Shifting the midpoint of the ab channels to 128. unshifting back to signed int on pack

Tests/test_format_lab.py

@@ -0,0 +1,41 @@
+from tester import *
+
+from PIL import Image
+
+def test_white():
+    i = Image.open('Tests/images/lab.tif')
+
+    bits = i.load()
+    
+    assert_equal(i.mode, 'LAB')
+
+    assert_equal(i.getbands(), ('L','A', 'B'))
+
+    k = i.getpixel((0,0))
+    assert_equal(k, (255,128,128))
+
+    L  = i.getdata(0)
+    a = i.getdata(1)
+    b = i.getdata(2)
+
+    assert_equal(list(L), [255]*100)
+    assert_equal(list(a), [128]*100)
+    assert_equal(list(b), [128]*100)
+    
+
+def test_green():
+    # l= 50 (/100), a = -100 (-128 .. 128) b=0 in PS
+    # == RGB: 0, 152, 117
+    i = Image.open('Tests/images/lab-green.tif')
+
+    k = i.getpixel((0,0))
+    assert_equal(k, (128,28,128))
+
+
+def test_red():
+    # l= 50 (/100), a = 100 (-128 .. 128) b=0 in PS
+    # == RGB: 255, 0, 124
+    i = Image.open('Tests/images/lab-red.tif')
+
+    k = i.getpixel((0,0))
+    assert_equal(k, (128,228,128))

Tests/test_image_lab.py

@@ -1,25 +0,0 @@
-from tester import *
-
-from PIL import Image
-
-def test_sanity():
-    i = Image.open('Tests/images/lab.tif')
-
-    bits = i.load()
-    
-    assert_equal(i.mode, 'LAB')
-
-    assert_equal(i.getbands(), ('L','A', 'B'))
-
-    k = i.getpixel((0,0))
-    assert_equal(k, (255,0,0))
-
-    L  = i.getdata(0)
-    a = i.getdata(1)
-    b = i.getdata(2)
-
-    assert_equal(list(L), [255]*100)
-    assert_equal(list(a), [0]*100)
-    assert_equal(list(b), [0]*100)
-    
-

libImaging/Pack.c

@@ -372,6 +372,19 @@ packI32S(UINT8* out, const UINT8* in, int pixels)
     }
 }
 
+void
+ImagingPackLAB(UINT8* out, const UINT8* in, int pixels)
+{
+    int i;
+    /* LAB triplets */
+    for (i = 0; i < pixels; i++) {
+	out[0] = in[0];
+	out[1] = in[1] ^ 128; /* signed in outside world */ 
+	out[2] = in[2] ^ 128;
+	out += 3; in += 4;
+    }
+}
+
 static void
 copy1(UINT8* out, const UINT8* in, int pixels)
 {
@@ -527,7 +540,7 @@ static struct {
     {"YCbCr",  	"Cr",           8,      band2},
 
     /* LAB Color */
-    {"LAB",	    "LAB",	       24,     ImagingPackRGB},
+    {"LAB",	    "LAB",	       24,     ImagingPackLAB},
     {"LAB",  	"L",           8,      band0},
     {"LAB",  	"A",           8,      band1},
     {"LAB",  	"B",           8,      band2},

libImaging/Unpack.c

@@ -660,6 +660,31 @@ unpackCMYKI(UINT8* out, const UINT8* in, int pixels)
     }
 }
 
+/* Unpack to "LAB" image */
+/* There are two representations of LAB images for whatever precision:
+   L: Uint (in PS, it's 0-100)
+   A: Int (in ps, -128 .. 128, or elsewhere 0..255, with 128 as middle. 
+           Channels in PS display a 0 value as middle grey, 
+           LCMS appears to use 128 as the 0 value for these channels)
+   B: Int (as above) 
+
+   Since we don't have any signed ints, we're going with the shifted versions 
+   internally, and we'll unshift for saving and whatnot. 
+*/
+void
+ImagingUnpackLAB(UINT8* out, const UINT8* in, int pixels)
+{
+    int i;
+    /* LAB triplets */
+    for (i = 0; i < pixels; i++) {
+        out[0] = in[0];
+        out[1] = in[1] ^ 128; /* signed in outside world */
+        out[2] = in[2] ^ 128;
+        out[3] = 255;
+        out += 4; in += 3;
+    }
+}
+
 static void
 copy1(UINT8* out, const UINT8* in, int pixels)
 {
@@ -965,7 +990,7 @@ static struct {
     {"YCbCr",   "YCbCrK",       32,     copy4},
 
     /* LAB Color */
-    {"LAB",	    "LAB",	        24,	    ImagingUnpackRGB},
+    {"LAB",	    "LAB",	        24,	    ImagingUnpackLAB},
     {"LAB",  	"L",            8,      band0},
     {"LAB",  	"A",            8,      band1},
     {"LAB",  	"B",            8,      band2},