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Commit 737e96c7 authored by Dmitry Chestnykh's avatar Dmitry Chestnykh Committed by Nigel Tao
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image/png: support encoding of paletted images with alpha channel. 

Encoder now writes tRNS chunk for non-opaque paletted images.
CL includes new test images (basn3a08-trns.[ps]ng).

R=nigeltao, rsc, r
CC=golang-dev
https://golang.org/cl/4432078
parent bcf3bfe2
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Inline Side-by-side
Showing with 339 additions and 9 deletions
src/pkg/image/png/reader_test.go
View file @ 737e96c7
......@@ -28,6 +28,7 @@ var filenames = []string{
"basn3p02",
"basn3p04",
"basn3p08",
"basn3p08-trns",
"basn4a08",
"basn4a16",
"basn6a08",
......@@ -98,17 +99,30 @@ func sng(w io.WriteCloser, filename string, png image.Image) {
// (the PNG spec section 11.3 says "Ancillary chunks may be ignored by a decoder").
io.WriteString(w, "gAMA {1.0000}\n")
// Write the PLTE (if applicable).
// Write the PLTE and tRNS (if applicable).
if cpm != nil {
lastAlpha := -1
io.WriteString(w, "PLTE {\n")
for i := 0; i < len(cpm); i++ {
r, g, b, _ := cpm[i].RGBA()
for i, c := range cpm {
r, g, b, a := c.RGBA()
if a != 0xffff {
lastAlpha = i
}
r >>= 8
g >>= 8
b >>= 8
fmt.Fprintf(w, " (%3d,%3d,%3d) # rgb = (0x%02x,0x%02x,0x%02x)\n", r, g, b, r, g, b)
}
io.WriteString(w, "}\n")
if lastAlpha != -1 {
io.WriteString(w, "tRNS {\n")
for i := 0; i <= lastAlpha; i++ {
_, _, _, a := cpm[i].RGBA()
a >>= 8
fmt.Fprintf(w, " %d", a)
}
io.WriteString(w, "}\n")
}
}
// Write the IMAGE.
......
src/pkg/image/png/testdata/pngsuite/README
View file @ 737e96c7
......@@ -10,6 +10,9 @@ The files basn0g01-30.png, basn0g02-29.png and basn0g04-31.png are in fact
not part of pngsuite but were created from files in pngsuite. Their non-power-
of-two sizes makes them useful for testing bit-depths smaller than a byte.
basn3a08.png was generated from basn6a08.png using the pngnq tool, which
converted it to the 8-bit paletted image with alpha values in tRNS chunk.
The *.sng files in this directory were generated from the *.png files
by the sng command-line tool and some hand editing. The files
basn0g0{1,2,4}.sng were actually generated by first converting the PNG
......
src/pkg/image/png/testdata/pngsuite/basn3p08-trns.sng 0 → 100644
View file @ 737e96c7
#SNG: from basn3p08-trns.png
IHDR {
width: 32; height: 32; bitdepth: 8;
using color palette;
}
gAMA {1.0000}
PLTE {
(255, 3, 7) # rgb = (0xff,0x03,0x07)
(255, 4, 7) # rgb = (0xff,0x04,0x07)
(255, 9, 7) # rgb = (0xff,0x09,0x07)
(217, 14, 7) # rgb = (0xd9,0x0e,0x07)
(255, 14, 7) # rgb = (0xff,0x0e,0x07)
( 2, 22, 19) # rgb = (0x02,0x16,0x13)
(255, 26, 7) # rgb = (0xff,0x1a,0x07)
(255, 31, 7) # rgb = (0xff,0x1f,0x07)
( 10, 37, 14) # rgb = (0x0a,0x25,0x0e)
(179, 37, 6) # rgb = (0xb3,0x25,0x06)
(254, 42, 7) # rgb = (0xfe,0x2a,0x07)
(255, 45, 7) # rgb = (0xff,0x2d,0x07)
( 25, 46, 9) # rgb = (0x19,0x2e,0x09)
( 0, 48,254) # rgb = (0x00,0x30,0xfe)
( 0, 48,255) # rgb = (0x00,0x30,0xff)
( 0, 49,255) # rgb = (0x00,0x31,0xff)
( 0, 51,254) # rgb = (0x00,0x33,0xfe)
( 0, 52,255) # rgb = (0x00,0x34,0xff)
(255, 53, 7) # rgb = (0xff,0x35,0x07)
( 0, 54,252) # rgb = (0x00,0x36,0xfc)
(254, 57, 7) # rgb = (0xfe,0x39,0x07)
(251, 57, 7) # rgb = (0xfb,0x39,0x07)
(247, 59, 7) # rgb = (0xf7,0x3b,0x07)
( 0, 59, 61) # rgb = (0x00,0x3b,0x3d)
( 0, 62,255) # rgb = (0x00,0x3e,0xff)
(142, 63, 5) # rgb = (0x8e,0x3f,0x05)
( 0, 63,250) # rgb = (0x00,0x3f,0xfa)
(255, 63, 7) # rgb = (0xff,0x3f,0x07)
(253, 68, 7) # rgb = (0xfd,0x44,0x07)
( 0, 73,255) # rgb = (0x00,0x49,0xff)
( 0, 73,246) # rgb = (0x00,0x49,0xf6)
(255, 75, 7) # rgb = (0xff,0x4b,0x07)
( 82, 85, 9) # rgb = (0x52,0x55,0x09)
(255, 85, 7) # rgb = (0xff,0x55,0x07)
( 0, 89,255) # rgb = (0x00,0x59,0xff)
( 0, 91,237) # rgb = (0x00,0x5b,0xed)
(255, 94, 7) # rgb = (0xff,0x5e,0x07)
(241,100, 7) # rgb = (0xf1,0x64,0x07)
( 0,101,255) # rgb = (0x00,0x65,0xff)
(253,105, 7) # rgb = (0xfd,0x69,0x07)
( 0,107,223) # rgb = (0x00,0x6b,0xdf)
(255,106, 7) # rgb = (0xff,0x6a,0x07)
( 1,110, 95) # rgb = (0x01,0x6e,0x5f)
(255,115, 7) # rgb = (0xff,0x73,0x07)
( 0,117,255) # rgb = (0x00,0x75,0xff)
(255,124, 7) # rgb = (0xff,0x7c,0x07)
(118,126, 10) # rgb = (0x76,0x7e,0x0a)
( 0,130,250) # rgb = (0x00,0x82,0xfa)
( 0,132,255) # rgb = (0x00,0x84,0xff)
( 0,134,207) # rgb = (0x00,0x86,0xcf)
(255,134, 7) # rgb = (0xff,0x86,0x07)
( 0,136,249) # rgb = (0x00,0x88,0xf9)
(219,140, 6) # rgb = (0xdb,0x8c,0x06)
( 0,140,252) # rgb = (0x00,0x8c,0xfc)
( 0,140,255) # rgb = (0x00,0x8c,0xff)
( 1,142,136) # rgb = (0x01,0x8e,0x88)
(255,143, 7) # rgb = (0xff,0x8f,0x07)
(243,150, 7) # rgb = (0xf3,0x96,0x07)
(198,152, 7) # rgb = (0xc6,0x98,0x07)
(165,153, 7) # rgb = (0xa5,0x99,0x07)
( 0,157,255) # rgb = (0x00,0x9d,0xff)
(255,158, 7) # rgb = (0xff,0x9e,0x07)
( 70,159, 4) # rgb = (0x46,0x9f,0x04)
( 0,160,251) # rgb = (0x00,0xa0,0xfb)
(203,163, 6) # rgb = (0xcb,0xa3,0x06)
( 0,163,239) # rgb = (0x00,0xa3,0xef)
( 1,164,178) # rgb = (0x01,0xa4,0xb2)
(255,166, 7) # rgb = (0xff,0xa6,0x07)
( 1,169,165) # rgb = (0x01,0xa9,0xa5)
( 1,170,255) # rgb = (0x01,0xaa,0xff)
(232,172, 6) # rgb = (0xe8,0xac,0x06)
(255,175, 7) # rgb = (0xff,0xaf,0x07)
(185,176,131) # rgb = (0xb9,0xb0,0x83)
( 1,179,225) # rgb = (0x01,0xb3,0xe1)
(188,179,118) # rgb = (0xbc,0xb3,0x76)
(199,180, 6) # rgb = (0xc7,0xb4,0x06)
( 1,182,255) # rgb = (0x01,0xb6,0xff)
( 1,184,249) # rgb = (0x01,0xb8,0xf9)
(255,184, 7) # rgb = (0xff,0xb8,0x07)
(207,186, 71) # rgb = (0xcf,0xba,0x47)
(193,187, 6) # rgb = (0xc1,0xbb,0x06)
(253,191, 7) # rgb = (0xfd,0xbf,0x07)
(218,193, 48) # rgb = (0xda,0xc1,0x30)
( 1,193,157) # rgb = (0x01,0xc1,0x9d)
( 1,196,244) # rgb = (0x01,0xc4,0xf4)
( 1,196,254) # rgb = (0x01,0xc4,0xfe)
( 48,199, 3) # rgb = (0x30,0xc7,0x03)
(164,199, 5) # rgb = (0xa4,0xc7,0x05)
(220,202, 6) # rgb = (0xdc,0xca,0x06)
(253,203, 7) # rgb = (0xfd,0xcb,0x07)
( 1,204,204) # rgb = (0x01,0xcc,0xcc)
(251,209, 7) # rgb = (0xfb,0xd1,0x07)
(231,208, 24) # rgb = (0xe7,0xd0,0x18)
( 1,210,254) # rgb = (0x01,0xd2,0xfe)
( 2,211,146) # rgb = (0x02,0xd3,0x92)
( 1,212,156) # rgb = (0x01,0xd4,0x9c)
( 1,213,252) # rgb = (0x01,0xd5,0xfc)
(237,219, 15) # rgb = (0xed,0xdb,0x0f)
( 1,218,240) # rgb = (0x01,0xda,0xf0)
(165,220, 5) # rgb = (0xa5,0xdc,0x05)
( 1,221,250) # rgb = (0x01,0xdd,0xfa)
(249,221, 6) # rgb = (0xf9,0xdd,0x06)
(146,222, 4) # rgb = (0x92,0xde,0x04)
( 1,224,184) # rgb = (0x01,0xe0,0xb8)
( 2,224,155) # rgb = (0x02,0xe0,0x9b)
(244,225, 10) # rgb = (0xf4,0xe1,0x0a)
(249,227, 7) # rgb = (0xf9,0xe3,0x07)
( 2,229,133) # rgb = (0x02,0xe5,0x85)
(192,228, 6) # rgb = (0xc0,0xe4,0x06)
( 37,230, 3) # rgb = (0x25,0xe6,0x03)
(246,230, 7) # rgb = (0xf6,0xe6,0x07)
(143,232, 4) # rgb = (0x8f,0xe8,0x04)
(244,233, 8) # rgb = (0xf4,0xe9,0x08)
( 2,236,139) # rgb = (0x02,0xec,0x8b)
( 1,236,227) # rgb = (0x01,0xec,0xe3)
( 1,238,238) # rgb = (0x01,0xee,0xee)
(101,241, 4) # rgb = (0x65,0xf1,0x04)
( 1,241,218) # rgb = (0x01,0xf1,0xda)
( 1,240,232) # rgb = (0x01,0xf0,0xe8)
(167,240, 5) # rgb = (0xa7,0xf0,0x05)
( 27,243, 2) # rgb = (0x1b,0xf3,0x02)
(126,243, 4) # rgb = (0x7e,0xf3,0x04)
( 2,246,113) # rgb = (0x02,0xf6,0x71)
(133,248, 5) # rgb = (0x85,0xf8,0x05)
( 22,250, 1) # rgb = (0x16,0xfa,0x01)
( 2,249,219) # rgb = (0x02,0xf9,0xdb)
(148,250, 5) # rgb = (0x94,0xfa,0x05)
( 2,250,199) # rgb = (0x02,0xfa,0xc7)
(183,252, 5) # rgb = (0xb7,0xfc,0x05)
(176,252, 5) # rgb = (0xb0,0xfc,0x05)
( 2,252,211) # rgb = (0x02,0xfc,0xd3)
( 2,252,190) # rgb = (0x02,0xfc,0xbe)
(164,251, 5) # rgb = (0xa4,0xfb,0x05)
( 12,254,128) # rgb = (0x0c,0xfe,0x80)
(192,253, 5) # rgb = (0xc0,0xfd,0x05)
(164,253, 5) # rgb = (0xa4,0xfd,0x05)
( 26,254, 85) # rgb = (0x1a,0xfe,0x55)
( 14,254, 1) # rgb = (0x0e,0xfe,0x01)
(133,253, 5) # rgb = (0x85,0xfd,0x05)
( 4,253,180) # rgb = (0x04,0xfd,0xb4)
(196,253, 5) # rgb = (0xc4,0xfd,0x05)
( 2,253,198) # rgb = (0x02,0xfd,0xc6)
( 3,255, 91) # rgb = (0x03,0xff,0x5b)
( 3,255, 80) # rgb = (0x03,0xff,0x50)
(186,255, 5) # rgb = (0xba,0xff,0x05)
( 9,255, 2) # rgb = (0x09,0xff,0x02)
( 3,255,118) # rgb = (0x03,0xff,0x76)
( 9,255, 3) # rgb = (0x09,0xff,0x03)
( 10,255, 1) # rgb = (0x0a,0xff,0x01)
( 3,255, 76) # rgb = (0x03,0xff,0x4c)
( 3,255, 86) # rgb = (0x03,0xff,0x56)
( 3,255, 82) # rgb = (0x03,0xff,0x52)
( 13,255, 1) # rgb = (0x0d,0xff,0x01)
( 3,255, 49) # rgb = (0x03,0xff,0x31)
( 3,255,101) # rgb = (0x03,0xff,0x65)
( 61,255, 32) # rgb = (0x3d,0xff,0x20)
(129,255, 5) # rgb = (0x81,0xff,0x05)
(177,255, 5) # rgb = (0xb1,0xff,0x05)
( 3,255, 37) # rgb = (0x03,0xff,0x25)
(149,255, 5) # rgb = (0x95,0xff,0x05)
( 7,255, 6) # rgb = (0x07,0xff,0x06)
(192,255, 5) # rgb = (0xc0,0xff,0x05)
( 2,255,131) # rgb = (0x02,0xff,0x83)
( 3,255, 98) # rgb = (0x03,0xff,0x62)
( 85,255, 11) # rgb = (0x55,0xff,0x0b)
( 2,255,163) # rgb = (0x02,0xff,0xa3)
( 2,255,149) # rgb = (0x02,0xff,0x95)
( 4,255, 23) # rgb = (0x04,0xff,0x17)
( 6,255, 12) # rgb = (0x06,0xff,0x0c)
( 3,255, 67) # rgb = (0x03,0xff,0x43)
(160,255, 5) # rgb = (0xa0,0xff,0x05)
(119,255, 6) # rgb = (0x77,0xff,0x06)
(102,255, 8) # rgb = (0x66,0xff,0x08)
(255,255,255) # rgb = (0xff,0xff,0xff)
(254,254,254) # rgb = (0xfe,0xfe,0xfe)
(254,254,254) # rgb = (0xfe,0xfe,0xfe)
(252,252,252) # rgb = (0xfc,0xfc,0xfc)
(252,252,252) # rgb = (0xfc,0xfc,0xfc)
(250,250,250) # rgb = (0xfa,0xfa,0xfa)
(250,250,250) # rgb = (0xfa,0xfa,0xfa)
(248,248,248) # rgb = (0xf8,0xf8,0xf8)
(248,248,248) # rgb = (0xf8,0xf8,0xf8)
(247,247,247) # rgb = (0xf7,0xf7,0xf7)
(245,245,245) # rgb = (0xf5,0xf5,0xf5)
(245,245,245) # rgb = (0xf5,0xf5,0xf5)
(243,243,243) # rgb = (0xf3,0xf3,0xf3)
(243,243,243) # rgb = (0xf3,0xf3,0xf3)
(241,241,241) # rgb = (0xf1,0xf1,0xf1)
(241,241,241) # rgb = (0xf1,0xf1,0xf1)
(239,239,239) # rgb = (0xef,0xef,0xef)
(238,238,238) # rgb = (0xee,0xee,0xee)
(238,238,238) # rgb = (0xee,0xee,0xee)
(236,236,236) # rgb = (0xec,0xec,0xec)
(236,236,236) # rgb = (0xec,0xec,0xec)
(234,234,234) # rgb = (0xea,0xea,0xea)
(234,234,234) # rgb = (0xea,0xea,0xea)
(232,232,232) # rgb = (0xe8,0xe8,0xe8)
(231,231,231) # rgb = (0xe7,0xe7,0xe7)
(231,231,231) # rgb = (0xe7,0xe7,0xe7)
(229,229,229) # rgb = (0xe5,0xe5,0xe5)
(229,229,229) # rgb = (0xe5,0xe5,0xe5)
(227,227,227) # rgb = (0xe3,0xe3,0xe3)
(226,226,226) # rgb = (0xe2,0xe2,0xe2)
(226,226,226) # rgb = (0xe2,0xe2,0xe2)
(224,224,224) # rgb = (0xe0,0xe0,0xe0)
(224,224,224) # rgb = (0xe0,0xe0,0xe0)
(222,222,222) # rgb = (0xde,0xde,0xde)
(222,222,222) # rgb = (0xde,0xde,0xde)
(220,220,220) # rgb = (0xdc,0xdc,0xdc)
(219,219,219) # rgb = (0xdb,0xdb,0xdb)
(219,219,219) # rgb = (0xdb,0xdb,0xdb)
(217,217,217) # rgb = (0xd9,0xd9,0xd9)
(217,217,217) # rgb = (0xd9,0xd9,0xd9)
(215,215,215) # rgb = (0xd7,0xd7,0xd7)
(214,214,214) # rgb = (0xd6,0xd6,0xd6)
(214,214,214) # rgb = (0xd6,0xd6,0xd6)
(212,212,212) # rgb = (0xd4,0xd4,0xd4)
(212,212,212) # rgb = (0xd4,0xd4,0xd4)
(210,210,210) # rgb = (0xd2,0xd2,0xd2)
(209,209,209) # rgb = (0xd1,0xd1,0xd1)
(209,209,209) # rgb = (0xd1,0xd1,0xd1)
(207,207,207) # rgb = (0xcf,0xcf,0xcf)
(205,205,205) # rgb = (0xcd,0xcd,0xcd)
(205,205,205) # rgb = (0xcd,0xcd,0xcd)
(204,204,204) # rgb = (0xcc,0xcc,0xcc)
(204,204,204) # rgb = (0xcc,0xcc,0xcc)
(202,202,202) # rgb = (0xca,0xca,0xca)
(201,201,201) # rgb = (0xc9,0xc9,0xc9)
(201,201,201) # rgb = (0xc9,0xc9,0xc9)
(199,199,199) # rgb = (0xc7,0xc7,0xc7)
(199,199,199) # rgb = (0xc7,0xc7,0xc7)
(197,197,197) # rgb = (0xc5,0xc5,0xc5)
(196,196,196) # rgb = (0xc4,0xc4,0xc4)
(196,196,196) # rgb = (0xc4,0xc4,0xc4)
(194,194,194) # rgb = (0xc2,0xc2,0xc2)
(193,193,193) # rgb = (0xc1,0xc1,0xc1)
(193,193,193) # rgb = (0xc1,0xc1,0xc1)
(191,191,191) # rgb = (0xbf,0xbf,0xbf)
(191,191,191) # rgb = (0xbf,0xbf,0xbf)
(189,189,189) # rgb = (0xbd,0xbd,0xbd)
(188,188,188) # rgb = (0xbc,0xbc,0xbc)
(188,188,188) # rgb = (0xbc,0xbc,0xbc)
(186,186,186) # rgb = (0xba,0xba,0xba)
(185,185,185) # rgb = (0xb9,0xb9,0xb9)
(185,185,185) # rgb = (0xb9,0xb9,0xb9)
(183,183,183) # rgb = (0xb7,0xb7,0xb7)
(182,182,182) # rgb = (0xb6,0xb6,0xb6)
(182,182,182) # rgb = (0xb6,0xb6,0xb6)
(180,180,180) # rgb = (0xb4,0xb4,0xb4)
(178,178,178) # rgb = (0xb2,0xb2,0xb2)
(178,178,178) # rgb = (0xb2,0xb2,0xb2)
(177,177,177) # rgb = (0xb1,0xb1,0xb1)
(177,177,177) # rgb = (0xb1,0xb1,0xb1)
(175,175,175) # rgb = (0xaf,0xaf,0xaf)
(174,174,174) # rgb = (0xae,0xae,0xae)
(174,174,174) # rgb = (0xae,0xae,0xae)
}
tRNS {
197 187 190 194 186 4 186 189 4 195 84 191 5 193 175 163 205 150 191 213 88 75 67 8 147 191 220 203 95 151 223 199 8 207 156 227 199 65 163 98 226 204 12 202 167 201 11 65 178 228 205 74 59 87 178 19 201 99 18 14 184 204 184 96 22 61 227 199 22 193 97 197 254 59 253 28 192 102 199 247 58 198 244 30 109 202 188 32 96 196 60 203 239 202 230 41 207 237 119 53 213 209 37 55 45 230 214 233 92 185 223 50 230 57 124 217 43 133 221 95 198 47 233 99 194 221 107 138 152 144 226 140 133 220 172 125 218 196 118 225 161 223 235 238 200 155 147 146 172 236 236 151 183 150 234 216 217 211 151 219 132 185 145 147 217 138 144 137 142 151 217 217 213}
IMAGE {
pixels hex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052e7474747474747474a0a0a0a0a0a09393936d6d6d6d787878787878787878
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052a53536871717171717171a9a9a9a9a9a9a9a9a9a9a9a99595959595959595
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05445353686871717171717171a5a2a2a2a2a2929292928585857a7a7a7a7a7a
054453535f68687171717171a5a5a5a5a5a5a6a6a6a6a68b8b8b8b8b8b8b8b6b
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052a37372f2f2f2f2f2f18111111110f0e0e0e0e0d0d0d0d0d0d0d0d0d0d0d13
}
src/pkg/image/png/writer.go
View file @ 737e96c7
......@@ -130,12 +130,8 @@ func (e *encoder) writePLTE(p image.PalettedColorModel) {
e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
return
}
for i := 0; i < len(p); i++ {
r, g, b, a := p[i].RGBA()
if a != 0xffff {
e.err = UnsupportedError("non-opaque palette color")
return
}
for i, c := range p {
r, g, b, _ := c.RGBA()
e.tmp[3*i+0] = uint8(r >> 8)
e.tmp[3*i+1] = uint8(g >> 8)
e.tmp[3*i+2] = uint8(b >> 8)
......@@ -143,6 +139,21 @@ func (e *encoder) writePLTE(p image.PalettedColorModel) {
e.writeChunk(e.tmp[0:3*len(p)], "PLTE")
}
func (e *encoder) maybeWritetRNS(p image.PalettedColorModel) {
last := -1
for i, c := range p {
_, _, _, a := c.RGBA()
if a != 0xffff {
last = i
}
e.tmp[i] = uint8(a >> 8)
}
if last == -1 {
return
}
e.writeChunk(e.tmp[:last+1], "tRNS")
}
// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls
// should be relatively infrequent, since writeIDATs uses a bufio.Writer.
......@@ -447,6 +458,7 @@ func Encode(w io.Writer, m image.Image) os.Error {
e.writeIHDR()
if pal != nil {
e.writePLTE(pal.Palette)
e.maybeWritetRNS(pal.Palette)
}
e.writeIDATs()
e.writeIEND()
......
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