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PNG decoder: Fix transparency handling

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This commit is contained in:
Lars Mueller 2021-10-30 16:49:06 +02:00
parent 7e57c55caf
commit 8e42d027a0
1 changed files with 18 additions and 8 deletions
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24
minetest/png.lua
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@ -167,6 +167,7 @@ local adam7_passes = {
end
elseif chunk_type == "tRNS" then
assert(not color_type.alpha, "unexpected tRNS chunk")
color_type.transparency = true
assert(idat_allowed, "tRNS after IDAT chunks")
if color_type.color == "palette" then
assert(palette, "PLTE chunk expected")
@ -180,7 +181,11 @@ local adam7_passes = {
else
assert(color_type.color == "truecolor")
assert(chunk_length == 6)
alpha = ((((byte() * 0x100 + byte()) * 0x100 + byte()) * 0x100 + byte()) * 0x100 + byte()) * 0x100 + byte() -- 16-bit RGB
alpha = 0
-- Read 16-bit RGB (6 bytes)
for _ = 1, 6 do
alpha = alpha * 0x100 + byte()
end
end
elseif chunk_type == "gAMA" then
assert(not palette, "gAMA after PLTE chunk")
@ -211,7 +216,10 @@ local adam7_passes = {
(64 bits required, packing non-mantissa bits isn't practical) => separate table with alpha values
]]
local data = {}
local alpha_data = color_type.color == "truecolor" and bit_depth == 16 and {} or nil
local alpha_data
if color_type.color == "truecolor" and bit_depth == 16 and (color_type.alpha or color_type.transparency) then
alpha_data = {}
end
if adam7 then
-- Allocate space in list part in order to not fill the hash part later
for i = 1, width * height do
@ -312,10 +320,12 @@ local adam7_passes = {
-- Pack only RGB in data, alpha goes in a different table
-- 3 * 16 = 48 bytes can still be held accurately by the double mantissa
data[data_index] = rgb16
if alpha_data then
alpha_data[data_index] = a
end
end
end
end
-- Each byte of the scanline must have been read from
assert(bit >= #scanline * 8 - 7)
previous_scanline = scanline
@ -347,15 +357,15 @@ local adam7_passes = {
}
end
local function rescale_depth(sample, source, target)
if source == target then
local function rescale_depth(sample, source_depth, target_depth)
if source_depth == target_depth then
return sample
end
return floor((sample / (2^source - 1) * (2^target - 1)) + 0.5)
return floor((sample * (2^target_depth - 1) / (2^source_depth - 1)) + 0.5)
end
-- In-place lossy (if bit depth = 16) conversion to ARGB8
(...).convert_png_to_argb8 = function(png)
local color, alpha, depth = png.color_type.color, png.color_type.alpha, png.color_type.depth
local color, transparency, depth = png.color_type.color, png.color_type.alpha or png.color_type.transparency, png.color_type.depth
if color == "palette" or (color == "truecolor" and depth == 8) then
return
end
@ -369,7 +379,7 @@ end
local g = rescale_depth(floor(value / 0x10000) % 0x10000, depth, 8)
local b = rescale_depth(value % 0x10000, depth, 8)
local a = 0xFF
if alpha then
if transparency then
a = rescale_depth(png.alpha_data[index], depth, 8)
end
png.data[index] = a * 0x1000000 + r * 0x10000 + g * 0x100 + b