bmp aspect ratio issue - lua

Question

I've been trying to understand how bmp files work so I can render some Mandelbrot set pictures and output them as bmp files since that seems to be one of the easiest methods but for some reason when I use an aspect ratio that isn't 1:1 even though its something to the power of 4 (so no padding is needed) I get weird artifacts like these 200:100 48:100 what I'm trying to do is turning an array of pixels that has white for even numbers and black for odd numbers into a bmp, this (100:100) is what it looks like with 1:1 aspect ratio.
I've tried reading through the wikipedia article to see if I can figure out what I'm doing wrong but I still don't get what I'm missing.
This is the script I've written in Lua so far:
ResolutionX = 100
ResolutionY = 100
local cos, atan, sin, atan2, sqrt, floor = math.cos, math.atan, math.sin, math.atan2, math.sqrt, math.floor
local insert, concat = table.insert, table.concat
local sub, char, rep = string.sub, string.char, string.rep
io.output("Test.bmp")
function Basen(n,b)
n = floor(n)
if not b or b == 10 then return tostring(n) end
local digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
local t = {}
repeat
local d = (n % b) + 1
n = floor(n / b)
insert(t, 1, digits:sub(d,d))
until n == 0
return rep("0",32-#t)..concat(t,"")
end
FileSize = Basen(ResolutionY*ResolutionX*3 + 54,2)
FileSize4 = tonumber(sub(FileSize,1,8),2) or 0
FileSize3 = tonumber(sub(FileSize,9,16),2) or 0
FileSize2 = tonumber(sub(FileSize,17,24),2) or 0
FileSize1 = tonumber(sub(FileSize,25,32),2) or 0
Width = Basen(ResolutionX,2)
print("Width: ",Width)
Width4 = tonumber(sub(Width,1,8),2) or 0
Width3 = tonumber(sub(Width,9,16),2) or 0
Width2 = tonumber(sub(Width,17,24),2) or 0
Width1 = tonumber(sub(Width,25,32),2) or 0
Height = Basen(ResolutionY,2)
print("Height: ",Height)
Height4 = tonumber(sub(Height,1,8),2) or 0
Height3 = tonumber(sub(Height,9,16),2) or 0
Height2 = tonumber(sub(Height,17,24),2) or 0
Height1 = tonumber(sub(Height,25,32),2) or 0
BMPSize = Basen(ResolutionY*ResolutionX*3,2)
BMPSize4 = tonumber(sub(BMPSize,1,8),2) or 0
BMPSize3 = tonumber(sub(BMPSize,9,16),2) or 0
BMPSize2 = tonumber(sub(BMPSize,17,24),2) or 0
BMPSize1 = tonumber(sub(BMPSize,25,32),2) or 0
print("TotalSize: ",FileSize1,FileSize2,FileSize3,FileSize4,"\nWidth: ",Width1,Width2,Width3,Width4,"\nHeight: ",Height1,Height2,Height3,Height4,"\nImage data: ",BMPSize1,BMPSize2,BMPSize3,BMPSize4)
Result = {"BM"..char( --File type
FileSize1,FileSize2,FileSize3,FileSize4,--File size
0,0,0,0, --Reserved
54,0,0,0, --Where the pixel data starts
40,0,0,0, --DIB header
Width1,Width2,Width3,Width4, --Width
Height1,Height2,Height3,Height4, --Height
1,0, --Color planes
24,00, --Bit depth
0,0,0,0, --Compression
BMPSize1,BMPSize2,BMPSize3,BMPSize4, --The amount of bytes pixel data will consume
Width1,Width2,Width3,Width4,
Height1,Height2,Height3,Height4,
0,0,0,0, --Number of colors in palatte
0,0,0,0
)}
for X = 0, ResolutionX - 1 do
for Y = 0, ResolutionY - 1 do
insert(Result,rep(char(255 * ((X + 1) % 2) * ((Y + 1) % 2)),3))
end
end
io.write(table.concat(Result))

Answer 1

Ok, here's a BMP version. I've put things in a module so it might be easier to use.
local writeBMP = {}
local floor = math.floor
local insert, concat = table.insert, table.concat
local sub, char, rep = string.sub, string.char, string.rep
local function Basen(n,b)
n = floor(n)
if not b or b == 10 then return tostring(n) end
local digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
local t = {}
repeat
local d = (n % b) + 1
n = floor(n / b)
insert(t, 1, digits:sub(d,d))
until n == 0
return rep("0",32-#t)..concat(t,"")
end
local function nextMul4(x)
if ( x % 4 == 0 ) then
return x
else
return x+4-(x%4)
end
end
local function clamp(x)
local y = x
if ( x > 255 ) then
y = 255
elseif ( x < 0 ) then
y = 0
end
return floor(y)
end
-- Accepts array of type pixelsXYC[X][Y][C] of numbers 0-255
-- C=1,2,3 are the red, green and blue channels respectively
-- X increases left to right, and Y increases top to bottom
function writeBMP.data(pixelsXYC, resolutionX, resolutionY)
local Pixels = pixelsXYC
local ResolutionX = resolutionX
local ResolutionY = resolutionY
assert(#Pixels == ResolutionX, "Table size and X resolution mismatch")
assert(#Pixels[1] == ResolutionY, "Table size and Y resolution mismatch")
local FileSize = Basen(ResolutionY*nextMul4(3*ResolutionX) + 54,2)
local FileSize4 = tonumber(sub(FileSize,1,8),2) or 0
local FileSize3 = tonumber(sub(FileSize,9,16),2) or 0
local FileSize2 = tonumber(sub(FileSize,17,24),2) or 0
local FileSize1 = tonumber(sub(FileSize,25,32),2) or 0
local Width = Basen(ResolutionX,2)
local Width4 = tonumber(sub(Width,1,8),2) or 0
local Width3 = tonumber(sub(Width,9,16),2) or 0
local Width2 = tonumber(sub(Width,17,24),2) or 0
local Width1 = tonumber(sub(Width,25,32),2) or 0
local Height = Basen(ResolutionY,2)
local Height4 = tonumber(sub(Height,1,8),2) or 0
local Height3 = tonumber(sub(Height,9,16),2) or 0
local Height2 = tonumber(sub(Height,17,24),2) or 0
local Height1 = tonumber(sub(Height,25,32),2) or 0
local BMPSize = Basen(ResolutionY*nextMul4(3*ResolutionX),2)
local BMPSize4 = tonumber(sub(BMPSize,1,8),2) or 0
local BMPSize3 = tonumber(sub(BMPSize,9,16),2) or 0
local BMPSize2 = tonumber(sub(BMPSize,17,24),2) or 0
local BMPSize1 = tonumber(sub(BMPSize,25,32),2) or 0
local Result = {}
Result[1] = "BM" .. char( --File type
FileSize1,FileSize2,FileSize3,FileSize4,--File size
0,0, --Reserved
0,0, --Reserved
54,0,0,0, --Where the pixel data starts
40,0,0,0, --DIB header
Width1,Width2,Width3,Width4, --Width
Height1,Height2,Height3,Height4, --Height
1,0, --Color planes
24,0, --Bit depth
0,0,0,0, --Compression
BMPSize1,BMPSize2,BMPSize3,BMPSize4, --The amount of bytes pixel data will consume
37,22,0,0, --Pixels per meter horizontal
37,22,0,0, --Pixels per meter vertical
0,0,0,0, --Number of colors in palatte
0,0,0,0
)
local Y = ResolutionY
while( Y >= 1 ) do
for X = 1, ResolutionX do
local r = clamp( Pixels[X][Y][1] )
local g = clamp( Pixels[X][Y][2] )
local b = clamp( Pixels[X][Y][3] )
Result[#Result+1] = char(b)
Result[#Result+1] = char(g)
Result[#Result+1] = char(r)
end
-- byte alignment
if ( ( (3*ResolutionX) % 4 ) ~= 0 ) then
local Padding = 4 - ((3*ResolutionX) % 4)
Result[#Result+1] = rep(char(0),Padding)
end
Y = Y - 1
end
return table.concat(Result)
end
function writeBMP.write(pixelsXYC, resolutionX, resolutionY, filename)
local file = io.open(filename,"wb")
local data = writeBMP.data(pixelsXYC, resolutionX, resolutionY)
file:write(data)
end
return writeBMP
A simple test:
-- writeBMP example
local writeBMP = require "writeBMP"
local resolutionX = 100
local resolutionY = 100
-- Pixel data
local pixels = {}
for x=1,resolutionX do
pixels[x] = {}
for y=1, resolutionY do
pixels[x][y] = {}
local red = 255*(resolutionX-x+resolutionY-y)/(resolutionX+resolutionY)
local green = 255*y/resolutionY
local blue = 255*x/resolutionX
pixels[x][y][1] = red
pixels[x][y][2] = green
pixels[x][y][3] = blue
end
end
writeBMP.write(pixels,resolutionX,resolutionY,"testwritebmp.bmp")
return
Note: In BMP, the Y axis starts on the bottom. I am more used to Y axis going from top down in computer graphics (so I wrote it that way).
Thanks HAX for the code.

Answer 2

Welcome to Stack Exchange :)
I suggest having a look at PPM files, they are easy. They can be converted with other tools to png or bmp with other tools.
Wikipedia PPM Specification
Here is a PPM solution:
ResolutionX = 48
ResolutionY = 100
local cos, atan, sin, atan2, sqrt, floor = math.cos, math.atan, math.sin, math.atan2, math.sqrt, math.floor
local insert, concat = table.insert, table.concat
local sub, char, rep = string.sub, string.char, string.rep
local file = io.open("Test.ppm","w")
-- PPM File headers
local Output = { }
Output[1] = "P3"
Output[2] = tostring(ResolutionX) .. " " .. tostring(ResolutionY)
Output[3] = "255"
-- Image body
for Y = 0, ResolutionY - 1 do
for X = 0, ResolutionX - 1 do
local value = 255 * ((X + 1) % 2) * ((Y + 1) % 2)
Output[#Output+1] = rep(tostring(floor(value)),3," ")
end
end
-- Join lines together
local Result = table.concat(Output,"\n")
file:write(Result)
Notes: I could not get your code to write to file (see my usage of file). The inner loop must be X (columns) and the outer loop must be Y (rows) if the write order is English reading order (left-right down-up).