98 lines
2.3 KiB
Go
98 lines
2.3 KiB
Go
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package color
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import (
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"image/color"
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)
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// ToNRGBA converts a color to RGBA values which are not premultiplied, unlike color.RGBA().
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func ToNRGBA(c color.Color) (r, g, b, a int) {
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// We use UnmultiplyAlpha with RGBA, RGBA64, and unrecognized implementations of Color.
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// It works for all Colors whose RGBA() method is implemented according to spec, but is only necessary for those.
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// Only RGBA and RGBA64 have components which are already premultiplied.
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switch col := c.(type) {
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// NRGBA and NRGBA64 are not premultiplied
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case color.NRGBA:
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r = int(col.R)
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g = int(col.G)
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b = int(col.B)
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a = int(col.A)
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case *color.NRGBA:
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r = int(col.R)
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g = int(col.G)
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b = int(col.B)
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a = int(col.A)
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case color.NRGBA64:
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r = int(col.R) >> 8
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g = int(col.G) >> 8
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b = int(col.B) >> 8
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a = int(col.A) >> 8
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case *color.NRGBA64:
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r = int(col.R) >> 8
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g = int(col.G) >> 8
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b = int(col.B) >> 8
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a = int(col.A) >> 8
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// Gray and Gray16 have no alpha component
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case *color.Gray:
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r = int(col.Y)
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g = int(col.Y)
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b = int(col.Y)
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a = 0xff
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case color.Gray:
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r = int(col.Y)
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g = int(col.Y)
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b = int(col.Y)
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a = 0xff
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case *color.Gray16:
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r = int(col.Y) >> 8
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g = int(col.Y) >> 8
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b = int(col.Y) >> 8
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a = 0xff
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case color.Gray16:
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r = int(col.Y) >> 8
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g = int(col.Y) >> 8
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b = int(col.Y) >> 8
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a = 0xff
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// Alpha and Alpha16 contain only an alpha component.
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case color.Alpha:
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r = 0xff
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g = 0xff
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b = 0xff
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a = int(col.A)
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case *color.Alpha:
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r = 0xff
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g = 0xff
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b = 0xff
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a = int(col.A)
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case color.Alpha16:
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r = 0xff
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g = 0xff
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b = 0xff
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a = int(col.A) >> 8
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case *color.Alpha16:
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r = 0xff
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g = 0xff
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b = 0xff
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a = int(col.A) >> 8
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default: // RGBA, RGBA64, and unknown implementations of Color
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r, g, b, a = unmultiplyAlpha(c)
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}
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return
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}
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// unmultiplyAlpha returns a color's RGBA components as 8-bit integers by calling c.RGBA() and then removing the alpha premultiplication.
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// It is only used by ToRGBA.
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func unmultiplyAlpha(c color.Color) (r, g, b, a int) {
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red, green, blue, alpha := c.RGBA()
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if alpha != 0 && alpha != 0xffff {
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red = (red * 0xffff) / alpha
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green = (green * 0xffff) / alpha
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blue = (blue * 0xffff) / alpha
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}
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// Convert from range 0-65535 to range 0-255
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r = int(red >> 8)
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g = int(green >> 8)
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b = int(blue >> 8)
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a = int(alpha >> 8)
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return
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}
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