问题
I am experimenting with JPEG compression using python. I load in a tiff image and store it as numpy uint8 RGB array. I was doing this for color mapping.
def rgb2ycbcr(im):
cbcr = np.empty_like(im)
r = im[:,:,0]
g = im[:,:,1]
b = im[:,:,2]
# Y
cbcr[:,:,0] = .299 * r + .587 * g + .114 * b
# Cb
cbcr[:,:,1] = 128 - .169 * r - .331 * g + .5 * b
# Cr
cbcr[:,:,2] = 128 + .5 * r - .419 * g - .081 * b
return np.uint8(cbcr)
def ycbcr2rgb(im):
rgb = np.empty_like(im)
y = im[:,:,0]
cb = im[:,:,1] - 128
cr = im[:,:,2] - 128
# R
rgb[:,:,0] = y + 1.402 * cr
# G
rgb[:,:,1] = y - .34414 * cb - .71414 * cr
# B
rgb[:,:,2] = y + 1.772 * cb
return np.uint8(rgb)
I did a simple RGB to YCbCr transformation followed with a inverse transformation.
img = rgb2ycbcr(img)
imshow(img)
img = ycbcr2rgb(img)
imshow(img)
I got these two output image as YCbCr and RGB output after the color space transformation.
It seems that something is wrong with my color conversion and I cannot figure out what is wrong. I was using the JPEG color space conversion provided by Wikipedia. Thanks you for the help.
回答1:
You have to do your intermediate calculations in floating point. The posterization should tip you off; you have a lot of "hot" (saturated) pixels.
def rgb2ycbcr(im):
xform = np.array([[.299, .587, .114], [-.1687, -.3313, .5], [.5, -.4187, -.0813]])
ycbcr = im.dot(xform.T)
ycbcr[:,:,[1,2]] += 128
return np.uint8(ycbcr)
def ycbcr2rgb(im):
xform = np.array([[1, 0, 1.402], [1, -0.34414, -.71414], [1, 1.772, 0]])
rgb = im.astype(np.float)
rgb[:,:,[1,2]] -= 128
rgb = rgb.dot(xform.T)
np.putmask(rgb, rgb > 255, 255)
np.putmask(rgb, rgb < 0, 0)
return np.uint8(rgb)
来源:https://stackoverflow.com/questions/34913005/color-space-mapping-ycbcr-to-rgb