70 lines
1.9 KiB
Go
70 lines
1.9 KiB
Go
package epd
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import (
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"image"
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"image/color"
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_ "image/gif"
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_ "image/jpeg"
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_ "image/png"
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"io"
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"golang.org/x/image/draw"
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)
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// Returns a new Rectangle that is resized and centered in `dst`
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func fitRectInto(src *image.Rectangle, dst *image.Rectangle) image.Rectangle {
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var targetWidth int
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var targetHeight int
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var scale float64
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srcRatio := float64(src.Max.X) / float64(src.Max.Y)
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dstRatio := float64(dst.Max.X) / float64(dst.Max.Y)
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if srcRatio < dstRatio {
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// center horizontally, scale vertically
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scale = float64(dst.Max.Y) / float64(src.Max.Y)
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} else {
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// center vertically, scale horizontally
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scale = float64(dst.Max.X) / float64(src.Max.X)
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}
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targetWidth = int(float64(src.Max.X) * scale)
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targetHeight = int(float64(src.Max.Y) * scale)
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targetX := (dst.Max.X - targetWidth) / 2
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targetY := (dst.Max.Y - targetHeight) / 2
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return image.Rect(targetX, targetY, targetWidth+targetX, targetHeight+targetY)
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}
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// Converts a GIF, JPEG or PNG into a 16-color grayscale image. A single pixel
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// is represented by 4 bits and each byte holds two pixels. The final 4-bits of
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// the last byte may be discarded if width * height is odd.
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func ConvertImage(input io.Reader, width int, height int) ([]byte, error) {
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src, _, err := image.Decode(input)
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if err != nil {
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return nil, err
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}
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dst := image.NewGray(image.Rect(0, 0, width, height))
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srcBounds := src.Bounds()
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targetRect := fitRectInto(&srcBounds, &dst.Rect)
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draw.Draw(dst, dst.Bounds(), &image.Uniform{color.White}, image.ZP, draw.Src)
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draw.CatmullRom.Scale(dst, targetRect, src, src.Bounds(), draw.Over, nil)
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// the actual conversion works by packing two nibbles together in a byte
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var result = make([]byte, (width*height+1)/2)
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for i, p := range dst.Pix {
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res := uint8((uint16(p) + 8) / 16)
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if i%2 == 0 {
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result[i/2] = (res << 4)
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} else {
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// note that integer division makes sure we're writing at the same
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// index for odd and even indices
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result[i/2] |= res
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}
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}
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return result, nil
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}
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