Started work on gif image loader.

src/pixie/fileformats/gif.nim

@@ -0,0 +1,159 @@
+import chroma, flatty/binny, pixie/common, pixie/images, math, bitty
+
+const gifSignatures* = @["GIF87a", "GIF89a"]
+
+# See: https://en.wikipedia.org/wiki/GIF
+
+proc decodeGIF*(data: string): Image =
+  ## Decodes GIF data into an Image.
+
+  let version = data[0 .. 5]
+  if version notin gifSignatures:
+    raise newException(PixieError, "Invalid GIF file signature.")
+
+  let
+    # Read information about the image.
+    width = data.readInt16(6).int
+    height = data.readInt16(8).int
+    flags = data.readUint8(10).int
+    hasColorTable = (flags and 0x80) != 0
+    originalDepth = ((flags and 0x70) shr 4) + 1
+    colorTableSorted = (flags and 0x8) != 0
+    colorTableSize = 2 ^ ((flags and 0x7) + 1)
+    bgColorIndex = data.readUint8(11)
+    pixelAspectRatio = data.readUint8(11)
+
+  result = newImage(width, height)
+
+  # Read the main color table.
+  var colors: seq[ColorRGBA]
+  var i = 0xD
+  if hasColorTable:
+    for c in 0 ..< colorTableSize:
+      let
+        r = data.readUint8(i + 0)
+        g = data.readUint8(i + 1)
+        b = data.readUint8(i + 2)
+      colors.add(rgba(r, g, b, 255))
+      i += 3
+
+  # Read the image blocks.
+  while true:
+    let blockType = data.readUint8(i)
+    i += 1
+    case blockType:
+    of 0x2c: # IMAGE
+      let
+        left = data.readUint16(i + 0)
+        top = data.readUint16(i + 2)
+        width = data.readUint16(i + 4)
+        height = data.readUint16(i + 6)
+        flags = data.readUint8(i + 8)
+
+        hasColorTable = (flags and 0x80) != 0
+        interlace = (flags and 0x40) != 0
+        colorTableSorted = (flags and 0x8) != 0
+        colorTableSize = 2 ^ ((flags and 0x7) + 1)
+
+      i += 9
+
+      # Make sure we support the GIF features.
+      if left != 0 and top != 0 and
+          width.int != result.width and height.int != result.height:
+        raise newException(PixieError, "Image block offsets not supported.")
+
+      if hasColorTable:
+        raise newException(PixieError, "Color table per block not supported.")
+
+      if interlace:
+        raise newException(PixieError, "Interlacing not supported.")
+
+      # Read the lzw data chunks.
+      let lzwMinBitSize = data.readUint8(i)
+      i += 1
+      var lzwData = ""
+      while true:
+        let lzwEncodedLen = data.readUint8(i)
+        i += 1
+        if lzwEncodedLen == 0:
+          # Stop reading when chunk len is 0.
+          break
+        lzwData.add data[i ..< i + lzwEncodedLen.int]
+        i += lzwEncodedLen.int
+
+      let
+        clearMark = 1 shl lzwMinBitSize
+        endMark = clearMark + 1
+
+      # Turn full lzw data into bit stream.
+      var
+        bs = newBitStream(lzwData)
+        bitSize = lzwMinBitSize + 1
+        currentCodeTableMax = (1 shl (bitSize)) - 1
+        codeLast: int = -1
+        codeTable: seq[seq[int]]
+        colorIndexes: seq[int]
+
+      # Main decode loop.
+      while codeLast != endMark:
+
+        var
+          # Read variable bits out of the table.
+          codeId = bs.read(bitSize.int)
+          # Some time we need to carry over table information.
+          carryOver: seq[int]
+
+        if codeId == clearMark:
+          # Clear and re-init the tables.
+          bitSize = lzwMinBitSize + 1
+          currentCodeTableMax = (1 shl (bitSize)) - 1
+          codeLast = -1
+          codeTable.setLen(0)
+          for x in 0 ..< endMark + 1:
+            codeTable.add(@[x])
+
+        elif codeId == endMark:
+          # Exit we are done.
+          break
+
+        elif codeId < codeTable.len and codeTable[codeId].len > 0:
+          # Its in the current table, use it.
+          let current = codeTable[codeId]
+          colorIndexes.add(current)
+          carryOver = @[current[0]]
+
+        elif codeLast != -1 and codeLast != clearMark and codeLast != endMark:
+          # Its in the current table use it.
+          var previous = codeTable[codeLast]
+          carryOver = @[previous[0]]
+          colorIndexes.add(previous & carryOver)
+
+        if codeTable.len == currentCodeTableMax and bitSize < 12:
+          # We need to expand the codeTable max and the bit size.
+          inc bitSize
+          currentCodeTableMax = (1 shl (bitSize)) - 1
+
+        if codeLast != -1 and codeLast != clearMark and codeLast != endMark:
+          # We had some left over and need to expand table.
+          codeTable.add(codeTable[codeLast] & carryOver)
+
+        codeLast = codeId
+
+      # Convert color indexes into real colors.
+      for j, idx in colorIndexes:
+        result.data[j] = colors[idx]
+
+    of 0x21:
+      # Skip over all extensions (mostly animation information)
+      let extentionType = data.readUint8(i)
+      inc i
+      let byteLen = data.readUint8(i)
+      inc i
+      i += byteLen.int
+      doAssert data.readUint8(i) == 0
+      inc i
+    of 0x3b:
+      # Exit block byte - we are done.
+      return
+    else:
+      raise newException(PixieError, "Invalid GIF block type.")

tests/test_gif.nim

@@ -0,0 +1,41 @@
+import pixie/fileformats/gif, pixie/fileformats/png, print, parseutils, flatty/hexprint, flatty/binlisting
+
+# let binary = decodeBinListing """
+#   0:     47 49 46 38 39 61
+#   6:     03 00
+#   8:     05 00
+#   A:     F7
+#   B:     00
+#   C:     00
+#   D:     00 00 00
+#   10:    80 00 00
+#   85:    00 00 00
+#   30A:   FF FF FF
+#   30D:   21 F9
+#   30F:   04
+#   310:   01
+#   311:   00 00
+#   313:   10 16
+#   314:   00
+#   315:   2C
+#   316:   00 00 00 00
+#   31A:   03 00 05 00
+#   31E:   00
+#   31F:   08
+#   320:   0B
+#   321:   00 51 FC 1B 28 70 A0 C1 83 01 01
+#   32C:   00
+#   32D:   3B
+# """
+
+# echo hexPrint(binary)
+# writeFile("tests/images/gif/3x5.gif", binary)
+
+var img = decodeGIF(readFile("tests/images/gif/3x5.gif"))
+writeFile("tests/images/gif/3x5.png", img.encodePng())
+
+var img2 = decodeGIF(readFile("tests/images/gif/audrey.gif"))
+writeFile("tests/images/gif/audrey.png", img2.encodePng())
+
+var img3 = decodeGIF(readFile("tests/images/gif/sunflower.gif"))
+writeFile("tests/images/gif/sunflower.png", img3.encodePng())