New `LetterBox(size)` `CenterCrop(size)`, `ToTensor()` transforms (#9213)

* New LetterBox transform

YOLOv5 LetterBox class for image preprocessing, i.e. T.Compose([T.ToTensor(), LetterBox(size)])

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Update augmentations.py

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Update augmentations.py

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Update augmentations.py

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* cleanup

* cleanup

* cleanup

* cleanup

* cleanup

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

utils/augmentations.py

@@ -8,6 +8,7 @@ import random
 
 import cv2
 import numpy as np
+import torch
 import torchvision.transforms as T
 import torchvision.transforms.functional as TF
 
@@ -345,4 +346,51 @@ def classify_albumentations(augment=True,
 def classify_transforms(size=224):
     # Transforms to apply if albumentations not installed
     assert isinstance(size, int), f'ERROR: classify_transforms size {size} must be integer, not (list, tuple)'
-    return T.Compose([T.ToTensor(), T.Resize(size), T.CenterCrop(size), T.Normalize(IMAGENET_MEAN, IMAGENET_STD)])
+    # T.Compose([T.ToTensor(), T.Resize(size), T.CenterCrop(size), T.Normalize(IMAGENET_MEAN, IMAGENET_STD)])
+    return T.Compose([CenterCrop(size), ToTensor(), T.Normalize(IMAGENET_MEAN, IMAGENET_STD)])
+
+
+class LetterBox:
+    # YOLOv5 LetterBox class for image preprocessing, i.e. T.Compose([LetterBox(size), ToTensor()])
+    def __init__(self, size=(640, 640), auto=False, stride=32):
+        super().__init__()
+        self.h, self.w = (size, size) if isinstance(size, int) else size
+        self.auto = auto  # pass max size integer, automatically solve for short side using stride
+        self.stride = stride  # used with auto
+
+    def __call__(self, im):  # im = np.array HWC
+        imh, imw = im.shape[:2]
+        r = min(self.h / imh, self.w / imw)  # ratio of new/old
+        h, w = round(imh * r), round(imw * r)  # resized image
+        hs, ws = (math.ceil(x / self.stride) * self.stride for x in (h, w)) if self.auto else self.h, self.w
+        top, left = round((hs - h) / 2 - 0.1), round((ws - w) / 2 - 0.1)
+        im_out = np.full((self.h, self.w, 3), 114, dtype=im.dtype)
+        im_out[top:top + h, left:left + w] = cv2.resize(im, (w, h), interpolation=cv2.INTER_LINEAR)
+        return im_out
+
+
+class CenterCrop:
+    # YOLOv5 CenterCrop class for image preprocessing, i.e. T.Compose([CenterCrop(size), ToTensor()])
+    def __init__(self, size=640):
+        super().__init__()
+        self.h, self.w = (size, size) if isinstance(size, int) else size
+
+    def __call__(self, im):  # im = np.array HWC
+        imh, imw = im.shape[:2]
+        m = min(imh, imw)  # min dimension
+        top, left = (imh - m) // 2, (imw - m) // 2
+        return cv2.resize(im[top:top + m, left:left + m], (self.w, self.h), interpolation=cv2.INTER_LINEAR)
+
+
+class ToTensor:
+    # YOLOv5 ToTensor class for image preprocessing, i.e. T.Compose([LetterBox(size), ToTensor()])
+    def __init__(self, half=False):
+        super().__init__()
+        self.half = half
+
+    def __call__(self, im):  # im = np.array HWC in BGR order
+        im = np.ascontiguousarray(im.transpose((2, 0, 1))[::-1])  # HWC to CHW -> BGR to RGB -> contiguous
+        im = torch.from_numpy(im)  # to torch
+        im = im.half() if self.half else im.float()  # uint8 to fp16/32
+        im /= 255.0  # 0-255 to 0.0-1.0
+        return im

utils/dataloaders.py

@@ -251,7 +251,7 @@ class LoadImages:
             s = f'image {self.count}/{self.nf} {path}: '
 
         if self.transforms:
-            im = self.transforms(cv2.cvtColor(im0, cv2.COLOR_BGR2RGB))  # transforms
+            im = self.transforms(im0)  # transforms
         else:
             im = letterbox(im0, self.img_size, stride=self.stride, auto=self.auto)[0]  # padded resize
             im = im.transpose((2, 0, 1))[::-1]  # HWC to CHW, BGR to RGB
@@ -386,7 +386,7 @@ class LoadStreams:
 
         im0 = self.imgs.copy()
         if self.transforms:
-            im = np.stack([self.transforms(cv2.cvtColor(x, cv2.COLOR_BGR2RGB)) for x in im0])  # transforms
+            im = np.stack([self.transforms(x) for x in im0])  # transforms
         else:
             im = np.stack([letterbox(x, self.img_size, stride=self.stride, auto=self.auto)[0] for x in im0])  # resize
             im = im[..., ::-1].transpose((0, 3, 1, 2))  # BGR to RGB, BHWC to BCHW
@@ -1113,18 +1113,18 @@ class ClassificationDataset(torchvision.datasets.ImageFolder):
 
     def __getitem__(self, i):
         f, j, fn, im = self.samples[i]  # filename, index, filename.with_suffix('.npy'), image
+        if self.cache_ram and im is None:
+            im = self.samples[i][3] = cv2.imread(f)
+        elif self.cache_disk:
+            if not fn.exists():  # load npy
+                np.save(fn.as_posix(), cv2.imread(f))
+            im = np.load(fn)
+        else:  # read image
+            im = cv2.imread(f)  # BGR
         if self.album_transforms:
-            if self.cache_ram and im is None:
-                im = self.samples[i][3] = cv2.imread(f)
-            elif self.cache_disk:
-                if not fn.exists():  # load npy
-                    np.save(fn.as_posix(), cv2.imread(f))
-                im = np.load(fn)
-            else:  # read image
-                im = cv2.imread(f)  # BGR
             sample = self.album_transforms(image=cv2.cvtColor(im, cv2.COLOR_BGR2RGB))["image"]
         else:
-            sample = self.torch_transforms(self.loader(f))
+            sample = self.torch_transforms(im)
         return sample, j