Color Space: RGB2BGR, RGB2GRAY
import cv2
import numpy as np
def iminvert(img):
"""Invert (negate) an image
Args:
img (ndarray): Image to be inverted.
Returns:
ndarray: The inverted image.
"""
return np.full_like(img, 255) - img
BGR2GRAY and GRAY2BGR with OpenCV
def bgr2gray(img, keepdim=False):
"""Convert a BGR image to grayscale image.
Args:
img (ndarray): The input image.
keepdim (bool): If False (by default), then return the grayscale image
with 2 dims, otherwise 3 dims.
Returns:
ndarray: The converted grayscale image.
"""
out_img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
if keepdim:
out_img = out_img[..., None]
return out_img
def gray2bgr(img):
"""Convert a grayscale image to BGR image.
Args:
img (ndarray or str): The input image.
Returns:
ndarray: The converted BGR image.
"""
img = img[..., None] if img.ndim == 2 else img
out_img = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
return out_img
def convert_color_factory(src, dst):
code = getattr(cv2, 'COLOR_{}2{}'.format(src.upper(), dst.upper()))
def convert_color(img):
out_img = cv2.cvtColor(img, code)
return out_img
convert_color.__doc__ = """Convert a {0} image to {1} image.
Args:
img (ndarray or str): The input image.
Returns:
ndarray: The converted {1} image.
""".format(src.upper(), dst.upper())
return convert_color
bgr2rgb = convert_color_factory('bgr', 'rgb')
rgb2bgr = convert_color_factory('rgb', 'bgr')
bgr2hsv = convert_color_factory('bgr', 'hsv')
hsv2bgr = convert_color_factory('hsv', 'bgr')
bgr2hls = convert_color_factory('bgr', 'hls')
hls2bgr = convert_color_factory('hls', 'bgr')
Geometry: Flip, Rotate, Padding, Crop
from __future__ import division
import cv2
import numpy as np
def imflip(img, direction='horizontal'):
"""Flip an image horizontally or vertically.
Args:
img (ndarray): Image to be flipped.
direction (str): The flip direction, either "horizontal" or "vertical".
Returns:
ndarray: The flipped image.
"""
assert direction in ['horizontal', 'vertical']
if direction == 'horizontal':
return np.flip(img, axis=1)
else:
return np.flip(img, axis=0)
Image Rotation with OpenCV
def imrotate(img,
angle,
center=None,
scale=1.0,
border_value=0,
auto_bound=False):
"""Rotate an image.
Args:
img (ndarray): Image to be rotated.
angle (float): Rotation angle in degrees, positive values mean
clockwise rotation.
center (tuple): Center of the rotation in the source image, by default
it is the center of the image.
scale (float): Isotropic scale factor.
border_value (int): Border value.
auto_bound (bool): Whether to adjust the image size to cover the whole
rotated image.
Returns:
ndarray: The rotated image.
"""
if center is not None and auto_bound:
raise ValueError('`auto_bound` conflicts with `center`')
h, w = img.shape[:2]
if center is None:
center = ((w - 1) * 0.5, (h - 1) * 0.5)
assert isinstance(center, tuple)
matrix = cv2.getRotationMatrix2D(center, -angle, scale)
if auto_bound:
cos = np.abs(matrix[0, 0])
sin = np.abs(matrix[0, 1])
new_w = h * sin + w * cos
new_h = h * cos + w * sin
matrix[0, 2] += (new_w - w) * 0.5
matrix[1, 2] += (new_h - h) * 0.5
w = int(np.round(new_w))
h = int(np.round(new_h))
rotated = cv2.warpAffine(img, matrix, (w, h), borderValue=border_value)
return rotated
def bbox_clip(bboxes, img_shape):
"""Clip bboxes to fit the image shape.
Args:
bboxes (ndarray): Shape (..., 4*k)
img_shape (tuple): (height, width) of the image.
Returns:
ndarray: Clipped bboxes.
"""
assert bboxes.shape[-1] % 4 == 0
clipped_bboxes = np.empty_like(bboxes, dtype=bboxes.dtype)
clipped_bboxes[..., 0::2] = np.maximum(
np.minimum(bboxes[..., 0::2], img_shape[1] - 1), 0)
clipped_bboxes[..., 1::2] = np.maximum(
np.minimum(bboxes[..., 1::2], img_shape[0] - 1), 0)
return clipped_bboxes
def bbox_scaling(bboxes, scale, clip_shape=None):
"""Scaling bboxes w.r.t the box center.
Args:
bboxes (ndarray): Shape(..., 4).
scale (float): Scaling factor.
clip_shape (tuple, optional): If specified, bboxes that exceed the
boundary will be clipped according to the given shape (h, w).
Returns:
ndarray: Scaled bboxes.
"""
if float(scale) == 1.0:
scaled_bboxes = bboxes.copy()
else:
w = bboxes[..., 2] - bboxes[..., 0] + 1
h = bboxes[..., 3] - bboxes[..., 1] + 1
dw = (w * (scale - 1)) * 0.5
dh = (h * (scale - 1)) * 0.5
scaled_bboxes = bboxes + np.stack((-dw, -dh, dw, dh), axis=-1)
if clip_shape is not None:
return bbox_clip(scaled_bboxes, clip_shape)
else:
return scaled_bboxes
Image Cropping with OpenCV
def imcrop(img, bboxes, scale=1.0, pad_fill=None):
"""Crop image patches.
3 steps: scale the bboxes -> clip bboxes -> crop and pad.
Args:
img (ndarray): Image to be cropped.
bboxes (ndarray): Shape (k, 4) or (4, ), location of cropped bboxes.
scale (float, optional): Scale ratio of bboxes, the default value
1.0 means no padding.
pad_fill (number or list): Value to be filled for padding, None for
no padding.
Returns:
list or ndarray: The cropped image patches.
"""
chn = 1 if img.ndim == 2 else img.shape[2]
if pad_fill is not None:
if isinstance(pad_fill, (int, float)):
pad_fill = [pad_fill for _ in range(chn)]
assert len(pad_fill) == chn
_bboxes = bboxes[None, ...] if bboxes.ndim == 1 else bboxes
scaled_bboxes = bbox_scaling(_bboxes, scale).astype(np.int32)
clipped_bbox = bbox_clip(scaled_bboxes, img.shape)
patches = []
for i in range(clipped_bbox.shape[0]):
x1, y1, x2, y2 = tuple(clipped_bbox[i, :])
if pad_fill is None:
patch = img[y1:y2 + 1, x1:x2 + 1, ...]
else:
_x1, _y1, _x2, _y2 = tuple(scaled_bboxes[i, :])
if chn == 1:
patch_shape = (_y2 - _y1 + 1, _x2 - _x1 + 1)
else:
patch_shape = (_y2 - _y1 + 1, _x2 - _x1 + 1, chn)
patch = np.array(
pad_fill, dtype=img.dtype) * np.ones(
patch_shape, dtype=img.dtype)
x_start = 0 if _x1 >= 0 else -_x1
y_start = 0 if _y1 >= 0 else -_y1
w = x2 - x1 + 1
h = y2 - y1 + 1
patch[y_start:y_start + h, x_start:x_start +
w, ...] = img[y1:y1 + h, x1:x1 + w, ...]
patches.append(patch)
if bboxes.ndim == 1:
return patches[0]
else:
return patches
Image Padding with OpenCV
def impad(img, shape, pad_val=0):
"""Pad an image to a certain shape.
Args:
img (ndarray): Image to be padded.
shape (tuple): Expected padding shape.
pad_val (number or sequence): Values to be filled in padding areas.
Returns:
ndarray: The padded image.
"""
if not isinstance(pad_val, (int, float)):
assert len(pad_val) == img.shape[-1]
if len(shape) < len(img.shape):
shape = shape + (img.shape[-1], )
assert len(shape) == len(img.shape)
for i in range(len(shape) - 1):
assert shape[i] >= img.shape[i]
pad = np.empty(shape, dtype=img.dtype)
pad[...] = pad_val
pad[:img.shape[0], :img.shape[1], ...] = img
return pad
def impad_to_multiple(img, divisor, pad_val=0):
"""Pad an image to ensure each edge to be multiple to some number.
Args:
img (ndarray): Image to be padded.
divisor (int): Padded image edges will be multiple to divisor.
pad_val (number or sequence): Same as :func:`impad`.
Returns:
ndarray: The padded image.
"""
pad_h = int(np.ceil(img.shape[0] / divisor)) * divisor
pad_w = int(np.ceil(img.shape[1] / divisor)) * divisor
return impad(img, (pad_h, pad_w), pad_val)
Image Normalization with Python
import numpy as np
from .colorspace import bgr2rgb, rgb2bgr
def imnormalize(img, mean, std, to_rgb=True):
img = img.astype(np.float32)
if to_rgb:
img = bgr2rgb(img)
return (img - mean) / std
def imdenormalize(img, mean, std, to_bgr=True):
img = (img * std) + mean
if to_bgr:
img = rgb2bgr(img)
return img
Scale: Resize Image with OpenCV
from __future__ import division
import cv2
def _scale_size(size, scale):
"""Rescale a size by a ratio.
Args:
size (tuple): w, h.
scale (float): Scaling factor.
Returns:
tuple[int]: scaled size.
"""
w, h = size
return int(w * float(scale) + 0.5), int(h * float(scale) + 0.5)
interp_codes = {
'nearest': cv2.INTER_NEAREST,
'bilinear': cv2.INTER_LINEAR,
'bicubic': cv2.INTER_CUBIC,
'area': cv2.INTER_AREA,
'lanczos': cv2.INTER_LANCZOS4
}
def imresize(img, size, return_scale=False, interpolation='bilinear'):
"""Resize image to a given size.
Args:
img (ndarray): The input image.
size (tuple): Target (w, h).
return_scale (bool): Whether to return `w_scale` and `h_scale`.
interpolation (str): Interpolation method, accepted values are
"nearest", "bilinear", "bicubic", "area", "lanczos".
Returns:
tuple or ndarray: (`resized_img`, `w_scale`, `h_scale`) or
`resized_img`.
"""
h, w = img.shape[:2]
resized_img = cv2.resize(
img, size, interpolation=interp_codes[interpolation])
if not return_scale:
return resized_img
else:
w_scale = size[0] / w
h_scale = size[1] / h
return resized_img, w_scale, h_scale
def imresize_like(img, dst_img, return_scale=False, interpolation='bilinear'):
"""Resize image to the same size of a given image.
Args:
img (ndarray): The input image.
dst_img (ndarray): The target image.
return_scale (bool): Whether to return `w_scale` and `h_scale`.
interpolation (str): Same as :func:`resize`.
Returns:
tuple or ndarray: (`resized_img`, `w_scale`, `h_scale`) or
`resized_img`.
"""
h, w = dst_img.shape[:2]
return imresize(img, (w, h), return_scale, interpolation)
def imrescale(img, scale, return_scale=False, interpolation='bilinear'):
"""Resize image while keeping the aspect ratio.
Args:
img (ndarray): The input image.
scale (float or tuple[int]): The scaling factor or maximum size.
If it is a float number, then the image will be rescaled by this
factor, else if it is a tuple of 2 integers, then the image will
be rescaled as large as possible within the scale.
return_scale (bool): Whether to return the scaling factor besides the
rescaled image.
interpolation (str): Same as :func:`resize`.
Returns:
ndarray: The rescaled image.
"""
h, w = img.shape[:2]
if isinstance(scale, (float, int)):
if scale <= 0:
raise ValueError(
'Invalid scale {}, must be positive.'.format(scale))
scale_factor = scale
elif isinstance(scale, tuple):
max_long_edge = max(scale)
max_short_edge = min(scale)
scale_factor = min(max_long_edge / max(h, w),
max_short_edge / min(h, w))
else:
raise TypeError(
'Scale must be a number or tuple of int, but got {}'.format(
type(scale)))
new_size = _scale_size((w, h), scale_factor)
rescaled_img = imresize(img, new_size, interpolation=interpolation)
if return_scale:
return rescaled_img, scale_factor
else:
return rescaled_img
Code from mmcv