|
|
The graphics can be
placed on top of your screen in two ways – raster graphics or vector
graphics. In Vector graphics mode, the mathematical functions or
formulas are used to represent a picture. The vector graphics can be
drawn on the screen using these mathematical formulas that can be
scaled up to any level without any distortion. But in raster
graphics mode, the picture is represented by an array of pixel
values or numbers that represent the color at the specified
location. The raster and vector mode are used or suitable for
different purposes. For example, the font library, geometrical
objects etc. can easily be represented in vector mode. But a picture
of a human, scenic view etc. can better be represented in raster
mode. Here we will discuss the methods to create and paste a bitmap
(a bitmap is a raster array which stores the pixel or color
information of a picture) on the computer screen. Before starting
the main discussion, let us see what is a Bitmap?
Bitmaps
A bitmap is an array of pixels, numbers which represents an image.
Bitmap structures are used to display images on most type of raster
device (like monitor, printer etc.). A bitmap contains information
about the size of image i.e. rows and columns, pointer to the pixel
data, and the color information in the image. The color is
represented by some number of bits in an image. The bits per pixel
is very important field to correctly draw the image on some raster
device. A 1-bit per pixel represents monochrome images that contain
only black and white colors. A 4-bits per pixel image can represent
at the most 16 distinct colors. Similarly 8-bits per pixel image can
represent 256 distinct colors. An image with 24-bits per pixel
represents the full range of colors i.e. 16.8 millions color. Such
images are called true-color images. When the bits per pixel are
less than or equal to 8, a color palette is used to store the number
of colors. Microsoft Windows uses two type of bitmap known as Device
Dependent Bitmap (DDB) and Device Independent Bitmap (DIB). The DDBs
were in use before the launch of Windows 3.0. Now DIBs are
frequently used to represent an image.
Device Dependent Bitmap (DDB)
The DDBs use the hardware palette to represent colors in the image.
But the inconsistency in the color palette was faced when the image
generated on one device was taken to another device. The colors were
different because the hardware palette of source and destination
device was different. To avoid such problem, DIBs were invented.
Device Independent Bitmap (DIB)
DIB has solved the problem of interoperability of bitmaps among
different raster device. Now the DIB contains the information of the
color format of the source device on which the DIB is created. It
also contains the complete palette information with it so that same
colors can be generated at the destination raster device. DIBs also
contain some other useful information that I don’t think is required
to be discussed here.
Steps required to create a DIB in Win32 environment
Step 1. Initialize the BITMAPINFO header.
typedef struct tagBITMAPINFO {
BITMAPINFOHEADER bmiHeader;
RGBQUAD bmiColors[1];
} BITMAPINFO;
typedef struct tagBITMAPINFOHEADER{
DWORD biSize;
LONG biWidth;
LONG biHeight;
WORD biPlanes;
WORD biBitCount
DWORD biCompression;
DWORD biSizeImage;
LONG biXPelsPerMeter;
LONG biYPelsPerMeter;
DWORD biClrUsed;
DWORD biClrImportant;
} BITMAPINFOHEADER;
As shown above, initialize the BITMAPINFO structure as per your
image parameters. This structure also contains the BITMAPINFOHEADER
structure. The fields are well understood and very clear from the
parameter name itself. The important point to note here while
initializing this structure is that if bit count or bits per pixel
is less than 8 then bmiColors[] array must be initialized properly.
The bmiColors[] represents the colors of the palette when bit count
is less than 8 otherwise it is not required. Secondly the biWidth
must me divisible by four. As internally it is represented by WORD
and it must finish at WORD boundaries. If your image width is not
multiple of four, you must provide padding while creating bitmap.
Step 2. Load the pixel data in a linear array of type unsigned
char *lpBits.
Step 3. Paste the bitmap on the computer screen
Use the following function to paste the bitmap on the screen at the
desired location.
int StretchDIBits(
HDC hdc, // handle to device context
int XDest, // x-coordinate of upper-left corner of dest. rectangle
int YDest, // y-coordinate of upper-left corner of dest. rectangle
int nDestWidth, // width of destination rectangle
int nDestHeight, // height of destination rectangle
int XSrc, // x-coordinate of upper-left corner of source rectangle
int YSrc, // y-coordinate of upper-left corner of source rectangle
int nSrcWidth, // width of source rectangle
int nSrcHeight, // height of source rectangle
CONST VOID *lpBits, // address of bitmap bits
CONST BITMAPINFO *lpBitsInfo, // address of bitmap data
UINT iUsage, // usage flags
DWORD dwRop // raster operation code
);
|
|
