Correcting image white balance with Python PIL and Numpy

White balance.ipynb

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   "source": [
    "from PIL import Image\n",
    "import numpy as np\n",
    "from urllib.request import urlopen\n",
    "\n",
    "im = Image.open(urlopen('https://codeandlife.com/wp-content/uploads/2019/07/pistore-8-768x654.jpg'))\n",
    "#im = Image.open('test.jpg') # For local files\n",
    "im"
   ]
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   "source": [
    "data = np.asarray(im)\n",
    "data.shape"
   ]
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   "source": [
    "refpos = (550,360) # top left corner\n",
    "refsize = (32,32) # reference sample size\n",
    "sub = data[refpos[0]:refpos[0]+refsize[0],refpos[1]:refpos[1]+refsize[1]]\n",
    "Image.fromarray(sub)"
   ]
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   "source": [
    "c = list(np.mean(sub[:,:,i]) for i in range(3))\n",
    "c"
   ]
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   "source": [
    "wb = data.astype(float)\n",
    "\n",
    "# RGB scaling just decreases oversaturated components to get average grey\n",
    "# See https://en.wikipedia.org/wiki/Color_balance\n",
    "\n",
    "for i in range(3): wb[:,:,i] /= c[i]/float(min(c))\n",
    "    \n",
    "Image.fromarray(wb.astype(np.uint8))"
   ]
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   "execution_count": null,
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   "source": [
    "# Conversion functions courtesy of https://stackoverflow.com/a/34913974/2721685\n",
    "def rgb2ycbcr(im):\n",
    "    xform = np.array([[.299, .587, .114], [-.1687, -.3313, .5], [.5, -.4187, -.0813]])\n",
    "    ycbcr = im.dot(xform.T)\n",
    "    ycbcr[:,:,[1,2]] += 128\n",
    "    return ycbcr #np.uint8(ycbcr)\n",
    "\n",
    "def ycbcr2rgb(im):\n",
    "    xform = np.array([[1, 0, 1.402], [1, -0.34414, -.71414], [1, 1.772, 0]])\n",
    "    rgb = im.astype(np.float)\n",
    "    rgb[:,:,[1,2]] -= 128\n",
    "    rgb = rgb.dot(xform.T)\n",
    "    np.putmask(rgb, rgb > 255, 255)\n",
    "    np.putmask(rgb, rgb < 0, 0)\n",
    "    return np.uint8(rgb)\n",
    "\n",
    "# Convert data and sample to YCbCr\n",
    "ycbcr = rgb2ycbcr(data)\n",
    "ysub = rgb2ycbcr(sub)\n",
    "\n",
    "# Calculate mean components\n",
    "yc = list(np.mean(ysub[:,:,i]) for i in range(3))\n",
    "\n",
    "# Center cb and cr components of image based on sample\n",
    "for i in range(1,3):\n",
    "    ycbcr[:,:,i] = np.clip(ycbcr[:,:,i] + (128-yc[i]), 0, 255)\n",
    "\n",
    "rgb = ycbcr2rgb(ycbcr) # Convert back\n",
    "Image.fromarray(rgb)"
   ]
  }
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