101 lines
2.8 KiB
Elixir
101 lines
2.8 KiB
Elixir
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defmodule Berrypod.Theme.Contrast do
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@moduledoc """
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WCAG contrast ratio calculations for accessibility checking.
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Used to warn shop owners when header background images might make text hard to read.
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"""
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@doc """
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Calculate relative luminance from sRGB values (0-255).
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Uses the WCAG 2.1 formula.
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"""
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def relative_luminance([r, g, b]) do
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[r, g, b]
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|> Enum.map(fn c ->
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c = c / 255
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if c <= 0.03928 do
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c / 12.92
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else
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:math.pow((c + 0.055) / 1.055, 2.4)
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end
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end)
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|> then(fn [r, g, b] -> 0.2126 * r + 0.7152 * g + 0.0722 * b end)
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end
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@doc """
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Calculate WCAG contrast ratio between two colors.
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Returns a ratio >= 1.0 (higher = better contrast).
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"""
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def contrast_ratio(color1, color2) do
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l1 = relative_luminance(color1)
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l2 = relative_luminance(color2)
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{lighter, darker} = if l1 > l2, do: {l1, l2}, else: {l2, l1}
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(lighter + 0.05) / (darker + 0.05)
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end
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@doc """
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Analyze header image colors against text color, return warning level.
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Returns:
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- `:ok` - All dominant colors have acceptable contrast
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- `:caution` - Some colors may be problematic
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- `:poor` - Most colors have poor contrast
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"""
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def analyze_header_contrast(nil, _text_color), do: :ok
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def analyze_header_contrast([], _text_color), do: :ok
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def analyze_header_contrast(dominant_colors, text_color) when is_list(dominant_colors) do
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text_rgb = hex_to_rgb(text_color)
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ratios = Enum.map(dominant_colors, &contrast_ratio(&1, text_rgb))
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min_ratio = Enum.min(ratios)
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avg_ratio = Enum.sum(ratios) / length(ratios)
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cond do
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# All colors have good contrast (3:1 is WCAG AA for large text)
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Enum.all?(ratios, &(&1 >= 3.0)) -> :ok
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# Most colors are problematic
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avg_ratio < 3.0 -> :poor
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# Some colors might be problematic
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min_ratio < 2.0 -> :caution
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true -> :ok
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end
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end
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@doc """
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Parse JSON-encoded dominant colors string.
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"""
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def parse_dominant_colors(nil), do: nil
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def parse_dominant_colors(""), do: nil
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def parse_dominant_colors(json) when is_binary(json) do
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case Jason.decode(json) do
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{:ok, colors} when is_list(colors) -> colors
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_ -> nil
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end
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end
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@doc """
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Get the text color for a given mood setting.
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"""
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def text_color_for_mood("dark"), do: "#fafafa"
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def text_color_for_mood("neutral"), do: "#171717"
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def text_color_for_mood("warm"), do: "#1c1917"
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def text_color_for_mood("cool"), do: "#0f172a"
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def text_color_for_mood(_), do: "#171717"
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@doc """
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Convert hex color to RGB list.
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"""
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def hex_to_rgb("#" <> hex), do: hex_to_rgb(hex)
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def hex_to_rgb(hex) when byte_size(hex) == 6 do
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[
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String.slice(hex, 0..1) |> Integer.parse(16) |> elem(0),
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String.slice(hex, 2..3) |> Integer.parse(16) |> elem(0),
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String.slice(hex, 4..5) |> Integer.parse(16) |> elem(0)
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]
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end
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end
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