Lua，位图读取的奇怪行为

Question

在我发现并修改的位图阅读器中，一些图像的前几种颜色非常不准确，在其他图像中，它读起来很完美，而在刚才，我必须让它读取的图像的前7种左右颜色根本不准确。我不懂字节顺序，所以请帮忙！

这是我修改后的代码副本：

---
-- (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

遗憾的是，我不能给你我用它运行的代码，因为它有太多的依赖关系，但它的输出是：

<254, 254, 254, 256>
<99, 254, 254, 256>
<49, 74, 91, 256>

使用以下bmp颜色运行时：

<90, 106, 113, 256>
<188, 194, 197, 256>
<254, 254, 254, 256>

我看不到任何图案，bmp阅读器似乎很有意义，它在读取时没有打印错误，我确保根据需要将bmp保存为24位。感谢所有帮助：-）

Answer 1

在上面的例子中，offset没有考虑到行宽度必须始终是4字节的倍数，如果低于4的倍数，则使用填充。您可以通过将行宽度四舍五入到4的最接近倍数来解决此问题，这解释了为什么该函数有时会准确读取图像，而有时则不会。

像素一开始的行为是错误的，但后来的其他行为是准确的，这是由于爬行，从逻辑上讲，第一个是准确的。最后一个是不准确的，但爬行的方向相反，因为位图是自下而上和从右到左读取的。