Add support for grayscale bmp image (#14296)

* Add support for grayscale bmp image

This fix tries to address the issue raised in 13942 to support grayscale
bmp image. Previously only channels of 3 or 4 are supported in bmp decoding.
This fix adds the support to have 1 channel grayscale image.

This fix fixes 13942.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test case for grayscale bmp image decoding

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

2 files changed, 44 insertions, 6 deletions

diff --git a/tensorflow/core/kernels/decode_bmp_op.cc b/tensorflow/core/kernels/decode_bmp_op.cc
index 086369a9f1..cd7956e1cb 100644
--- a/tensorflow/core/kernels/decode_bmp_op.cc
+++ b/tensorflow/core/kernels/decode_bmp_op.cc
@@ -33,9 +33,10 @@ class DecodeBmpOp : public OpKernel {
  public:
   explicit DecodeBmpOp(OpKernelConstruction* context) : OpKernel(context) {
     OP_REQUIRES_OK(context, context->GetAttr("channels", &channels_));
-    OP_REQUIRES(
-        context, channels_ == 0 || channels_ == 3 || channels_ == 4,
-        errors::InvalidArgument("channels must be 0, 3 or 4, got ", channels_));
+    OP_REQUIRES(context, channels_ == 0 || channels_ == 1 || channels_ == 3 ||
+                             channels_ == 4,
+                errors::InvalidArgument("channels must be 0, 1, 3 or 4, got ",
+                                        channels_));
   }
 
   void Compute(OpKernelContext* context) override {
@@ -66,11 +67,11 @@ class DecodeBmpOp : public OpKernel {
       channels_ = bpp / 8;
     }
 
-    // Current implementation only supports 3 or 4 channel
+    // Current implementation only supports 1, 3 or 4 channel
     // bitmaps.
-    OP_REQUIRES(context, (channels_ == 3 || channels_ == 4),
+    OP_REQUIRES(context, (channels_ == 1 || channels_ == 3 || channels_ == 4),
                 errors::InvalidArgument(
-                    "Number of channels must be 3 or 4, was ", channels_));
+                    "Number of channels must be 1, 3 or 4, was ", channels_));
 
     // if height is negative, data layout is top down
     // otherwise, it's bottom up
@@ -117,6 +118,9 @@ uint8* DecodeBmpOp::Decode(const uint8* input, uint8* const output,
       dst_pos = (i * width + j) * channels;
 
       switch (channels) {
+        case 1:
+          output[dst_pos] = input[src_pos];
+          break;
         case 3:
           // BGR -> RGB
           output[dst_pos] = input[src_pos + 2];
diff --git a/tensorflow/python/kernel_tests/decode_bmp_op_test.py b/tensorflow/python/kernel_tests/decode_bmp_op_test.py
index 783492a6f2..e7b472240e 100644
--- a/tensorflow/python/kernel_tests/decode_bmp_op_test.py
+++ b/tensorflow/python/kernel_tests/decode_bmp_op_test.py
@@ -64,6 +64,40 @@ class DecodeBmpOpTest(test.TestCase):
       decoded = decode.eval()
       self.assertAllEqual(decoded, img_bytes)
 
+  def testGrayscale(self):
+    img_bytes = [[[255], [0]], [[255], [0]]]
+    encoded_bytes = [
+        0x42, 0x40,
+        0x3d, 0, 0, 0,
+        0, 0,
+        0, 0,
+        0x36, 0, 0, 0,
+        0x28, 0, 0, 0,
+        0x2, 0, 0, 0,
+        0x2, 0, 0, 0,
+        0x1, 0,
+        0x8, 0,
+        0, 0, 0, 0,
+        0x10, 0, 0, 0,
+        0x13, 0xb, 0, 0,
+        0x13, 0xb, 0, 0,
+        0, 0, 0, 0,
+        0, 0, 0, 0,
+        0xff,
+        0,
+        0, 0,
+        0xff,
+        0,
+        0, 0,
+    ]
+
+    byte_string = bytes(bytearray(encoded_bytes))
+    img_in = constant_op.constant(byte_string, dtype=dtypes.string)
+    decode = image_ops.decode_bmp(img_in)
+
+    with self.test_session():
+      decoded = decode.eval()
+      self.assertAllEqual(decoded, img_bytes)
 
 if __name__ == "__main__":
   test.main()