diff --git a/init.lua b/init.lua
index dbf1d1d..18699f4 100644
--- a/init.lua
+++ b/init.lua
@@ -225,7 +225,11 @@ function image:encode_data(properties)
 			end
 		else
 			if "RAW" == compression then
-				self:encode_data_R8G8B8_as_A1R5G5B5_raw()
+				if 24 == self.pixel_depth then
+					self:encode_data_R8G8B8_as_A1R5G5B5_raw()
+				elseif 32 == self.pixel_depth then
+					self:encode_data_R8G8B8A8_as_A1R5G5B5_raw()
+				end
 			elseif "RLE" == compression then
 				self:encode_data_R8G8B8_as_A1R5G5B5_rle()
 			end
@@ -320,6 +324,27 @@ function image:encode_data_R8G8B8_as_A1R5G5B5_raw()
 	self.data = self.data .. table.concat(raw_pixels)
 end
 
+function image:encode_data_R8G8B8A8_as_A1R5G5B5_raw()
+	assert(32 == self.pixel_depth)
+	local raw_pixels = {}
+	-- Sample depth rescaling is done according to the algorithm presented in:
+	-- <https://www.w3.org/TR/2003/REC-PNG-20031110/#13Sample-depth-rescaling>
+	local max_sample_in = math.pow(2, 8) - 1
+	local max_sample_out = math.pow(2, 5) - 1
+	for _, row in ipairs(self.pixels) do
+		for _, pixel in ipairs(row) do
+			local colorword = 0 +
+				((math.floor((pixel[4] / max_sample_in) + 0.5)) * 32768) +  -- transparent if alpha < 128
+				((math.floor((pixel[1] * max_sample_out / max_sample_in) + 0.5)) * 1024) +
+				((math.floor((pixel[2] * max_sample_out / max_sample_in) + 0.5)) * 32) +
+				((math.floor((pixel[3] * max_sample_out / max_sample_in) + 0.5)) * 1)
+			local raw_pixel = string.char(colorword % 256, math.floor(colorword / 256))
+			raw_pixels[#raw_pixels + 1] = raw_pixel
+		end
+	end
+	self.data = self.data .. table.concat(raw_pixels)
+end
+
 function image:encode_data_R8G8B8_as_A1R5G5B5_rle()
 	assert(24 == self.pixel_depth)
 	local colorword = nil