diff --git a/src/pixie.nim b/src/pixie.nim
index f4a1004..a3e4fe3 100644
--- a/src/pixie.nim
+++ b/src/pixie.nim
@@ -18,24 +18,36 @@ converter autoPremultipliedAlpha*(c: ColorRGBA): ColorRGBX {.inline, raises: [].
   ## Convert a straight alpha RGBA to a premultiplied alpha RGBA.
   c.rgbx()
 
+proc decodeImageDimensions*(
+  data: pointer, len: int
+): ImageDimensions {.raises: [PixieError].} =
+  ## Decodes an image's dimensions from memory.
+  if len > 8 and equalMem(data, pngSignature[0].unsafeAddr, 8):
+    decodePngDimensions(data, len)
+  elif len > 2 and equalMem(data, jpegStartOfImage[0].unsafeAddr, 2):
+    decodeJpegDimensions(data, len)
+  elif len > 2 and equalMem(data, bmpSignature.cstring, 2):
+    decodeBmpDimensions(data, len)
+  elif len > 6 and (
+    equalMem(data, gifSignatures[0].cstring, 6) or
+    equalMem(data, gifSignatures[1].cstring, 6)
+  ):
+    decodeGifDimensions(data, len)
+  elif len > (14 + 8) and equalMem(data, qoiSignature.cstring, 4):
+    decodeQoiDimensions(data, len)
+  elif len > 9 and (
+    equalMem(data, ppmSignatures[0].cstring, 2) or
+    equalMem(data, ppmSignatures[1].cstring, 2)
+  ):
+    decodePpmDimensions(data, len)
+  else:
+    raise newException(PixieError, "Unsupported image file format")
+
 proc decodeImageDimensions*(
   data: string
 ): ImageDimensions {.raises: [PixieError].} =
   ## Decodes an image's dimensions from memory.
-  if data.len > 8 and data.readUint64(0) == cast[uint64](pngSignature):
-    decodePngDimensions(data)
-  elif data.len > 2 and data.readUint16(0) == cast[uint16](jpegStartOfImage):
-    decodeJpegDimensions(data)
-  elif data.len > 2 and data.readStr(0, 2) == bmpSignature:
-    decodeBmpDimensions(data)
-  elif data.len > 6 and data.readStr(0, 6) in gifSignatures:
-    decodeGifDimensions(data)
-  elif data.len > (14+8) and data.readStr(0, 4) == qoiSignature:
-    decodeQoiDimensions(data)
-  elif data.len > 9 and data.readStr(0, 2) in ppmSignatures:
-    decodePpmDimensions(data)
-  else:
-    raise newException(PixieError, "Unsupported image file format")
+  decodeImageDimensions(data.cstring, data.len)
 
 proc decodeImage*(data: string): Image {.raises: [PixieError].} =
   ## Loads an image from memory.
diff --git a/src/pixie/fileformats/jpeg.nim b/src/pixie/fileformats/jpeg.nim
index 60e7380..50c3301 100644
--- a/src/pixie/fileformats/jpeg.nim
+++ b/src/pixie/fileformats/jpeg.nim
@@ -24,7 +24,6 @@ import chroma, flatty/binny, ../common, ../images, ../internal,
 
 const
   fastBits = 9
-  jpegStartOfImage* = [0xFF.uint8, 0xD8]
   deZigZag = [
     uint8 00, 01, 08, 16, 09, 02, 03, 10,
     uint8 17, 24, 32, 25, 18, 11, 04, 05,
@@ -40,6 +39,9 @@ const
     1023, 2047, 4095, 8191, 16383, 32767, 65535
   ]
 
+let
+  jpegStartOfImage* = [0xFF.uint8, 0xD8]
+
 type
   Huffman = object
     codes: array[256, uint16]
@@ -1155,13 +1157,13 @@ proc decodeJpeg*(data: string): Image {.raises: [PixieError].} =
   state.buildImage()
 
 proc decodeJpegDimensions*(
-  data: string
+  data: pointer, len: int
 ): ImageDimensions {.raises: [PixieError].} =
   ## Decodes the JPEG dimensions.
 
   var state = DecoderState()
-  state.buffer = cast[ptr UncheckedArray[uint8]](data.cstring)
-  state.len = data.len
+  state.buffer = cast[ptr UncheckedArray[uint8]](data)
+  state.len = len
 
   while true:
     if state.readUint8() != 0xFF:
@@ -1209,5 +1211,11 @@ proc decodeJpegDimensions*(
     else:
       failInvalid("invalid orientation")
 
+proc decodeJpegDimensions*(
+  data: string
+): ImageDimensions {.raises: [PixieError].} =
+  ## Decodes the JPEG dimensions.
+  decodeJpegDimensions(data.cstring, data.len)
+
 when defined(release):
   {.pop.}
diff --git a/src/pixie/fileformats/png.nim b/src/pixie/fileformats/png.nim
index 228351b..a8ff15e 100644
--- a/src/pixie/fileformats/png.nim
+++ b/src/pixie/fileformats/png.nim
@@ -3,7 +3,7 @@ import chroma, flatty/binny, math, ../common, ../images, ../internal,
 
 # See http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
 
-const
+let
   pngSignature* = [137.uint8, 80, 78, 71, 13, 10, 26, 10]
 
 type