104 lines
2.3 KiB
Go
104 lines
2.3 KiB
Go
package clip
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import (
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"encoding/binary"
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"errors"
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"image"
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"math/bits"
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)
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var errBMP = errors.New("unsupported or truncated bmp")
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func channel(v, m uint32) uint8 {
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if m == 0 {
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return 0
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}
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s := bits.TrailingZeros32(m)
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n := bits.OnesCount32(m)
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x := (v & m) >> s
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if n >= 8 {
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return uint8(x >> (n - 8))
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}
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return uint8(x * 255 / (1<<n - 1))
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}
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func DecodeBMP(data []byte) (*image.NRGBA, error) {
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le := binary.LittleEndian
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b := data
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off := -1
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if len(b) >= 14 && b[0] == 'B' && b[1] == 'M' {
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off = int(le.Uint32(b[10:])) - 14
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b = b[14:]
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}
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if len(b) < 40 {
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return nil, errBMP
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}
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hs := int(le.Uint32(b))
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w := int(int32(le.Uint32(b[4:])))
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h := int(int32(le.Uint32(b[8:])))
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bpp := int(le.Uint16(b[14:]))
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comp := le.Uint32(b[16:])
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if hs < 40 || hs > len(b) || w <= 0 || h == 0 || (bpp != 24 && bpp != 32) || (comp != 0 && comp != 3) {
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return nil, errBMP
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}
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masks := [4]uint32{0x00FF0000, 0x0000FF00, 0x000000FF, 0}
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if bpp == 32 {
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masks[3] = 0xFF000000
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}
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end := hs
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if comp == 3 {
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switch {
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case hs >= 56:
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masks = [4]uint32{le.Uint32(b[40:]), le.Uint32(b[44:]), le.Uint32(b[48:]), le.Uint32(b[52:])}
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case hs >= 52:
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masks = [4]uint32{le.Uint32(b[40:]), le.Uint32(b[44:]), le.Uint32(b[48:]), 0}
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default:
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if len(b) < hs+12 {
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return nil, errBMP
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}
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masks = [4]uint32{le.Uint32(b[hs:]), le.Uint32(b[hs+4:]), le.Uint32(b[hs+8:]), 0}
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end = hs + 12
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}
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}
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if off < 0 {
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off = end + int(le.Uint32(b[32:]))*4
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}
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top := h < 0
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if top {
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h = -h
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}
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stride := (w*bpp/8 + 3) &^ 3
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if off < 0 || w > 1<<15 || h > 1<<15 || off+stride*h > len(b) {
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return nil, errBMP
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}
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img := image.NewNRGBA(image.Rect(0, 0, w, h))
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alpha := false
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for y := 0; y < h; y++ {
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row := h - 1 - y
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if top {
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row = y
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}
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src := b[off+row*stride:]
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dst := img.Pix[y*img.Stride:]
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for x := 0; x < w; x++ {
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o := x * 4
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if bpp == 24 {
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dst[o], dst[o+1], dst[o+2], dst[o+3] = src[x*3+2], src[x*3+1], src[x*3], 255
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continue
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}
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v := le.Uint32(src[x*4:])
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dst[o], dst[o+1], dst[o+2], dst[o+3] = channel(v, masks[0]), channel(v, masks[1]), channel(v, masks[2]), 255
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if masks[3] != 0 {
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dst[o+3] = channel(v, masks[3])
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alpha = alpha || dst[o+3] != 0
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}
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}
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}
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if bpp == 32 && masks[3] != 0 && !alpha {
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for i := 3; i < len(img.Pix); i += 4 {
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img.Pix[i] = 255
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}
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}
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return img, nil
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}
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