remove unnecessary checks from pad_image_tensor

torchvision/prototype/transforms/functional/_geometry.py

@@ -4,6 +4,7 @@
 
 import PIL.Image
 import torch
+from torch.nn.functional import pad as torch_pad
 from torchvision.prototype import features
 from torchvision.transforms import functional_pil as _FP, functional_tensor as _FT
 from torchvision.transforms.functional import (
@@ -14,7 +15,6 @@
     pil_to_tensor,
     to_pil_image,
 )
-from torchvision.transforms.functional_tensor import _parse_pad_padding
 
 from ._meta import convert_format_bounding_box, get_spatial_size_image_pil
 
@@ -645,7 +645,32 @@ def rotate(
         return rotate_image_pil(inpt, angle, interpolation=interpolation, expand=expand, fill=fill, center=center)
 
 
-pad_image_pil = _FP.pad
+def _parse_pad_padding(padding: Union[int, List[int]]) -> List[int]:
+    if not isinstance(padding, (int, tuple, list)):
+        raise TypeError(f"`padding` should be an integer or tuple or list of integers, but got {padding}")
+
+    if isinstance(padding, int):
+        if torch.jit.is_scripting():
+            # This maybe unreachable
+            raise ValueError("padding can't be an int while torchscripting, set it as a list [value, ]")
+        pad_left = pad_right = pad_top = pad_bottom = padding
+    else:
+        if len(padding) == 1:
+            pad_left = pad_right = pad_top = pad_bottom = padding[0]
+        elif len(padding) == 2:
+            pad_left = pad_right = padding[0]
+            pad_top = pad_bottom = padding[1]
+        elif len(padding) == 4:
+            pad_left = padding[0]
+            pad_top = padding[1]
+            pad_right = padding[2]
+            pad_bottom = padding[3]
+        else:
+            raise ValueError(
+                f"Padding must be an int or a 1, 2, or 4 element tuple, not a {len(padding)} element tuple"
+            )
+
+    return [pad_left, pad_right, pad_top, pad_bottom]
 
 
 def pad_image_tensor(
@@ -654,50 +679,86 @@ def pad_image_tensor(
     fill: features.FillTypeJIT = None,
     padding_mode: str = "constant",
 ) -> torch.Tensor:
+    # Be aware that while `padding` has order `[left, top, right, bottom]` has order, `torch_padding` uses
+    # `[left, right, top, bottom]`. This stems from the fact that we align our API with PIL, but need to use `torch_pad`
+    # internally.
+    torch_padding = _parse_pad_padding(padding)
+
+    if padding_mode not in ["constant", "edge", "reflect", "symmetric"]:
+        raise ValueError(
+            f"`padding_mode` should be either `'constant'`, `'edge'`, `'reflect'` or `'symmetric'`, "
+            f"but got `'{padding_mode}'`."
+        )
+
     if fill is None:
         # This is a JIT workaround
-        return _pad_with_scalar_fill(image, padding, fill=None, padding_mode=padding_mode)
-    elif isinstance(fill, (int, float)) or len(fill) == 1:
-        fill_number = fill[0] if isinstance(fill, list) else fill
-        return _pad_with_scalar_fill(image, padding, fill=fill_number, padding_mode=padding_mode)
+        return _pad_with_scalar_fill(image, torch_padding, fill=0, padding_mode=padding_mode)
+    elif isinstance(fill, (int, float)):
+        return _pad_with_scalar_fill(image, torch_padding, fill=fill, padding_mode=padding_mode)
+    elif len(fill) == 1:
+        return _pad_with_scalar_fill(image, torch_padding, fill=fill[0], padding_mode=padding_mode)
     else:
-        return _pad_with_vector_fill(image, padding, fill=fill, padding_mode=padding_mode)
+        return _pad_with_vector_fill(image, torch_padding, fill=fill, padding_mode=padding_mode)
 
 
 def _pad_with_scalar_fill(
     image: torch.Tensor,
-    padding: Union[int, List[int]],
-    fill: Union[int, float, None],
-    padding_mode: str = "constant",
+    torch_padding: List[int],
+    fill: Union[int, float],
+    padding_mode: str,
 ) -> torch.Tensor:
     shape = image.shape
     num_channels, height, width = shape[-3:]
 
     if image.numel() > 0:
-        image = _FT.pad(
-            img=image.reshape(-1, num_channels, height, width), padding=padding, fill=fill, padding_mode=padding_mode
-        )
+        image = image.reshape(-1, num_channels, height, width)
+
+        if padding_mode == "edge":
+            # Similar to the padding order, `torch_pad`'s PIL's padding modes don't have the same names. Thus, we map
+            # the PIL name for the padding mode, which we are also using for our API, to the corresponding `torch_pad`
+            # name.
+            padding_mode = "replicate"
+
+        if padding_mode == "constant":
+            image = torch_pad(image, torch_padding, mode=padding_mode, value=float(fill))
+        elif padding_mode in ("reflect", "replicate"):
+            # `torch_pad` only supports `"reflect"` or `"replicate"` padding for floating point inputs.
+            # TODO: See https://github.com/pytorch/pytorch/issues/40763
+            dtype = image.dtype
+            if not image.is_floating_point():
+                needs_cast = True
+                image = image.to(torch.float32)
+            else:
+                needs_cast = False
+
+            image = torch_pad(image, torch_padding, mode=padding_mode)
+
+            if needs_cast:
+                image = image.to(dtype)
+        else:  # padding_mode == "symmetric"
+            image = _FT._pad_symmetric(image, torch_padding)
+
         new_height, new_width = image.shape[-2:]
     else:
-        left, right, top, bottom = _FT._parse_pad_padding(padding)
+        left, right, top, bottom = torch_padding
         new_height = height + top + bottom
         new_width = width + left + right
 
     return image.reshape(shape[:-3] + (num_channels, new_height, new_width))
 
 
-# TODO: This should be removed once pytorch pad supports non-scalar padding values
+# TODO: This should be removed once torch_pad supports non-scalar padding values
 def _pad_with_vector_fill(
     image: torch.Tensor,
-    padding: Union[int, List[int]],
+    torch_padding: List[int],
     fill: List[float],
-    padding_mode: str = "constant",
+    padding_mode: str,
 ) -> torch.Tensor:
     if padding_mode != "constant":
         raise ValueError(f"Padding mode '{padding_mode}' is not supported if fill is not scalar")
 
-    output = _pad_with_scalar_fill(image, padding, fill=0, padding_mode="constant")
-    left, right, top, bottom = _parse_pad_padding(padding)
+    output = _pad_with_scalar_fill(image, torch_padding, fill=0, padding_mode="constant")
+    left, right, top, bottom = torch_padding
     fill = torch.tensor(fill, dtype=image.dtype, device=image.device).reshape(-1, 1, 1)
 
     if top > 0:
@@ -711,6 +772,9 @@ def _pad_with_vector_fill(
     return output
 
 
+pad_image_pil = _FP.pad
+
+
 def pad_mask(
     mask: torch.Tensor,
     padding: Union[int, List[int]],