Use default palettes for indexed BMPs

src/pixie/fileformats/bmp.nim

@@ -77,8 +77,8 @@ proc decodeDib*(
   if colorPaletteSize < 0 or colorPaletteSize > 256:
     failInvalid()
 
-  if bits == 8 and colorPaletteSize == 0:
-    colorPaletteSize = 256
+  if bits in [1, 4, 8] and colorPaletteSize == 0:
+    colorPaletteSize = 1 shl bits
 
   var colorPalette = newSeq[ColorRGBA](colorPaletteSize)
   if colorPaletteSize > 0:

tests/test_bmp.nim

@@ -1,5 +1,46 @@
 import os, pixie, pixie/fileformats/bmp
 
+proc addLe16(data: var string, value: int) =
+  data.add(char(value and 0xff))
+  data.add(char((value shr 8) and 0xff))
+
+proc addLe32(data: var string, value: int) =
+  data.add(char(value and 0xff))
+  data.add(char((value shr 8) and 0xff))
+  data.add(char((value shr 16) and 0xff))
+  data.add(char((value shr 24) and 0xff))
+
+proc makeIndexedBmp(bits: int, pixelData: string): string =
+  let
+    paletteEntries = 1 shl bits
+    pixelDataOffset = 14 + 40 + paletteEntries * 4
+    fileSize = pixelDataOffset + pixelData.len
+
+  result.add("BM")
+  result.addLe32(fileSize)
+  result.addLe16(0)
+  result.addLe16(0)
+  result.addLe32(pixelDataOffset)
+  result.addLe32(40) # BITMAPINFOHEADER
+  result.addLe32(1) # Width
+  result.addLe32(1) # Height
+  result.addLe16(1) # Planes
+  result.addLe16(bits)
+  result.addLe32(0) # BI_RGB
+  result.addLe32(pixelData.len)
+  result.addLe32(0)
+  result.addLe32(0)
+  result.addLe32(0) # clrUsed = 0 means use the default palette size.
+  result.addLe32(0)
+
+  for i in 0 ..< paletteEntries:
+    if i == paletteEntries - 1:
+      result.add("\xff\xff\xff\x00")
+    else:
+      result.add("\x00\x00\x00\x00")
+
+  result.add(pixelData)
+
 # block:
 #   var image = newImage(4, 2)
 
@@ -37,6 +78,13 @@ block:
       image = decodeBmp(readFile(path))
     writeFile("tests/fileformats/bmp/knight." & $bits & ".bmp", encodeBmp(image))
 
+block:
+  for fixture in [(1, "\x80\x00\x00\x00"), (4, "\xf0\x00\x00\x00")]:
+    let image = decodeBmp(makeIndexedBmp(fixture[0], fixture[1]))
+    doAssert image.width == 1
+    doAssert image.height == 1
+    doAssert image.data[0] == rgbx(255, 255, 255, 255)
+
 block:
   let image = decodeBmp(readFile(
     "tests/fileformats/bmp/rgb.24.master.bmp"