Graphical Device Interface GDI

1
Graphical Device Interface (GDI)

• High level interface to graphics engine.
• Functions to
• Obtain and release device contexts
• Obtain (e.g., GetTextMetrics) and set attributes
  of device contexts (SetTextColor, SetTextAlign).
• That work with GDI objects (e.g., logical pens,
  logical brushes).
• Primitives for drawing lines, curves, filled
  areas, bitmaps, Text, etc.
• To reduce complexity, many graphics attributes
  are contained in device context.

2
Color

• Three primary colors are Red, Green, and Blue.
• Each pixel has color based on the intensity of
  each color.
• full color display 24 bits per pixel. R8 G8
  B8.
• Can display 224th 16 Million.
• high color 16 bits per pixel. R5 G6 B5
• Can display 216th 64K colors.
• How many bits per pixel for 16 colors? 4.
• 24th 16.
• For 256 colors need 8 bits per pixel. (28th
  256).

3
Color Pallet
0
8 bit value
256
4
31..24 23..16 15..8 7..0 B G R
Predefined Macro RGB(0-255,0-255,0-255)
RGB(R,G,B) (0-255) RGB(255,255,255) White
RGB(0,0,0) Black RGB(255,255,0) Yellow
5
SetPixel(hdc, x, y, RGB(0,0,0) Color
GetPixel(hdc, x, y) Lines Drawn from current
position maintained in DC. Want line from
startx, starty to endx, endy POINT pt
MovetoEx(startx, starty, pt) LineTo(hdc,
endx, endy) Now current position is endx,
endy.
6
Filled functions Filled with current
area-filling brush. Rectangle(hdc, xleft, ytop,
xright, ybottom)
xleft xright
ytop ybottom
Filled with Current brush
7
Filling Brush

• Not the brush associated with class
• Wndclass.hbrBackground .
• Can change brush, need handle to brush
• HBRUSH hBrush
• Several predefined
• LTGRAY_BRUSH
• GRAY_BRUSH
• BLACK_BRUSH
• NULL_BRUSH

8
HBRUSH

• Not attribute of Device Context.
• Selected into DC.
• Hdc BeginPaint(hwnd, ps)
• hBrush GetStockObject(BLACK_BRUSH)
• OldBrush SelectObject(hdc, hBrush)
• Rectangle(hdc, xleft, ytop, xright, ybottom)
  
• Rectangle will be filled with black.

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Create Brush

• hBrush CreateSolidBrush(RGB(x,x,x))
• SelectObject(hdc, hBrush)
• Also create background out of various patterns
• hBrush CreateHatchBrush(Style, RGB(x,x,x))
• SelectObject(hdc, hBrush)
• Style HS_HORIZONTAL, HS_VERTICAL, ..
• DeleteObject(hBrush)

10
Logical Pens

• current pen determines appearance of lines
• color, width, style.
• Default Black Pen one pixel wide.
• White Pen, Null Pen
• Need handle to a pen HPEN hPen, oldPen
• hPen GetStockObject(WHITE_PEN)
• oldPen SelectObject(hdc, hPen)

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Create Logical Pen

• HPEN hPen
• hPen CreatePen(Penstyle, width, RGB(x,x,x))
• Penstyle
• PS_SOLID, PS_DASH, PS_DOT, PS_DASHDOT,
• PS_NULL
• Width
• 0 1 pixel. If style is dotted or dashed, and
  width gt 1, will draw solid.
• Must select into device context.

12
Raster Operations

• Appearance of lines is function of pen and
  drawing mode.
• Windows performs a bitwise Boolean operation
  between pixels of pen and pixels of display
  surface.

13
Raster operation.

• Since involves only two pixel patterns is binary
  raster operation or ROP2.
• Drawing mode is attribute of device context.

14
Drawing Modes

• Default R2_COPYPEN. Copies from pen onto
  display.
• Also
• R2_COPYBLACK Draws black line regardless of
  color in pen or display.
• R2_NOP Leaves display unchanged regardless of
  color of pen or display.

15
P 1 1 0 0 Boolean Op Drawing Mode D 1
0 1 0
0 0 0 0
0 R2_BLACK
1 1 1 1
1 R2_WHITE
0 0 1 1
P R2_NOTCOPYPEN
0 0 0 1
(P D) R2_NOTMERGEPEN
D R2_NOT
0 1 0 1
P R2_COPYPEN
1 1 0 0
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Setting Mode

• SetROP2(hdc, mode)