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Reduce TGA filesize by 25% by using 16-bit colors

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Nils Dagsson Moskopp 2021-11-05 18:58:01 +01:00
parent 4f6ea5d035
commit 13552f16f2
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GPG key ID: A3BC671C35191080
1 changed files with 26 additions and 8 deletions
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34
init.lua
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@ -30,7 +30,7 @@ function image:encode_image_spec()
.. string.char(0, 0) -- Y-origin
.. string.char(self.width % 256, math.floor(self.width / 256)) -- width
.. string.char(self.height % 256, math.floor(self.height / 256)) -- height
.. string.char(24) -- pixel depth (RGB = 3 bytes = 24 bits)
.. string.char(16) -- pixel depth (ARRRRRGGGGGBBBBB = 2 bytes = 16 bits)
.. string.char(0) -- image descriptor
end
@ -44,24 +44,42 @@ function image:encode_header()
end
function image:encode_data()
local current_pixel = ''
local previous_pixel = ''
local colorword = nil
local previous_r = nil
local previous_g = nil
local previous_b = nil
local count = 1
local packets = {}
local rle_packet = ''
-- 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
current_pixel = string.char(pixel[3], pixel[2], pixel[1])
if current_pixel ~= previous_pixel or count == 128 then
packets[#packets +1] = rle_packet
if pixel[1] ~= previous_r or pixel[2] ~= previous_g or pixel[3] ~= previous_b or count == 128 then
if nil ~= previous_r then
colorword = 32768 +
((math.floor((previous_r * max_sample_out / max_sample_in) + 0.5)) * 1024) +
((math.floor((previous_g * max_sample_out / max_sample_in) + 0.5)) * 32) +
((math.floor((previous_b * max_sample_out / max_sample_in) + 0.5)) * 1)
rle_packet = string.char(128 + count - 1, colorword % 256, math.floor(colorword / 256))
packets[#packets +1] = rle_packet
end
count = 1
previous_pixel = current_pixel
previous_r = pixel[1]
previous_g = pixel[2]
previous_b = pixel[3]
else
count = count + 1
end
rle_packet = string.char(128 + count - 1) .. current_pixel
end
end
colorword = 32768 +
((math.floor((previous_r * max_sample_out / max_sample_in) + 0.5)) * 1024) +
((math.floor((previous_g * max_sample_out / max_sample_in) + 0.5)) * 32) +
((math.floor((previous_b * max_sample_out / max_sample_in) + 0.5)) * 1)
rle_packet = string.char(128 + count - 1, colorword % 256, math.floor(colorword / 256))
packets[#packets +1] = rle_packet
self.data = self.data .. table.concat(packets)
end