From 823df34f35344e7affec516ec6adbf7873c0716a Mon Sep 17 00:00:00 2001
From: Alexander Mankuta <cheba@pointlessone.org>
Date: Sat, 21 Nov 2015 23:09:57 +0200
Subject: [PATCH 1/2] Added support for transparency in indexed PNG images

---
 data/images/indexed_transparency.png       | Bin 0 -> 494 bytes
 data/images/indexed_transparency_alpha.dat | Bin 0 -> 253 bytes
 data/images/indexed_transparency_color.dat | Bin 0 -> 206 bytes
 data/images/pal_bk.png                     | Bin 1152 -> 0 bytes
 lib/prawn/images/png.rb                    |  45 ++++++++++++++++++---
 spec/png_spec.rb                           |  30 ++++++++++++--
 6 files changed, 66 insertions(+), 9 deletions(-)
 create mode 100644 data/images/indexed_transparency.png
 create mode 100644 data/images/indexed_transparency_alpha.dat
 create mode 100644 data/images/indexed_transparency_color.dat
 delete mode 100644 data/images/pal_bk.png

diff --git a/data/images/indexed_transparency.png b/data/images/indexed_transparency.png
new file mode 100644
index 0000000000000000000000000000000000000000..12656481ac23279da81f0c3e432de5d228372c24
GIT binary patch
literal 494
zcmeAS@N?(olHy`uVBq!ia0vp^CqS5k8A#4*i(3PvSkfJR9T^xl_H+M9WCijSl0AZa
z85pY67#JE_7#My5g&JNkFq9fFFuY1&V6d9Oz#v{QXIG#NP=Y_eC&cwX0|Ns6|9_(J
z-*=!Cb4ie2@cw#xt*xdYE@P6ny9-Ny#?3$=hqJ&VvY3H^TNs2H8D`Cq01BS=ba4!+
zxb^ltqhNyr!{H6=f2!Ts_D<BuIB)!@MmFJy$DD~$%BF5vJy#@F8U`gVIp)!eS75U-
z6VOx$_+Qd%K!g@FLvKp%bSKU(6pd$k_MBvSm8i4BUKwb+YKdz^NlIc#s#S7PDv)9@
zGB7mJH8jvQunaLYurfBaGBVXRFtRc*I3^NmilQMmKP5A*61Rr!o^Lxr{_}M8b6Mw<
G&;$VA1fH(|

literal 0
HcmV?d00001

diff --git a/data/images/indexed_transparency_alpha.dat b/data/images/indexed_transparency_alpha.dat
new file mode 100644
index 0000000000000000000000000000000000000000..06a06e6eb60eedf78906c37b7adbd97c49bc71f2
GIT binary patch
literal 253
zcmb=J^Y%O=Z;J!NkpsnTTc)>}7<}ZM?Cb7WJx}oY>xTMFm&+Z?1bvlfxta84NL@5p
kL|DM`dk67~;l|CKc%M9N*)F6w4r1TgGbgzBL<klD02+8~JOBUy

literal 0
HcmV?d00001

diff --git a/data/images/indexed_transparency_color.dat b/data/images/indexed_transparency_color.dat
new file mode 100644
index 0000000000000000000000000000000000000000..2d2343ec1db53a88862e1875eb1193f720e94cf0
GIT binary patch
literal 206
zcmb=J^Y%O=Z-WERfddDBfA8TGnpL=b!Q9P9YGgl%DVw@w^<0ryX&98e<d{eAL@9y-
nvltnmprL-(Gy|gb!A-ttT%?~!q-AhThC!Rto-m$dd6ftNKKNDJ

literal 0
HcmV?d00001

diff --git a/data/images/pal_bk.png b/data/images/pal_bk.png
deleted file mode 100644
index 1923b0381d1445e658201cf8dbed8cf96cddc103..0000000000000000000000000000000000000000
GIT binary patch
literal 0
HcmV?d00001

literal 1152
zcmeAS@N?(olHy`uVBq!ia0vp^p+IcM!3-pGC6<3=U|{kO@Ck7Ra)&f<XlNMH=>HH0
zz{CR#f1oEC7#RLD{9iir`W>Lqfs!D<U>ge)104+&1sMqu0Uiz(1_u`ppMa2vn1qy!
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zF)*>Pp|}3va|b5_{lb{!?e20!gh%=zkS*-#;uumfC)vV>`#`}D2ALbTZ{CWCj*OZy
zd*&<&X-TOUuV22($j;1Kv3lhy4Q)-W6Q@s}^6>Wb>gew5;^5}wGB7YQ+`M7a#^Qpa
z!p|Q*eLQ?{#mdFY`es>Wy#g^Vow?Fn5D*_89UL1P85kE96%-Q^F=77n*^}o^ojGyd
zv{{qpOqn4dFD@-CD=H}{CnhB%BO>wO{qxt4-#&f$@ZGalkKR0a!N87WV&5z;<6!=S
z2mjSpe}8jtYw_`KxzXEm?rkYN{Hrwk`kJ_{MMpoSdQaDh+fs1wPi67*GxJP~kA2Au
zUhXr`q|m88KHtAQJl(xII@`NAIGI_d2j)_cleNzJEM;Y2=z4$p@F~{$>A<jJ@O1Ta
JS?83{1ON&x7{dSn

diff --git a/lib/prawn/images/png.rb b/lib/prawn/images/png.rb
index 67dcdc4c0..33c679d3f 100644
--- a/lib/prawn/images/png.rb
+++ b/lib/prawn/images/png.rb
@@ -68,9 +68,7 @@ def initialize(data)
             @transparency = {}
             case @color_type
             when 3
-              fail Errors::UnsupportedImageType,
-                   "Pallete-based transparency in PNG is not currently supported.\n" \
-                   "See https://github.com/prawnpdf/prawn/issues/783"
+              @transparency[:palette] = data.read(chunk_size).unpack('C*')
             when 0
               # Greyscale. Corresponding to entries in the PLTE chunk.
               # Grey is two bytes, range 0 .. (2 ^ bit-depth) - 1
@@ -109,11 +107,17 @@ def colors
       # where it's required.
       #
       def split_alpha_channel!
-        split_image_data if alpha_channel?
+        if alpha_channel?
+          if color_type == 3
+            generate_alpha_channel
+          else
+            split_image_data
+          end
+        end
       end
 
       def alpha_channel?
-        @color_type == 4 || @color_type == 6
+        [3, 4, 6].include? color_type
       end
 
       # Build a PDF object representing this image in +document+, and return
@@ -285,6 +289,37 @@ def split_image_data
 
         @img_data = color_data
       end
+
+      def generate_alpha_channel
+        alpha_palette = Hash.new(0xff)
+        0.upto(palette.bytesize / 3) do |n|
+          alpha_palette[n] = @transparency[:palette][n] || 0xff
+        end
+
+        scanline_length = width + 1
+        scanlines = @img_data.bytesize / scanline_length
+        pixels = width * height
+
+        data = StringIO.new(@img_data)
+        data.binmode
+
+        @alpha_channel = [0x00].pack('C') * (pixels + scanlines)
+        alpha = StringIO.new(@alpha_channel)
+        alpha.binmode
+
+        scanlines.times do |line|
+          data.seek(line * scanline_length)
+
+          filter = data.getbyte
+
+          alpha.putc filter
+
+          width.times do
+            color = data.read(1).unpack('C').first
+            alpha.putc alpha_palette[color]
+          end
+        end
+      end
     end
   end
 end
diff --git a/spec/png_spec.rb b/spec/png_spec.rb
index be870827d..2d236bc15 100644
--- a/spec/png_spec.rb
+++ b/spec/png_spec.rb
@@ -95,10 +95,32 @@
 end
 
 describe "When reading an indexed color PNG file with transparency (color type 3)" do
-  it "raises a not supported error" do
-    bin = File.binread("#{Prawn::DATADIR}/images/pal_bk.png")
-    expect { Prawn::Images::PNG.new(bin) }.to(
-      raise_error(Prawn::Errors::UnsupportedImageType))
+  let(:filename) { "#{Prawn::DATADIR}/images/indexed_transparency.png" }
+  let(:color_filename) { "#{Prawn::DATADIR}/images/indexed_transparency_color.dat" }
+  let(:transparency_filename) { "#{Prawn::DATADIR}/images/indexed_transparency_alpha.dat" }
+  let(:img_data) { File.binread(filename) }
+  let(:png) { Prawn::Images::PNG.new(img_data) }
+
+  it "reads the attributes from the header chunk correctly" do
+    expect(png.width).to eq(200)
+    expect(png.height).to eq(200)
+    expect(png.bits).to eq(8)
+    expect(png.color_type).to eq(3)
+    expect(png.compression_method).to eq(0)
+    expect(png.filter_method).to eq(0)
+    expect(png.interlace_method).to eq(0)
+  end
+
+  it "reads the image data correctly" do
+    data = Zlib::Inflate.inflate(File.binread(color_filename))
+    expect(png.img_data).to eq(data)
+  end
+
+  it "reads the image transparency correctly" do
+    png.split_alpha_channel!
+
+    data = Zlib::Inflate.inflate(File.binread(transparency_filename))
+    expect(png.alpha_channel).to eq(data)
   end
 end
 

From ba8cbc1a0ed996c3e7f988e42713560d0ebdaef1 Mon Sep 17 00:00:00 2001
From: Alexander Mankuta <cheba@pointlessone.org>
Date: Sun, 22 Nov 2015 13:25:41 +0200
Subject: [PATCH 2/2] Added Changelog entry for indexed PNG transparency

[skip ci]
---
 CHANGELOG.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cd8b2bc82..ef6c52cfa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,12 @@
 
 ## PrawnPDF master branch
 
+### Added support for PNG images with indexed transparency
+
+Prawn now properly hadles transparency in PNG images with indexed color.
+
+(Maciej Mucha, [#783](https://github.com/prawnpdf/prawn/issues/783); Alexander Mankuta, [#920](https://github.com/prawnpdf/prawn/pull/920))
+
 ### Prawn no longer generates IRB warnings
 
 Fix a few issues with code style that were triggering warnings in IRB when run in verbose mode (`irb -w`).