Graphics subsystem is a complex software and hardware stack in our personal computer. Clear understanding of this subsystem is often necessary before starting graphics programming. A VGA capable graphics card is a second processor after CPU. Its job is to convert image frame from frame buffer address 0xA0000 to analog VGA signal frames. It is basically a digital to analog converter. It converts 2D pixel array bytes to pixel scan frame and feeds this VGA analog signal to output monitor. It can work in character mode. Character mode works in frame address 0xB8000. Host driver transfers 80x25 character matrix and VGA card puts font image bitmap for each characters. Output of this frame looks character console with 80 characters columns in each lines and total 25 lines.
VGA card has I/O registers and memory mapped frame buffer. Host side graphics program or graphics library can access I/O registers to control many operations. Mode switching and accessing cursor and mouse pointer are the most often used control operations. Frame buffer hardware memory is mapped from 0xA0000 to 0xBFFFF physical address. This is graphics card memory shared between host CPU. Graphics card operates in two modes. Text mode is the simplest one. It consists of a character matrix of 80x25. One character in VGA buffer is represented by an ASCII byte and next byte contains 4bit foreground and 4bit background color. So total 80x25x2 bytes are consumed from 0xB8000.
Graphic mode is different form console mode. It has 640x480 pixels and each pixel consists of 16 colors. 16 colors can be represented by 4bits. VGA splits color planes in 4 different planes. Host driver selects one plane at a time and sets respective bits. It has to access all 4 planes and has to set all respective color bits to complete one pixel operation. VGA card converts pixel bits to color signals and constructs VGA frames. These VGA analog frames then feed to display monitor.
There are many ways to interruct with a VGA graphics card. Application can directly access VGA I/O registers and framebuffer. Else it can use BIOS software interrupts or it can use Turbo C BGI graphics driver. BGI driver can talk to BIOS subsystem as well.
What is graphics.h in C? C Graphics library and driver?
Graphics in C is mainly VGA drawing program runs under DOS operating system. TurboC, Borland C are some compilers that provide a software stack for developing graphics-based programs. This graphics programs run under DOS or virtual mode in Windows. These programs should not be confused with Windows graphics programs. They are developed with Win32 APIs and graphics subsystem provided by DirectX, openGL or DRM.
A graphics program should include graphics.h using #include
Now the linking process starts with the Object. Graphics.h file contains the prototypes of the graphics routines but the actual definitions are not included. The definition of these graphics calls are provided by the static library file graphics.lib. Graphics.lib are provided by TurboC and located in standard library folder \TurboC\Lib. The linker resolves the definitions and generates an executable file (.exe).
This final executable is DOS executable and requires DOS operating system to run. DOS shell can be run under Win95/98/XP and virtual mode in Windows 8./8.1/10. DosBox, Virtual box and QMU are some alternatives for running DOS OS inside Windows. This executable should be run under DOS shell and during running it looks for DOS runtime driver EGAVGA.BGI graphics driver. The library routine initializes the graphics mode via this graphics library located at \TurboC\BGI folder. Library call returns successful if driver file is loaded from this path. After this graphics library switches the video mode to VGA and handles all the rendering routines.
Turbo C graphics program should start with initializing graphics mode with initgraph. This function must be called in a graphics program. A successful initialization puts output console to VGA graphic mode. VGA mode consists of 640x480 pixels with 16 colors for each pixel. initgraph initializes the graphics system by loading a graphics driver from disk (or validating a registered driver) then putting the system into graphics mode. initgraph also resets all graphics settings (color, palette, current position, viewport, etc.) to their defaults, then resets graphresult to 0.
void initgraph(int *graph_driver, int *graph_mode, const char *pathtodriver);
- graph_driver: Integer that specifies the graphics driver to be used. You can give graphdriver a value using a constant of the graphics drivers enumeration type ( DETECT, /* requests autodetection */ CGA, MCGA, EGA, EGA64, EGAMONO, IBM8514,HERCMONO, ATT400, VGA, PC3270,CURRENT_DRIVER)
- *graph_mode : Integer that specifies the initial graphics mode (unless *graphdriver = DETECT). If *graphdriver = DETECT, initgraph sets *graphmode to the highest resolution available for the detected driver. You can give *graphmode a value using a constant of the graphics_modes enumeration type ( CGA MCGA EGA EGAMONOHI, HERCMONOHI, ATT VGA PC3270 IBM8514)
- pathtodriver : Specifies the directory path where initgraph looks for graphics drivers (*.BGI) first. If they're not there, initgraph looks in the current directory. If pathtodriver is null, the driver files must be in the current directory. This is also the path settextstyle searches for the stroked character font files (*.CHR).
Closegraph does the termination of graphics mode and puts console mode again to text mode. A graphics program should have a closegraph function at the end of graphics. Otherwise DOS screen will not go to text mode after running the program.
void closegraph(void);Here, closegraph() is called after getch() since screen should not clear until user hits a key. Additional functions:
Graph Error handling
Above program does not contain any error handling cases. It may possible that initgraph may fail. There could be many reasons and one of the main reason is the absence of EGAVGA.BGI runtime library driver. graphresult returns the error code which graph subsystem encountered in the last call. An equivalent error string for each error code can be obtained passing the error code to grapherrormsg. It returns a human understandable error message for the error code. This is necessary to explain the error to the user.
initgraph error -3 Device driver file not found (EGAVGA.BGI)
To run this program, you need graphics.h header file, graphics.lib library file and Graphics driver (TC\BGI\EGAVGA.BGI file) in the program folder. These files are part of Turbo C package. In all our programs we used 640x480 VGA monitor and graphics driver used is EGAVGA.BGI. So all the programs are according to that specification. You need to make necessary changes to your programs according to your screen resolution.
Here is list of error code and their explanations
- grOk 0 No error
- grNoInitGraph -1 (BGI) graphics not installed
- grNotDetected -2 Graphics hardware not detected
- grFileNotFound -3 Device driver file not found (EGAVGA.BGI)
- grInvalidDriver -4 Invalid device driver file (EGAVGA.BGI)
- grNoLoadMem -5 Not enough memory to load driver
- grNoScanMem -6 Out of memory in scan fill
- grNoFloodMem -7 Out of memory in flood fill
- grFontNotFound -8 Font file not found ()
- grNoFontMem -9 Not enough memory to load font
- grInvalidMode -10 Invalid graphics mode for selected driver
- grError -11 Graphics error
- grIOerror -12 Graphics I/O error
- grInvalidFont -13 Invalid font file ()
- grInvalidFontNum -14 Invalid font number
- grError15 -15 Graphics error (65521)
- grError16 -16 Invalid Printer Initialize
- grError17 -17 Printer Module Not Linked
- grInvalidVersion -18 Invalid File Version Number
Graphic mode in Turbo C has maximum 16 colors. Color range depends on the mode. Other modes can have 2, 4, 8, 256 colors. Color values are defined as enum COLOR and the values are-
Line drawings are the fundamental steps for any simple graphic objects. Graphic library provides standalone line drawing as well as drawing outer lines of basic shapes. We can modify line style and width. Here are the list of possibilities. Line Style:
- SOLID_LINE = 0
- DOTTED_LINE = 1
- CENTER_LINE = 2
- DASHED_LINE = 3
- USERBIT_LINE = 4
- NORM_WIDTH = 1
- THICK_WIDTH = 3
Texture and filling
Graphical shapes and background can be filled with color and textures. Graphic library provides fill routines to do this work. Below are the list of fill patterns
- EMPTY_FILL - fills area in background color
- SOLID_FILL - fills area in solid fill color
- LINE_FILL - fills area with horizontal lines/
- LTSLASH_FILL - fills area with /// like lines
- SLASH_FILL - fills area with /// like thick lines
- BKSLASH_FILL - fills area with \\\ like thick lines
- LTBKSLASH_FILL - fills area with \\\ like lines
- HATCH_FILL -light hatch fill
- XHATCH_FILL - heavy cross hatch fill
- INTERLEAVE_FILL - interleaving line fill
- WIDE_DOT_FILL - Widely spaced dot fill
- CLOSE_DOT_FILL - Closely spaced dot fill
- USER_FILL - user defined fill
Graphic library also provides Text display with outtext. Text display requires fonts to be installed and some fonts are installed by default. Font names:
- DEFAULT_FONT = 0,
- TRIPLEX_FONT = 1,
- SMALL_FONT = 2,
- SANS_SERIF_FONT = 3,
- GOTHIC_FONT= 4,
- SCRIPT_FONT= 5,
- SIMPLEX_FONT = 6,
- TRIPLEX_SCR_FONT = 7,
- COMPLEX_FONT = 8,
- EUROPEAN_FONT = 9,
- BOLD_FONT = 10
- LEFT_TEXT = 0,
- CENTER_TEXT = 1,
- RIGHT_TEXT = 2,
- TOP_TEXT = 2
- HORIZ_DIR 0
- VERT_DIR 1
Font size can be given from 1 to n. User defined fonts takes USER_CHAR_SIZE 0.
Circle, ellipse, rectangle, cube etc are basic shapes. Function names and their arguments are self explanatory.
Bar chart, 3D bar chart, poly shape, pie chart, sectors etc are extended shapes. Function names and their arguments are self explanatory.
Image bit block transfer functions are to copy image from one location to another. getimage function copies image to byte stream and putimage transfers byte image to screen location. Bitblock image putting operations are -
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