54 行
2.0 KiB
Python
54 行
2.0 KiB
Python
import hashlib
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from website.constants import BACKGROUND_COLOUR
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def _contrast(c1, c2):
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# https://www2.cs.sfu.ca/CourseCentral/820/li/material/RGB-YUV.pdf
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l1 = (0.299 * c1[0] + 0.587 * c1[1] + 0.114 * c1[2]) / 256
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l2 = (0.299 * c2[0] + 0.587 * c2[1] + 0.114 * c2[2]) / 256
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# https://www.accessibility-developer-guide.com/knowledge/colours-and-contrast/how-to-calculate/
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return (l1 + 0.05) / (l2 + 0.05)
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def _distance_sq(c1, c2):
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return sum((i / 256 - j / 256) ** 2 for i, j in zip(c1, c2))
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def _gen_colour(seed, background=BACKGROUND_COLOUR):
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'''
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Returns a colour with sufficient contrast to the background colour
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'''
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while True:
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seed = hashlib.sha256(seed).digest()
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for i in range(0, len(seed) - len(seed) % 3, 3):
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colour = seed[i:i+3]
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if 1.5 < _contrast(colour, background) < 3:
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return colour
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def gen_colour(seed, background=BACKGROUND_COLOUR, *avoid):
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'''
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Returns a colour with sufficient contrast to the background colour
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Tries to make the colour contrast with all the colours in `avoid`
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This function hasn't been analysed for efficiency or anything
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'''
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best_colour, best_score = None, None
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for _ in range(16384):
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colour = _gen_colour(seed, background)
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score = float('inf') if len(avoid) == 0 else sum(_contrast(colour, c) for c in avoid) / len(avoid)
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if colour in avoid:
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score = float('-inf')
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if 2.5 < score:
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return colour
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if best_score == None or score > best_score:
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best_colour = colour
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return best_colour
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# old-style tag generation: similar colours make similar tags
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#def tag(colour):
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# tag = ((colour[2] & 0xf0) >> 4) | (colour[1] & 0xf0) | ((colour[0] & 0xf0) << 4)
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# return f'#{tag:03x}'
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def tag(token, length=3):
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'''
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Generates a deterministic pseudorandom tag from a given token
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'''
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digest = hashlib.sha256(token.encode()).digest()
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return f'#{digest.hex()[:length]}'
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