Information Header Structures

The BITMAPINFOHEADER and BITMAPCOREHEADER structures contain information about the dimensions and color format of Windows 3.0 and Presentation Manager 1.2 DIBs, respectively. They are defined as follows:

Windows 3.0 DIB

typedef struct tagBITMAPINFOHEADER {
    ULONG  biSize;
    ULONG  biWidth;
    ULONG  biHeight;
    USHORT biPlanes;
    USHORT biBitCount;
    ULONG  biCompression;
    ULONG  biSizeImage;
    ULONG  biXPelsPerMeter;
    ULONG  biYPelsPerMeter;
    ULONG  biClrUsed;
    ULONG  biClrImportant;
 } BITMAPINFOHEADER;

Presentation Manager 1.2 DIB

typedef struct tagBITMAPCOREHEADER {
    ULONG   bcSize;
    USHORT  bcWidth;
    USHORT  bcHeight;
    USHORT  bcPlanes;
    USHORT  bcBitCount;
 } BITMAPCOREHEADER;

Again, because of the similarly of the Windows 3.0 and Presentation Manager structures, their common fields are described together. Each field name for the Windows structure is followed by the corresponding field name for the Presentation Manager structure, in parentheses.

Common Fields
The following fields are used in both the Windows 3.0 and Presentation Manager 1.2 formats:

Windows (PM) Field

Description
biSize (bcSize)
Specifies the number of bytes required by the BITMAPINFOHEADER structure. You can use this field to distinguish between Windows 3.0 and Presentation Manager 1.2 DIBs.
biWidth (bcWidth)
Specifies the width of the DIB in pixels.
biHeight (bcHeight)
Specifies the height of the DIB in pixels.
biPlanes (bcPlanes)
Specifies the number of planes for the target device. Must be set to 1.
biBitCount (bcBitCount)
Specifies the number of bits-per-pixel.

Windows Fields The following fields are used only in the Windows 3.0 BITMAPINFOHEADER structure:

Field

Description
biCompression
Specifies the type of compression for a compressed bitmap. It can be one of the following values:

BI_RGB

Specifies that the bitmap is not compressed.
BI_RLE4
Specifies a run-length encoded format for bitmaps with 4 bits-per-pixel. The compression format is a 2-byte format consisting of a count byte followed by two word-length color indexes.
BI_RLE8
Specifies a run-length encoded format for bitmaps with 8 bits-per-pixel. The compression format is a 2-byte format consisting of a count byte followed by a color-index byte.
See Windows 3.0 Bitmap Compression Formats for information about the encoding schemes.
biSizeImage
Specifies the size of the image (in bytes).
biXPelsPerMeter
Specifies the horizontal resolution (in pixels per meter) of the target device for the bitmap. An application can use this value to select a bitmap from a resource group that most closely matches the characteristics of the current device.
biYPelsPerMeter
Specifies the vertical resolution (in pixels per meter) of the target device for the bitmap.
biClrUsed
Specifies the number of color values in the color table actually used by the bitmap. Possible values follow.
• Bitmap uses the maximum number of colors corresponding to the value of the usBitCount field.

  Nonzero

  If the usBitCount value is less than 24, the biClrUsed value indicates the actual number of colors that the graphics engine or device driver will access.

  If the usBitCount value is 24, the biClrUsed value indicates the size of the reference color table used to optimize performance of Windows color palettes.

  If the bitmap is packed (that is, a bitmap in which the bitmap array immediately follows the BITMAPINFO header and which is referenced by a single pointer), the biClrUsed field must be set to 0 or to the actual size of the color table. Interpreting the Color Table for more information on how this field affects the interpretation of the color table.
biClrImportant
Specifies the number of color indexes that are considered important for displaying the bitmap. If this value is 0, then all colors are important.

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