Lua, Odd behavior with bitmap reading - image

Question

In a bitmap reader I found and modified, the first few colors of some images are innacurate by a large amount, in other images it reads perfectly, in just now, the first 7 or so colors of an image that I must have it read are not accurate at all. I don't understand byte orders, so please help!
Heres my modified copy of the code:
---
-- (Evil Steve)Because I'm a kind and wonderful person: http://www.gamedev.net/topic/572784-lua-read-bitmap/
---
function error(err)
-- Replace with your own error output method:
print(err);
end
-- Helper function: Parse a 16-bit WORD from the binary string
function ReadWORD(str, offset)
local loByte = str:byte(offset);
local hiByte = str:byte(offset+1);
return hiByte*256 + loByte;
end
-- Helper function: Parse a 32-bit DWORD from the binary string
function ReadDWORD(str, offset)
local loWord = ReadWORD(str, offset);
local hiWord = ReadWORD(str, offset+2);
return hiWord*65536 + loWord;
end
-- Process a bitmap file in a string, and call DrawPoint for each pixel
function OpenBitmap(File, Stream)
if Stream == nil then Stream = false end
local bytecode = File:read("*a")
-------------------------
-- Parse BITMAPFILEHEADER
-------------------------
local offset = 1;
local bfType = ReadWORD(bytecode, offset);
if(bfType ~= 0x4D42) then
error("Not a bitmap file (Invalid BMP magic value)");
return;
end
local bfOffBits = ReadWORD(bytecode, offset+10);
-------------------------
-- Parse BITMAPINFOHEADER
-------------------------
offset = 15; -- BITMAPFILEHEADER is 14 bytes long
local biWidth = ReadDWORD(bytecode, offset+4);
local biHeight = ReadDWORD(bytecode, offset+8);
local biBitCount = ReadWORD(bytecode, offset+14);
local biCompression = ReadDWORD(bytecode, offset+16);
if(biBitCount ~= 24) then
error("Only 24-bit bitmaps supported (Is " .. biBitCount .. "bpp)");
return;
end
if(biCompression ~= 0) then
error("Only uncompressed bitmaps supported (Compression type is " .. biCompression .. ")");
return;
end
---------------------
-- Parse bitmap image
---------------------
local TmpImg = {}
if Stream == false then
for y = biHeight-1, 0, -1 do
offset = bfOffBits + (biWidth*biBitCount/8)*y + 1;
for x = 0, biWidth-1 do
local b = bytecode:byte(offset);
local g = bytecode:byte(offset+1);
local r = bytecode:byte(offset+2);
offset = offset + 3;
TmpImg[#TmpImg+1] = {r,g,b}
end
end
else
for y = biHeight-1, 0, -1 do
offset = bfOffBits + (biWidth*biBitCount/8)*y + 1;
for x = 0, biWidth-1 do
local b = bytecode:byte(offset);
local g = bytecode:byte(offset+1);
local r = bytecode:byte(offset+2);
offset = offset + 3;
TmpImg[#TmpImg+1] = r
TmpImg[#TmpImg+1] = g
TmpImg[#TmpImg+1] = b
end
end
end
return TmpImg, biWidth, biHeight
end
function OpenBmp(FileName, Stream)
if Stream == nil then Stream = false end
if FileName == nil then
return false
end
local File = assert(io.open(FileName, 'rb'))
local Data, Width, Height = OpenBitmap(File, Stream)
File:close()
return Data, Width, Height
end
I cannot give you the code I run with this, sadly, because it has too many dependencies to bother with, but its output is:
<254, 254, 254, 256>
<99, 254, 254, 256>
<49, 74, 91, 256>
When ran with the following bmp colors:
<90, 106, 113, 256>
<188, 194, 197, 256>
<254, 254, 254, 256>
I don't see any pattern, and the bmp reader seems to make sense, it prints no errors when reading, and I made sure to save the bmp as 24 bit as required. all help appreciated :-)

Answer 1

In the example above, offset wasn't considering that row widths must always be a multiple of 4 bytes wide, with it padded if it was below the multiple of 4. You can resolve this by rounding up row width to the nearest multiple of 4, which explains why the function sometimes read images accurately, and other times not.
the behavior from the pixels in the beginning being false, but the rest later on being accurate was due to creeping, logically, the first would be accurate, with the last inaccurate, but the creeping went the opposite way due to bitmaps being read bottom-up and right-left.