我有大量来自从事一个项目的艺术家提供的图像。这些图像有不同的规格，但都是PNG。有些是 8 位索引（调色板），有些是带有 alpha 32 位和 64 位 PNG 的真彩色。
现在，我正在尝试使用 imagemagick 将所有这些图像推送到单一的连贯格式，但我面临着一个痛苦的问题。我注意到从带有 alpha（16 位通道深度）的真彩色转换而来的图像非常有用。但是，每通道 8 位图像，或者可能更低/更高的图像会有些损坏，并且我们拥有的渲染引擎（在 iOS 上）将无法正确显示图像。
有没有办法简单地将所有图像可靠地转换为具有 alpha PNG 格式（RGBA8888）的每通道 8 位真彩色？
（注意：我在 wikipedia 链接中读到真彩色和 alpha 每个通道至少 32 位，我需要通过修剪来减少深度以减小文件大小。渲染引擎也会处理这个问题。）
我尝试了以下命令：
/opt/local/bin/convert -depth 8 {} {}
/opt/local/bin/convert {} -depth 8 {}
/opt/local/bin/convert {} -colorspace sRGB -depth 8 {}
/opt/local/bin/convert {} -type truecolormatte -depth 8 {}
这是运行最后一个命令的两个图像，第二个不在引擎上运行：
Image: bu_hu_townhall_l01.png
Format: PNG (Portable Network Graphics)
Class: DirectClass
Geometry: 640x558+0+0
Resolution: 28.35x28.35
Print size: 22.575x19.6825
Units: PixelsPerCentimeter
Type: TrueColorAlpha
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
alpha: 8-bit
Channel statistics:
Red:
min: 0 (0)
max: 255 (1)
mean: 207.468 (0.8136)
standard deviation: 60.7017 (0.238046)
kurtosis: 1.18939
skewness: -1.39109
Green:
min: 0 (0)
max: 255 (1)
mean: 205.419 (0.805565)
standard deviation: 56.0321 (0.219734)
kurtosis: 1.15995
skewness: -1.22368
Blue:
min: 0 (0)
max: 255 (1)
mean: 174.337 (0.683673)
standard deviation: 79.2051 (0.310608)
kurtosis: -0.8628
skewness: -0.578582
Alpha:
min: 0 (0)
max: 255 (1)
mean: 96.4893 (0.37839)
standard deviation: 123.454 (0.484134)
kurtosis: -1.74557
skewness: -0.501528
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 186.434 (0.731112)
standard deviation: 84.1704 (0.33008)
kurtosis: 0.210139
skewness: -1.19818
Alpha: srgba(255,255,255,0) #FFFFFF00
Rendering intent: Perceptual
Gamma: 0.45455
Chromaticity:
red primary: (0.63999,0.33001)
green primary: (0.3,0.6)
blue primary: (0.15,0.05999)
white point: (0.31269,0.32899)
Background color: white
Border color: srgba(223,223,223,1)
Matte color: grey74
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 640x558+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2013-07-19T12:48:10-07:00
date:modify: 2013-07-19T12:48:10-07:00
png:bKGD: chunk was found (see Background color, above)
png:cHRM: chunk was found (see Chromaticity, above)
png:gAMA: gamma=0.45454544 (See Gamma, above)
png:IHDR.bit-depth-orig: 8
png:IHDR.bit_depth: 8
png:IHDR.color-type-orig: 6
png:IHDR.color_type: 6 (RGBA)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 640, 558
png:pHYs: x_res=2835, y_res=2835, units=1
png:sRGB: intent=0 (Perceptual Intent)
png:text: 2 tEXt/zTXt/iTXt chunks were found
signature: 547acae3cbfddef87d1ec49fb4dbe259772343ed121a8f9bfe2f1846fc542cf0
Artifacts:
filename: bu_hu_townhall_l01.png
verbose: true
Tainted: False
Filesize: 333KB
Number pixels: 357K
Pixels per second: 17.86MB
User time: 0.010u
Elapsed time: 0:01.019
Version: ImageMagick 6.8.6-0 2013-07-03 Q16 http://www.imagemagick.org
和...
Image: red_circle.png
Format: PNG (Portable Network Graphics)
Class: DirectClass
Geometry: 256x256+0+0
Resolution: 28.35x28.35
Print size: 9.02998x9.02998
Units: PixelsPerCentimeter
Type: PaletteAlpha
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 1-bit
green: 1-bit
blue: 1-bit
alpha: 8-bit
Channel statistics:
Red:
min: 255 (1)
max: 255 (1)
mean: 255 (1)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
Green:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
Blue:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
Alpha:
min: 0 (0)
max: 40 (0.156863)
mean: 6.30113 (0.0247103)
standard deviation: 10.7071 (0.0419887)
kurtosis: 1.47694
skewness: -1.66221
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 125.925 (0.493822)
standard deviation: 5.35356 (0.0209944)
kurtosis: 309547
skewness: 77.6945
Alpha: srgba(255,0,0,0) #FF000000
Colors: 41
Histogram:
37164: (255, 0, 0, 0) #FF000000 srgba(255,0,0,0)
4416: (255, 0, 0, 1) #FF000001 srgba(255,0,0,0.00392157)
1974: (255, 0, 0, 2) #FF000002 srgba(255,0,0,0.00784314)
1394: (255, 0, 0, 4) #FF000004 srgba(255,0,0,0.0156863)
1154: (255, 0, 0, 5) #FF000005 srgba(255,0,0,0.0196078)
982: (255, 0, 0, 8) #FF000008 srgba(255,0,0,0.0313725)
942: (255, 0, 0, 7) #FF000007 srgba(255,0,0,0.027451)
893: (255, 0, 0, 10) #FF00000A srgba(255,0,0,0.0392157)
758: (255, 0, 0, 3) #FF000003 srgba(255,0,0,0.0117647)
754: (255, 0, 0, 11) #FF00000B srgba(255,0,0,0.0431373)
752: (255, 0, 0, 14) #FF00000E srgba(255,0,0,0.054902)
749: (255, 0, 0, 13) #FF00000D srgba(255,0,0,0.0509804)
698: (255, 0, 0, 16) #FF000010 srgba(255,0,0,0.0627451)
684: (255, 0, 0, 17) #FF000011 srgba(255,0,0,0.0666667)
657: (255, 0, 0, 19) #FF000013 srgba(255,0,0,0.0745098)
636: (255, 0, 0, 20) #FF000014 srgba(255,0,0,0.0784314)
635: (255, 0, 0, 38) #FF000026 srgba(255,0,0,0.14902)
630: (255, 0, 0, 32) #FF000020 srgba(255,0,0,0.12549)
604: (255, 0, 0, 23) #FF000017 srgba(255,0,0,0.0901961)
598: (255, 0, 0, 26) #FF00001A srgba(255,0,0,0.101961)
589: (255, 0, 0, 28) #FF00001C srgba(255,0,0,0.109804)
574: (255, 0, 0, 34) #FF000022 srgba(255,0,0,0.133333)
574: (255, 0, 0, 31) #FF00001F srgba(255,0,0,0.121569)
568: (255, 0, 0, 22) #FF000016 srgba(255,0,0,0.0862745)
551: (255, 0, 0, 29) #FF00001D srgba(255,0,0,0.113725)
542: (255, 0, 0, 6) #FF000006 srgba(255,0,0,0.0235294)
541: (255, 0, 0, 25) #FF000019 srgba(255,0,0,0.0980392)
516: (255, 0, 0, 37) #FF000025 srgba(255,0,0,0.145098)
485: (255, 0, 0, 35) #FF000023 srgba(255,0,0,0.137255)
387: (255, 0, 0, 9) #FF000009 srgba(255,0,0,0.0352941)
381: (255, 0, 0, 12) #FF00000C srgba(255,0,0,0.0470588)
327: (255, 0, 0, 36) #FF000024 srgba(255,0,0,0.141176)
327: (255, 0, 0, 24) #FF000018 srgba(255,0,0,0.0941176)
326: (255, 0, 0, 15) #FF00000F srgba(255,0,0,0.0588235)
316: (255, 0, 0, 21) #FF000015 srgba(255,0,0,0.0823529)
305: (255, 0, 0, 18) #FF000012 srgba(255,0,0,0.0705882)
304: (255, 0, 0, 27) #FF00001B srgba(255,0,0,0.105882)
270: (255, 0, 0, 33) #FF000021 srgba(255,0,0,0.129412)
254: (255, 0, 0, 30) #FF00001E srgba(255,0,0,0.117647)
251: (255, 0, 0, 39) #FF000027 srgba(255,0,0,0.152941)
74: (255, 0, 0, 40) #FF000028 srgba(255,0,0,0.156863)
Rendering intent: Perceptual
Gamma: 0.454545
Chromaticity:
red primary: (0.64733,0.33636)
green primary: (0.28904,0.6052)
blue primary: (0.1235,0.0424)
white point: (0.28021,0.296)
Background color: white
Border color: srgba(223,223,223,1)
Matte color: grey74
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 256x256+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2013-07-19T12:48:29-07:00
date:modify: 2013-07-19T12:48:29-07:00
png:bKGD: chunk was found (see Background color, above)
png:cHRM: chunk was found (see Chromaticity, above)
png:iCCP: chunk was found
png:IHDR.bit-depth-orig: 8
png:IHDR.bit_depth: 8
png:IHDR.color-type-orig: 3
png:IHDR.color_type: 3 (Indexed)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 256, 256
png:pHYs: x_res=2835, y_res=2835, units=1
png:PLTE.number_colors: 42
png:text: 2 tEXt/zTXt/iTXt chunks were found
png:tRNS: chunk was found
signature: f41a73e5fe161c06aa147572380ab95b6e43a411fa533b6533af37e2d19a161e
Profiles:
Profile-icc: 3224 bytes
Description: Display
Manufacturer: Display
Model: Display
Copyright: Copyright Apple, Inc., 2013
Artifacts:
filename: red_circle.png
verbose: true
Tainted: False
Filesize: 8.44KB
Number pixels: 65.5K
Pixels per second: 0B
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.8.6-0 2013-07-03 Q16 http://www.imagemagick.org
这是它被破坏之前的样子：
Image: red_circle.png
Format: PNG (Portable Network Graphics)
Class: DirectClass
Geometry: 256x256+0+0
Resolution: 28.35x28.35
Print size: 9.02998x9.02998
Units: PixelsPerCentimeter
Type: PaletteAlpha
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 1-bit
green: 1-bit
blue: 1-bit
alpha: 8-bit
Channel statistics:
Red:
min: 255 (1)
max: 255 (1)
mean: 255 (1)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
Green:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
Blue:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
Alpha:
min: 0 (0)
max: 40 (0.156863)
mean: 6.30113 (0.0247103)
standard deviation: 10.7071 (0.0419887)
kurtosis: 1.47694
skewness: -1.66221
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 125.925 (0.493822)
standard deviation: 5.35356 (0.0209944)
kurtosis: 309547
skewness: 77.6945
Alpha: srgba(255,0,0,0) #FF000000
Colors: 41
Histogram:
37164: (255, 0, 0, 0) #FF000000 srgba(255,0,0,0)
4416: (255, 0, 0, 1) #FF000001 srgba(255,0,0,0.00392157)
1974: (255, 0, 0, 2) #FF000002 srgba(255,0,0,0.00784314)
1394: (255, 0, 0, 4) #FF000004 srgba(255,0,0,0.0156863)
1154: (255, 0, 0, 5) #FF000005 srgba(255,0,0,0.0196078)
982: (255, 0, 0, 8) #FF000008 srgba(255,0,0,0.0313725)
942: (255, 0, 0, 7) #FF000007 srgba(255,0,0,0.027451)
893: (255, 0, 0, 10) #FF00000A srgba(255,0,0,0.0392157)
758: (255, 0, 0, 3) #FF000003 srgba(255,0,0,0.0117647)
754: (255, 0, 0, 11) #FF00000B srgba(255,0,0,0.0431373)
752: (255, 0, 0, 14) #FF00000E srgba(255,0,0,0.054902)
749: (255, 0, 0, 13) #FF00000D srgba(255,0,0,0.0509804)
698: (255, 0, 0, 16) #FF000010 srgba(255,0,0,0.0627451)
684: (255, 0, 0, 17) #FF000011 srgba(255,0,0,0.0666667)
657: (255, 0, 0, 19) #FF000013 srgba(255,0,0,0.0745098)
636: (255, 0, 0, 20) #FF000014 srgba(255,0,0,0.0784314)
635: (255, 0, 0, 38) #FF000026 srgba(255,0,0,0.14902)
630: (255, 0, 0, 32) #FF000020 srgba(255,0,0,0.12549)
604: (255, 0, 0, 23) #FF000017 srgba(255,0,0,0.0901961)
598: (255, 0, 0, 26) #FF00001A srgba(255,0,0,0.101961)
589: (255, 0, 0, 28) #FF00001C srgba(255,0,0,0.109804)
574: (255, 0, 0, 34) #FF000022 srgba(255,0,0,0.133333)
574: (255, 0, 0, 31) #FF00001F srgba(255,0,0,0.121569)
568: (255, 0, 0, 22) #FF000016 srgba(255,0,0,0.0862745)
551: (255, 0, 0, 29) #FF00001D srgba(255,0,0,0.113725)
542: (255, 0, 0, 6) #FF000006 srgba(255,0,0,0.0235294)
541: (255, 0, 0, 25) #FF000019 srgba(255,0,0,0.0980392)
516: (255, 0, 0, 37) #FF000025 srgba(255,0,0,0.145098)
485: (255, 0, 0, 35) #FF000023 srgba(255,0,0,0.137255)
387: (255, 0, 0, 9) #FF000009 srgba(255,0,0,0.0352941)
381: (255, 0, 0, 12) #FF00000C srgba(255,0,0,0.0470588)
327: (255, 0, 0, 36) #FF000024 srgba(255,0,0,0.141176)
327: (255, 0, 0, 24) #FF000018 srgba(255,0,0,0.0941176)
326: (255, 0, 0, 15) #FF00000F srgba(255,0,0,0.0588235)
316: (255, 0, 0, 21) #FF000015 srgba(255,0,0,0.0823529)
305: (255, 0, 0, 18) #FF000012 srgba(255,0,0,0.0705882)
304: (255, 0, 0, 27) #FF00001B srgba(255,0,0,0.105882)
270: (255, 0, 0, 33) #FF000021 srgba(255,0,0,0.129412)
254: (255, 0, 0, 30) #FF00001E srgba(255,0,0,0.117647)
251: (255, 0, 0, 39) #FF000027 srgba(255,0,0,0.152941)
74: (255, 0, 0, 40) #FF000028 srgba(255,0,0,0.156863)
Rendering intent: Perceptual
Gamma: 0.454545
Chromaticity:
red primary: (0.64733,0.33636)
green primary: (0.28904,0.6052)
blue primary: (0.1235,0.0424)
white point: (0.28021,0.296)
Background color: white
Border color: srgba(223,223,223,1)
Matte color: grey74
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 256x256+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2013-07-18T20:55:49-07:00
date:modify: 2013-07-18T20:55:49-07:00
png:cHRM: chunk was found (see Chromaticity, above)
png:iCCP: chunk was found
png:IHDR.bit-depth-orig: 8
png:IHDR.bit_depth: 8
png:IHDR.color-type-orig: 6
png:IHDR.color_type: 6 (RGBA)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 256, 256
png:pHYs: x_res=2835, y_res=2835, units=1
signature: f41a73e5fe161c06aa147572380ab95b6e43a411fa533b6533af37e2d19a161e
Profiles:
Profile-icc: 3224 bytes
Description: Display
Manufacturer: Display
Model: Display
Copyright: Copyright Apple, Inc., 2013
Artifacts:
filename: red_circle.png
verbose: true
Tainted: False
Filesize: 11.2KB
Number pixels: 65.5K
Pixels per second: 0B
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.8.6-0 2013-07-03 Q16 http://www.imagemagick.org