BMP文件格式

bmp 文件　　 bmp（bitmap 的缩写）文件格式是 windows 本身的位图文件格式，所谓本身是指 windows 内部存储位图即采用这种格式。一个 .bmp 格式的文件通常有 .bmp 的扩展名，但有一些是以 .rle 为扩展名的， rle 的意思是行程长度编码（run length encoding）。这样的文件意味着其使用的数据压缩方法是 .bmp 格式文件支持的两种 rle 方法中的一种。　　 bmp 文件可用每象素 1、 4、 8、 16 或 24 位来编码颜色信息，这个位数称作图象的颜色深度，它决定了图象所含的最大颜色数。一幅 1-bpp（位每象素， bit per pixel）的图象只能有两种颜色。而一幅 24-bpp 的图象可以有超过 16 兆种不同的颜色。　　下一页的图说明了一个典型 .bmp 文件的结构。它是以 256 色也就是 8-bpp 为例的，文件被分成四个主要的部分：一个位图文件头，一个位图信息头，一个色表和位图数据本身。位图文件头包含关于这个文件的信息。如从哪里开始是位图数据的定位信息，位图信息头含有关于这幅图象的信息，例如以象素为单位的宽度和高度。色表中有图象颜色的 rgb 值。对显示卡来说，如果它不能一次显示超过 256 种颜色，读取和显示 .bmp 文件的程序能够把这些 rgb 值转换到显示卡的调色板来产生准确的颜色。　　 bmp 文件的位图数据格式依赖于编码每个象素颜色所用的位数。对于一个 256 色的图象来说，每个象素占用文件中位图数据部分的一个字节。象素的值不是 rgb 颜色值，而是文件中色表的一个索引。所以在色表中如果第一个 r/g/b 值是 255/0/0，那么象素值为 0 表示它是鲜红色，象素值按从左到右的顺序存储，通常从最后一行开始。所以在一个 256 色的文件中，位图数据中第一个字节就是图象左下角的象素的颜色索引，第二个就是它右边的那个象素的颜色索引。如果位图数据中每行的字节数是奇数，就要在每行都加一个附加的字节来调整位图数据边界为 16 位的整数倍。　　并不是所有的 bmp 文件结构都象表中所列的那样，例如 16 和 24-bpp，文件就没有色表，象素值直接表示 rgb 值，另外文件私有部分的内部存储格式也是可以变化的。例如，在 16 和 256 色 .bmp 文件中的位图数据采用 rle 算法来压缩，这种算法用颜色加象素个数来取代一串颜色相同的序列，而且， windows 还支持 os/2 下的 .bmp 文件，尽管它使用了不同的位图信息头和色表格式。

1. BMP文件组成
BMP文件由文件头、位图信息头、颜色信息和图形数据四部分组成。
2. BMP文件头
BMP文件头数据结构含有BMP文件的类型、文件大小和位图起始位置等信息。其结构定义如下: typedef struct tagBITMAPFILEHEADER
{
WORDbfType; // 位图文件的类型，必须为BM
DWORD bfSize; // 位图文件的大小，以字节为单位
WORDbfReserved1; // 位图文件保留字，必须为0
WORDbfReserved2; // 位图文件保留字，必须为0
DWORD bfOffBits; // 位图数据的起始位置，以相对于位图
// 文件头的偏移量表示，以字节为单位
} BITMAPFILEHEADER;3. 位图信息头 BMP位图信息头数据用于说明位图的尺寸等信息。
typedef struct tagBITMAPINFOHEADER{
DWORD biSize; // 本结构所占用字节数
LONGbiWidth; // 位图的宽度，以像素为单位
LONGbiHeight; // 位图的高度，以像素为单位
WORD biPlanes; // 目标设备的级别，必须为1
WORD biBitCount// 每个像素所需的位数，必须是1(双色),
// 4(16色)，8(256色)或24(真彩色)之一
DWORD biCompression; // 位图压缩类型，必须是 0(不压缩),
// 1(BI_RLE8压缩类型)或2(BI_RLE4压缩类型)之一
DWORD biSizeImage; // 位图的大小，以字节为单位
LONGbiXPelsPerMeter; // 位图水平分辨率，每米像素数
LONGbiYPelsPerMeter; // 位图垂直分辨率，每米像素数
DWORD biClrUsed;// 位图实际使用的颜色表中的颜色数
DWORD biClrImportant;// 位图显示过程中重要的颜色数
} BITMAPINFOHEADER;4. 颜色表
颜色表用于说明位图中的颜色，它有若干个表项，每一个表项是一个RGBQUAD类型的结构，定义一种颜色。RGBQUAD结构的定义如下: typedef struct tagRGBQUAD {
BYTErgbBlue;// 蓝色的亮度(值范围为0-255)
BYTErgbGreen; // 绿色的亮度(值范围为0-255)
BYTErgbRed; // 红色的亮度(值范围为0-255)
BYTErgbReserved;// 保留，必须为0
} RGBQUAD;
颜色表中RGBQUAD结构数据的个数有biBitCount来确定:
当biBitCount=1,4,8时，分别有2,16,256个表项;
当biBitCount=24时，没有颜色表项。
位图信息头和颜色表组成位图信息，BITMAPINFO结构定义如下:
typedef struct tagBITMAPINFO {
BITMAPINFOHEADER bmiHeader; // 位图信息头
RGBQUAD bmiColors[1]; // 颜色表
} BITMAPINFO;
5. 位图数据
位图数据记录了位图的每一个像素值，记录顺序是在扫描行内是从左到右,扫描行之间是从下到上。位图的一个像素值所占的字节数: 当biBitCount=1时，8个像素占1个字节;
当biBitCount=4时，2个像素占1个字节;
当biBitCount=8时，1个像素占1个字节;
当biBitCount=24时,1个像素占3个字节;
Windows规定一个扫描行所占的字节数必须是
4的倍数(即以long为单位),不足的以0填充，
一个扫描行所占的字节数计算方法:
DataSizePerLine= (biWidth* biBitCount+31)/8;
// 一个扫描行所占的字节数
DataSizePerLine= DataSizePerLine/4*4; // 字节数必须是4的倍数
位图数据的大小(不压缩情况下):
DataSize= DataSizePerLine* biHeight;

'The Bitmap Info project demonstrates one means of obtaining from a bitmap file on the disk its image information without loading the actual bitmap.
'
'Within a bitmap file are 2 header groupings of information describing the bitmap image in that file.  This data is contained in the file's BITMAPFILEHEADER and BITMAPINFOHEADER structures. By using the VB Type to recreate these structures, the details of the bitmap contained in the file can be obtained with a simple binary read of this data.
'(Note: due to the formatting of the code to allow pasting into VB, the strings used here will necessitate horizontal scrolling of the browser window on displays under 1024x768 full screen.)
'
'
' Form Code
'
'To a form add the controls as indicated in the illustration. To see a template of this form with control names substituted for the control captions, click here to open the image in a new window.
'
'The names of the description labels is unimportant, but the data returned from the routine below posts the information to the lblInfo() label array. Also on the form is an image control (Image1), with its Stretch property set to True, two command buttons (cmdSelect and cmdEnd), and another label to contain the selected filename and path (lblFileName). Finally, the routine needs a common dialog control (CMDialog1) added to the form.
'
'Paste the following code into the general declarations section of the form:'--------------------------------------------------------------------------------

Option Explicit  Private Const CANCELERR = 32755
  Private Const BI_RGB = 0&
  Private Const BI_RLE8 = 1&
  Private Const BI_RLE4 = 2&
  Private Const BI_BITFIELDS = 3&

  Private Type BITMAPINFOHEADER '40 bytes
    biSize            As Long
    biWidth           As Long
    biHeight          As Long
    biPlanes          As Integer
    biBitCount        As Integer
    biCompression     As Long
    biSizeImage       As Long
    biXPelsPerMeter   As Long
    biYPelsPerMeter   As Long
    biClrUsed         As Long
    biClrImportant    As Long
  End Type

  Private Type BITMAPFILEHEADER
    bfType            As Integer
    bfSize            As Long
    bfReserved1       As Integer
    bfReserved2       As Integer
    bfOffBits         As Long
  End Type
Private Sub Form_Load()

'initialize the form controls
  lblFileName = "Select a bitmap or RLE file to detail..."
  lblInfo(0) = ""
  lblInfo(1) = ""
  lblInfo(2) = ""
  lblInfo(3) = ""
  lblInfo(4) = ""
  lblInfo(5) = ""
  lblInfo(6) = ""
  lblInfo(7) = ""

'position the form
  Me.Move (Screen.Width - Me.Width) \ 2, (Screen.Height - Me.Height) \ 2End Sub
Private Sub cmdEnd_Click()  Unload MeEnd Sub
Private Sub cmdSelect_Click()

'create some working variables
  Dim ff As Integer
  Dim tmp As String

'create the variables to hold the bitmap info
  Dim FileHeader As BITMAPFILEHEADER
  Dim InfoHeader As BITMAPINFOHEADER  On Error GoTo cmdSelect_FileErrorHandler

'show the common dialog
  CMDialog1.CancelError = True
  CMDialog1.ShowOpen

'display a rendition of the loaded bitmap
  Image1 = LoadPicture((CMDialog1.filename))
  Image1.ZOrder 1

'read the file header info
  ff = FreeFile
  Open CMDialog1.filename For Binary Access Read As #ff
    Get #ff, , FileHeader
    Get #ff, , InfoHeader
  Close #ff

'display the file info
  lblFileName = CMDialog1.filename

  lblInfo(0) = InfoHeader.biWidth & " pixels"
  lblInfo(1) = InfoHeader.biHeight & " pixels"

'select the appropriate string based on the value of biCompression
  Select Case InfoHeader.biSizeImage
    Case 0:    tmp$ = "BI_RGB bitmap; size variable not filled in."
    Case Else: tmp$ = Format$(InfoHeader.biSizeImage, "#,###,###") & " bytes"
  End Select
  lblInfo(2) = tmp$

  lblInfo(3) = InfoHeader.biPlanes
  lblInfo(4) = InfoHeader.biBitCount & " (" & 2 ^ InfoHeader.biBitCount & " colours)" 'select the appropriate string based on the value of biCompression
  Select Case InfoHeader.biCompression
    Case BI_RGB:  tmp$ = "Uncompressed bitmap."
    Case BI_RLE8: tmp$ = "Run-length encoded (RLE) format for bitmaps with 8 bits per pixel."
    Case BI_RLE4: tmp$ = "Run-length encoded (RLE) format for bitmaps with 4 bits per pixel."
    Case BI_BITFIELDS: tmp$ = "Uncompressed 16- or 32-bit-per-pixel format."
  End Select
  lblInfo(5) = tmp$ 'select the appropriate string based on the value of biClrUsed
  Select Case InfoHeader.biClrUsed
    Case 0:
        tmp$ = "Bitmap uses the maximum number of colours corresponding to the"
        tmp$ = tmp$ & " bits-per-pixel for the compression mode."    Case Is <> 0 And InfoHeader.biBitCount = 16:
         tmp$ = "The size of the colour table used to optimize performance"
         tmp$ = tmp$ & "of Windows colour palettes is " & Str$(InfoHeader.biClrUsed)
  End Select
  lblInfo(6) = tmp$

'select the appropriate string based on the value of biClrImportant
  Select Case InfoHeader.biClrImportant
    Case 0:
        tmp$ = "All " & 2 ^ InfoHeader.biBitCount & " colour"
        tmp$ = tmp$ & " indices are considered important for displaying this bitmap."
    Case Is <> 0
        tmp$ = "The number of colours that are considered important for displaying"
        tmp$ = tmp$ & " this bitmap are " & Str$(InfoHeader.biClrImportant)
  End Select
  lblInfo(7) = tmp$

Exit Sub'handle file errors or the user choosing cancel
cmdSelect_FileErrorHandler:   If Err <> CANCELERR Then MsgBox Error$(Err), 48, "Image Info"
   lblFileName = "No file was selected."End Sub

一个奇怪的现象：
我用savepicture方法保存一副图，可保存的结果是：不仅大小和原图不一样比原图大了不少，而且保存图的宽度和高度也和原图不一样。这是怎么回事呢？
请指教！！！

关于BMP文件的格式可参考机械工业出版社出版的《Win32开发人员参考库（第三卷）》，里面有非常详细的描述。另外对于spirit2001(巍巍)的问题：BMP文件的宽度必须确保32位的字长，所以必须能被4整除。由于这个原因，BMP实际的宽度与图片宽度有差别的。但读出来显示的时候却是准确的宽度。

我曾经写过一个读取BMP的模块：
Option Explicit
Private Declare Function SetPixelV Lib "gdi32" (ByVal hDC As Long, ByVal X As Long, ByVal Y As Long, ByVal crColor As Long) As LongPublic Type ZMapInfoType
    Headle As Integer
    MapType As String * 3
    MapCount As Integer
End TypePrivate Type BitMapAllHeader
    bfType As String * 2
    bfSize As Long
    bfReserved1 As Integer
    bfReserved2 As Integer
    bfOffBits As Long
    '---------------------
    biSize As Long
    biWidth As Long
    biHeight As Long
    biPlanes As Integer
    biBitCount As Integer
    biCompression As Long
    biSizeImage As Long
    biXRes As Long
    biYRes As Long
    biClrUsed As Long
    biClrImportant As Long
End TypePrivate Type BitMapPalette
    peBlue As Byte
    peGreen As Byte
    peRed As Byte
    peFlags As Byte
End TypePublic Sub ChkMapType(MapInfo As ZMapInfoType)
    Dim LoadByt() As Byte

    ReDim LoadByt(0 To 1) As Byte
    Get MapInfo.Headle, 1, LoadByt
    If UCase$(StrConv(LoadByt, vbUnicode)) = "BM" Then
        MapInfo.MapType = "BMP"
    Else
        MapInfo.MapType = ""
    End If

End SubPublic Function GetMapCount(MapInfo As ZMapInfoType) As Integer
    Select Case UCase$(RTrim$(MapInfo.MapType))
    Case "BMP"
        MapInfo.MapCount = 1
    Case Else
        MapInfo.MapCount = 0
    End Select

    GetMapCount = MapInfo.MapCount

End FunctionPublic Function GetMapSize(MapInfo As ZMapInfoType, _
        Width As Long, _
        Height As Long, _
        Optional MapIndex As Integer) As Integer
    On Error GoTo LoadErr

    Select Case UCase$(RTrim$(MapInfo.MapType))
    Case "BMP"
        'MapInfo.MapCount = 1
        Dim BMah As BitMapAllHeader

        Get MapInfo.Headle, 1, BMah

        Width = BMah.biWidth
        Height = BMah.biHeight

    Case Else
        On Error GoTo 0
        GetMapSize = 0: Exit Function
    End Select

    On Error GoTo 0
    GetMapSize = -1
    Exit Function

LoadErr:
    GetMapSize = 0

End FunctionPublic Function LoadBmpMap(hDC As Long, _
        PutX As Long, PutY As Long, _
        MapInfo As ZMapInfoType, _
        FileX As Long, FileY As Long, _
        ByVal FileWidth As Long, ByVal FileHeight As Long) As Integer
    Dim Width As Long, Height As Long
    Dim I As Long, J As Long, K As Long
    Dim BMah As BitMapAllHeader
    Dim LoadPalette As BitMapPalette
    Dim LoadByt1 As Byte, LoadByt() As Byte
    Dim BMPPalette(0 To 255) As Long
    Dim PutColor As Long
    Dim LineByt As Long

    If FileX + FileWidth <= 0 Or FileY + FileHeight <= 0 Then GoTo LoadErr

    On Error GoTo LoadErr

    Get MapInfo.Headle, 1, BMah
    If BMah.biPlanes <> 1 Or BMah.biCompression <> 0 Then LoadBmpMap = 1: Exit Function

    Width = BMah.biWidth
    Height = BMah.biHeight
    If FileX + FileWidth > Width Then FileWidth = Width - FileX
    If FileY + FileHeight > Height Then FileHeight = Height - FileY
    'Debug.Print FileX, FileWidth, PutX, Width
    'Debug.Print FileY, FileHeight, PutY, Height

    Select Case BMah.biBitCount
    Case 24
        'Get MapInfo.Headle, BMah.bfOffBits, LoadByt1
        LineByt = Int((Width * 3 + 3) / 4) * 4
        ReDim LoadByt(0 To FileWidth * 3 - 1) As Byte

        For I = 0 To FileHeight - 1
            'Get MapInfo.Headle, BMah.bfOffBits + LineByt * (Height - (I + FileY) - 1) + FileX * 3, LoadByt1
            Get MapInfo.Headle, BMah.bfOffBits + LineByt * (Height - (I + FileY) - 1) + FileX * 3 + 1, LoadByt
            For J = 0 To FileWidth - 1
                PutColor = 0
                For K = 0 To 2
                    'Get MapInfo.Headle, , LoadByt1
                    LoadByt1 = LoadByt(J * 3 + K)
                    PutColor = PutColor Or LoadByt1 * &H100& ^ (2 - K)
                Next K
                SetPixelV hDC, J + PutX, I + PutY, PutColor
            Next J
        Next I

    Case 8
        For I = 0 To 255
            Get MapInfo.Headle, , LoadPalette
            BMPPalette(I) = RGB(LoadPalette.peRed, LoadPalette.peGreen, LoadPalette.peBlue)
        Next I
        LineByt = Int((Width + 3) / 4) * 4
        ReDim LoadByt(0 To FileWidth - 1) As Byte

        For I = 0 To FileHeight - 1
            Get MapInfo.Headle, BMah.bfOffBits + LineByt * (Height - (I + FileY) - 1) + FileX + 1, LoadByt
            For J = 0 To FileWidth - 1
                SetPixelV hDC, J + PutX, I + PutY, BMPPalette(LoadByt(J))
            Next J
        Next I

    Case 4
        For I = 0 To 16
            Get MapInfo.Headle, , LoadPalette
            BMPPalette(I) = RGB(LoadPalette.peRed, LoadPalette.peGreen, LoadPalette.peBlue)
        Next I
        LineByt = Int((Int((Width + 1) / 2) + 3) / 4) * 4
        ReDim LoadByt(0 To Int((Width + 1) / 2) - 1) As Byte

        For I = 0 To FileHeight - 1
            Get MapInfo.Headle, BMah.bfOffBits + LineByt * (Height - (I + FileY) - 1) + 1, LoadByt
            For J = 0 To FileWidth - 1 Step 2
                SetPixelV hDC, J + PutX, I + PutY, BMPPalette((LoadByt(Int(J / 2)) And &HF0) \ &H10)
                SetPixelV hDC, J + PutX + 1, I + PutY, BMPPalette(LoadByt(Int(J / 2)) And &HF)
            Next J
        Next I

    Case 1
        Dim BitAnd(0 To 15) As Integer
        For I = 0 To 7
            BitAnd(I) = 2 ^ (7 - I)
        Next I
        BitAnd(8) = &H8000
        For I = 0 To 6
            BitAnd(9 + I) = 2 ^ (14 - I)
        Next I
        For I = 0 To 1
            Get MapInfo.Headle, , LoadPalette
            BMPPalette(I) = RGB(LoadPalette.peRed, LoadPalette.peGreen, LoadPalette.peBlue)
        Next I

        LineByt = Int((Int((Width + 7) / 8) + 3) / 4) * 4
        ReDim LoadByt(0 To Int((Width + 7) / 8) - 1) As Byte

        For I = 0 To FileHeight - 1
            Get MapInfo.Headle, BMah.bfOffBits + LineByt * (Height - (I + FileY) - 1) + 1, LoadByt
            For J = 0 To FileWidth - 1 Step 8
                For K = 0 To 7
                    SetPixelV hDC, J + PutX + K, I + PutY, BMPPalette((((LoadByt(Int(J / 8)) And BitAnd(K)) <> 0) And 1))
                Next K
            Next J
        Next I

    Case Else
        LoadBmpMap = 2
        Exit Function

    End Select

    On Error GoTo 0
    LoadBmpMap = 0
    Exit Function

LoadErr:
    LoadBmpMap = -1

End Function

使用方法：
Private Sub LoadBMPSub()
    Dim ZMap As ZMapInfoType
    Dim Wi As Long, He As Long

    If ChkFileRead(Text1.Text) = False Then Exit Sub
    ZMap.Headle = FreeFile
    Open Text1.Text For Binary As ZMap.Headle

    Me.MousePointer = 11
    Me.Caption = MeName + " (Loading)"
    ChkMapType ZMap
    If GetMapCount(ZMap) = 0 Then Exit Sub
    If GetMapSize(ZMap, Wi, He) = 0 Then Exit Sub
    SetSize Pic2, Wi, He
    DoEvents
    LoadBmpMap Pic2.hDC, 0, 0, ZMap, 0, 0, Wi, He
    Me.MousePointer = 0
    Me.Caption = MeName

    Close ZMap.Headle

    Pic2.Picture = Pic2.Image
    JSS

End Sub

我们有时候说某张BMP图为32＊32的图是什么意思？
32＊32代表什么意思？
如用数组来保存它，那么这个二维数组要有多大？
谢谢

解决方案 »