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Lecture No.4 Point
Pixel: The smallest dot illuminated that can be seen on screen.
Picture: Composition of pixels makes picture that forms on whole screen
Resolution We know that Graphics images on the screen are built up from tiny dots called picture elements or pixels. The display resolution is defined by the number of rows from top to bottom, and number of pixels from left to right on each scan line.
Since each mode uses a particular resolution. For example mode 19 uses a resolution of 200 scan lines, each containing 320 pixels across. This is often referred to as 320* resolution. In general, higher the resolution, more pleasing is the picture. Higher resolution means a sharper, clearer picture, with less pronounced ‘staircase’ effect on lines drawn diagonally and better looking text characters. On the other hand, higher resolution also means more memory requirement for the display.
Text and Graphics Modes We discussed different video hardware devices that include VGA cards and monitors. Video cards are responsible to send picture data to monitor each time it refresh itself. Video cards support both different text and graphics modes. Modes consist of their own refresh rate, number of colors and resolutions (number of rows multiply by number of columns). The following famous video modes that we can set in today’s VGA cards on different refresh rate:
25 * 80 with 16 colors support (text mode) 320 * 200 with 8 bit colors support (graphics mode) 640 * 480 with 16 colors support (graphics mode) 640 * 480 with 8, 16, 24, 32 bit color support (graphics mode) 800 * 600 with 8, 16, 24, 32 bit color support (graphics mode)
Text and Graphics All modes are fundamentally of two types, text or graphics. Some modes display only text and some are made only for graphics. As seen earlier, the display adapter continuously dumps the contents of the VDU (video display unit) memory on the screen.
The amount of memory required representing a character on screen in text mode and a pixel in graphics mode varies from mode to mode.
Mode No. Type Resolution Memory Required 3 Text 80 x 25 2 bytes per char 6 Graphics 640 x 200 1 bit per pixel 7 Text 80 x 25 2 bytes per char 18 Graphics 640 x 480 1 bit per pixel 19 Graphics 320 x 200 1 byte per pixel
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In mode 6 each pixel displayed on the screen occupies one bit in VDU memory. Since this bit can take only two values, either 0 or 1, only two colors can be used with each pixel.
How text displays
As seen previously text modes need two bytes in VDU memory to represent one character on screen; of these two bytes, the first byte contains the ASCII value of the character being displayed, whereas the second byte is the attribute byte. The attribute byte controls the color in which the character is being displayed.
The ASCII value present in VDU memory must be translated into a character and drawn on the screen. This drawing is done by a character generator this is part of the display adapter or in VBIOS. The CGA has a character generator that uses 8 scan lines and 8 pixels in each of these scan lines to produce a character on screen; whereas the MA’s character generator uses 9 scan lines and 14 pixels in each of these scan lines to produce a character. This larger format of MA makes the characters generated by MA much sharper and hence easier to read.
On older display adapters like MA and CGA, the character generator is located in ROM (Read Only Memory). EGA and VGA do not have a character generator ROM. Instead, character generator data is loaded into plane 2 of display RAM. This feature makes it easy for custom character set to be loaded. Multiple character sets (up to 4 for EGA and up to 8 for VGA) may reside in RAM simultaneously.
A set of BIOS services is available for easy loading of character sets. Each character set can contain 256 characters. Either one or two character sets may be active giving these adapters on the screen simultaneously. When two character sets are active, a bit in each character attribute byte selects which character set will be used for that character.
Using a ROM-BIOS service we can select the active character set. Each character in the standard character set provided with the EGA is 8 pixels wide and 14 pixels tall. Since VGA has higher resolution, it provides a 9 pixel wide by 16 pixels tall character set. Custom character set can also be loaded using BIOS VDU services.
The graphics modes can also display characters, but they are produced quite differently. The graphics modes can only store information bit by bit. The big advantage of this method is that you design characters of desired style, shape and size.
Text mode colors
In mode 3, for each character on screen there are two bytes in VDU memory, one containing the ACCII value of the character and other containing its attribute. The attribute byte controls the color of the character. The attribute byte contains three components: the foreground color (color of the character itself), the background color (color of the area not covered by the character) and the blinking component of the character. The next slide shows the breakup of the attribute byte.
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MOV AH,
MOV AL,13h ;;mode number from 0- INT 10H
To insert in C language above code will be inserted with key word asm and curly braces. asm{ MOV AH, MOV AL,13h ;;mode number from 0- INT 10H }
Description Line #1: mov ah,0 is the service number for setting video mode that is in register ah Line #2: mov al,13h is the mode number that is in register al Line #3: int 10h is the video bios interrupt number that will set mode 13h
Source code for writing pixel The following code can be used to write pixel using video bios interrupt 10h and service number 0ch.
MOV AH,0Ch MOV AL,COLOR_NUM MOV BH, MOV CX,ROW_NUM MOV DX,COLUMN_NUM INT 10h
Description Line#1: service number in register Ah Line#2: color value, since it is 13h mode so it has 0-255 colors range. You can assign any color number from 0 to 255 to all register. Color will be selected from default palette setting against the number you have used. Line#3: page number in Bh register. This mode supports only one page. So 0 is used in Bh register. 0 mean default page. Line#4: column number will be used in CX register Line#5: row number will be used in DX register Line#6: BIOS interrupt number 10h
Writing pixel by accessing memory directly So far we used BIOS to draw pixel. Here we will draw pixel by accessing direct pointer to the video memory and write color value. The following steps are involved to write direct pixel without using BIOS:
Set video mode by using video BIOS routine as discussed earlier Set any pointer to the video graphics memory address 0x0A0000. Now write any color value in the video memory addressing
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Direct Graphics Memory Access Code
Mov ax,0a000h Mov ds,ax ;;segment address changed Mov si,10 ;; column number Mov [si],COLOR_NUM
Work to do: Write pixel at 12th row and 15th column Hint: use formula (row * 320 + column) in si register.
Writing character directly on screen You can also write direct text by setting any text mode using BIOS service and then setting direct pointer at text memory address 0x0b8000.
Example Set mode Number 3. using BIOS service and then use this code to write character
Mov ax,0b8000h Mov ds,ax Mov si,10 ;;column number Mov [si],’a’ ;;character to write
Using Library functions While working in C language, you can use graphics library functions to write pixel on screen. These graphics library functions then use BIOS routines or use direct memory access drivers to draw pixel on screen.
initgraph(&gdriver, &gmode, ""); /* read result of initialization / errorcode = graphresult(); if (errorcode != grOk) / an error occurred / { printf("Graphics error: %s\n", getch()); exit(1); / return with error code / } / draw a pixel on 10th row and 10 column / putpixel(10, 10, BLUE); / clean up */ closegraph();
Steps in C language First call Initgraph() function and then call putpixel() function to draw pixel on screen. It takes row, column and color value as parameters. after drawing pixel use closegraph() function to close the graphics routines provided by built in driver by Borland.
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// TODO: Place code here. MSG msg; HACCEL hAccelTable;
// Initialize global strings LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING); LoadString(hInstance, IDC_A, szWindowClass, MAX_LOADSTRING); MyRegisterClass(hInstance);
// Perform application initialization: if (!InitInstance (hInstance, nCmdShow)) { return FALSE; }
hAccelTable = LoadAccelerators(hInstance, (LPCTSTR)IDC_A);
// Main message loop: while (GetMessage(&msg, NULL, 0, 0)) { if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg)) { TranslateMessage(&msg); DispatchMessage(&msg); } }
return msg.wParam; }
// FUNCTION: MyRegisterClass() // // PURPOSE: Registers the window class. // // COMMENTS: // // This function and its usage is only necessary if you want this code // to be compatible with Win32 systems prior to the 'RegisterClassEx' // function that was added to Windows 95. It is important to call this function // so that the application will get 'well formed' small icons associated // with it. // ATOM MyRegisterClass(HINSTANCE hInstance) { WNDCLASSEX wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
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wcex.lpfnWndProc = (WNDPROC)WndProc; wcex.cbClsExtra = 0; wcex.cbWndExtra = 0; wcex.hInstance = hInstance; wcex.hIcon = LoadIcon(hInstance, (LPCTSTR)IDI_A); wcex.hCursor = LoadCursor(NULL, IDC_ARROW); wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1); wcex.lpszMenuName = (LPCSTR)IDC_A; wcex.lpszClassName = szWindowClass; wcex.hIconSm = LoadIcon(wcex.hInstance, (LPCTSTR)IDI_SMALL);
return RegisterClassEx(&wcex); }
// // FUNCTION: InitInstance(HANDLE, int) // // PURPOSE: Saves instance handle and creates main window // // COMMENTS: // // In this function, we save the instance handle in a global variable and // create and display the main program window. // BOOL InitInstance(HINSTANCE hInstance, int nCmdShow) { HWND hWnd;
hInst = hInstance; // Store instance handle in our global variable
hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
if (!hWnd) { return FALSE; }
ShowWindow(hWnd, nCmdShow); UpdateWindow(hWnd);
return TRUE; }
// // FUNCTION: WndProc(HWND, unsigned, WORD, LONG) // // PURPOSE: Processes messages for the main window. // // WM_COMMAND - process the application menu
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case WM_DESTROY: PostQuitMessage(0); break; default: return DefWindowProc(hWnd, message, wParam, lParam); } return 0; } // Mesage handler for about box. LRESULT CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam) { switch (message) { case WM_INITDIALOG: return TRUE;
case WM_COMMAND: if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL) { EndDialog(hDlg, LOWORD(wParam)); return TRUE; } break; } return FALSE; }
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