in[1] << 8));
+}
+
+
+static void
+I16B_F(UINT8* out_, const UINT8* in, int xsize)
+{
+    int x;
+    FLOAT32* out = (FLOAT32*) out_;
+    for (x = 0; x < xsize; x++, in += 2)
+        *out++ = (FLOAT32) (in[1] + ((int) in[0] << 8));
+}
+
 static void
 L_I16L(UINT8* out, const UINT8* in, int xsize)
 {
@@ -630,6 +649,10 @@ static struct {
     { "L", "I;16B", L_I16B },
     { "I;16B", "L", I16B_L },
 
+    { "I;16", "F", I16L_F },
+    { "I;16L", "F", I16L_F },
+    { "I;16B", "F", I16B_F },
+
     { NULL }
 };