From 25b7b30945241cb22529bb60fdb44248a64c19a6 Mon Sep 17 00:00:00 2001
From: Nils Dagsson Moskopp <nils@dieweltistgarnichtso.net>
Date: Sat, 14 May 2022 22:41:52 +0200
Subject: [PATCH] Allow encoding grayscale images as BW8

---
 examples.lua | 16 +++++++++++++--
 init.lua     | 58 +++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 58 insertions(+), 16 deletions(-)

diff --git a/examples.lua b/examples.lua
index 5e16722f3..e21e4db95 100644
--- a/examples.lua
+++ b/examples.lua
@@ -29,6 +29,18 @@ for x = 1,56,1 do
 end
 tga_encoder.image(pixels):save("bitmap_large.tga")
 
+-- note that the uncompressed grayscale TGA file written in this
+-- example is 80 bytes – but an optimized PNG file is 81 bytes …
+local pixels = {}
+for x = 1,6,1 do -- left to right
+	for z = 1,6,1 do -- bottom to top
+		local color = { math.min(x * z * 4 - 1, 255) }
+		pixels[z] = pixels[z] or {}
+		pixels[z][x] = color
+	end
+end
+tga_encoder.image(pixels, {colors="BW", pixel_depth=8}):save("gradients_8bpp.tga")
+
 local pixels = {}
 for x = 1,16,1 do -- left to right
 	for z = 1,16,1 do -- bottom to top
@@ -46,5 +58,5 @@ for x = 1,16,1 do -- left to right
 		pixels[z][x] = color
 	end
 end
-tga_encoder.image(pixels, {pixel_depth=16}):save("gradients_16bpp.tga")
-tga_encoder.image(pixels, {pixel_depth=24}):save("gradients_24bpp.tga")
+tga_encoder.image(pixels, {colors="RGB", pixel_depth=16}):save("gradients_16bpp.tga")
+tga_encoder.image(pixels, {colors="RGB", pixel_depth=24}):save("gradients_24bpp.tga")
diff --git a/init.lua b/init.lua
index d8a044244..aad7f2cac 100644
--- a/init.lua
+++ b/init.lua
@@ -10,14 +10,15 @@ local image = setmetatable({}, {
 
 function image:constructor(pixels, properties)
 	local properties = properties or {}
-	local pixel_depth = properties.pixel_depth or 16
+	properties.colors = properties.colors or "RGB"
+	properties.pixel_depth = properties.pixel_depth or 16
 
 	self.data = ""
 	self.pixels = pixels
 	self.width = #pixels[1]
 	self.height = #pixels
 
-	self:encode(pixel_depth)
+	self:encode(properties)
 end
 
 function image:encode_colormap_spec()
@@ -27,10 +28,13 @@ function image:encode_colormap_spec()
 		.. string.char(0) -- bits per pixel
 end
 
-function image:encode_image_spec(pixel_depth)
+function image:encode_image_spec(properties)
+	local colors = properties.colors
+	local pixel_depth = properties.pixel_depth
 	assert(
-		16 == pixel_depth or -- (A1R5G5B5 = 2 bytes = 16 bits)
-		24 == pixel_depth -- (B8G8R8 = 3 bytes = 24 bits)
+		"BW" == colors and 8 == pixel_depth or -- (8 bit grayscale = 1 byte = 8 bits)
+		"RGB" == colors and 16 == pixel_depth or -- (A1R5G5B5 = 2 bytes = 16 bits)
+		"RGB" == colors and 24 == pixel_depth -- (B8G8R8 = 3 bytes = 24 bits)
 	)
 	self.data = self.data
 		.. string.char(0, 0) -- X-origin
@@ -41,23 +45,49 @@ function image:encode_image_spec(pixel_depth)
 		.. string.char(0) -- image descriptor
 end
 
-function image:encode_header(pixel_depth)
+function image:encode_header(properties)
+	local colors = properties.colors
+	local pixel_depth = properties.pixel_depth
+	local image_type
+	if "BW" == colors and 8 == pixel_depth then
+		image_type = 3 -- grayscale
+	elseif (
+		"RGB" == colors and 16 == pixel_depth or
+		"RGB" == colors and 24 == pixel_depth
+	) then
+		image_type = 10 -- RLE RGB
+	end
 	self.data = self.data
 		.. string.char(0) -- image id
 		.. string.char(0) -- color map type
-		.. string.char(10) -- image type (RLE RGB = 10)
+		.. string.char(image_type)
 	self:encode_colormap_spec() -- color map specification
-	self:encode_image_spec(pixel_depth) -- image specification
+	self:encode_image_spec(properties) -- image specification
 end
 
-function image:encode_data(pixel_depth)
-	if 16 == pixel_depth then
+function image:encode_data(properties)
+	local colors = properties.colors
+	local pixel_depth = properties.pixel_depth
+	if "BW" == colors and 8 == pixel_depth then
+		self:encode_data_bw8()
+	elseif "RGB" == colors and 16 == pixel_depth then
 		self:encode_data_a1r5g5b5_rle()
-	elseif 24 == pixel_depth then
+	elseif "RGB" == colors and 24 == pixel_depth then
 		self:encode_data_r8g8b8_rle()
 	end
 end
 
+function image:encode_data_bw8()
+	local raw_pixels = {}
+	for _, row in ipairs(self.pixels) do
+		for _, pixel in ipairs(row) do
+			local raw_pixel = string.char(pixel[1])
+			raw_pixels[#raw_pixels + 1] = raw_pixel
+		end
+	end
+	self.data = self.data .. table.concat(raw_pixels)
+end
+
 function image:encode_data_a1r5g5b5_rle()
 	local colorword = nil
 	local previous_r = nil
@@ -232,10 +262,10 @@ function image:encode_footer()
 		.. string.char(0)
 end
 
-function image:encode(pixel_depth)
-	self:encode_header(pixel_depth) -- header
+function image:encode(properties)
+	self:encode_header(properties) -- header
 	-- no color map and image id data
-	self:encode_data(pixel_depth) -- encode data
+	self:encode_data(properties) -- encode data
 	-- no extension or developer area
 	self:encode_footer() -- footer
 end