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Windows GDI Programming

CIS 487/587

Bruce R. Maxim

UM-Dearborn

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Validating

• Whenever either you or Windows disturbs a window a WM_PAINT message is sent

• The area to be repainted is the invalid rectangle

• The coordinates of the invalid rectangle are shipped with the WM_PAINT message as part of the validating process

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Processing WM_PAINT

• BeginPaint( ) … EndPaint( )– Advantage: all needed validating done

automatically– Disadvantage: invalid rectangle is same as

clipping rectangle (can’t draw outside invalid rectangle)

– This approach fools Windows into believing you fixed the invalid rectangle whether you have drawn anything or not

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Processing WM_PAINT

• GetDC( ) … ReleaseDC( )– Advantage:you can draw outside the invalid

rectangle – Disadvantage: you need to validate every

rectangle manually using ValidateRect( )

• Note: With either method you need to be sure that windows does not fill in your Background to avoid losing your drawing

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Invalidating Entire Windowcase WM_PAINT:

{

// NULL is pointer to invalid rectangle

// meaning entire window

ValidateRect(hwnd,NULL);

// simply validate the window

hdc = BeginPaint(hwnd,&ps);

// fixes go here

EndPaint(hwnd,&ps); // end painting

  return(0); // return success

} break;

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Validating Entire Windowcase WM_PAINT:

{

// NULL is pointer to invalid rectangle

// meaning entire window

ValidateRect(hwnd,NULL);

// simply validate the window without

// doing any drawing

  return(0); // return success

} break;

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Avoid Repaintcase WM_PAINT:

{

hdc = GetDC(hwnd);

// do graphics here

ReleaseDC(hwnd,hdc);

// validate the window to clear message

ValidateRect(hwnd,hdc);

  return(0); // return success

} break;

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Display Surface Basics

• Resolutions in pixels640x480, 800x600, 1024x768, …

• Color depth (# color bits per pixel)6, 16, 24, 32

• Color space (actual # available colors)8 bits = 256 colors, 16 bits = 65536

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Display Modes

• RGB mode– True RGB values are encoded for each

pixel displayed

• Palletized mode– Color indirection scheme– 256 colors available– Value 0 to 255 is stored with each pixel– Color value looked up in CLUT

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Color Objects

• Windows uses to data structures– COLOREF: byte structure allows you to

create colors on the fly– PALETTEENTRY: all creation of pallets

with control flag components

• Complete control of pallets is best done using DirectX

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Drawing Text

• TextOut( )– No formatting or processing of text string is

done before it is displayed

• DrawText( )– Allows justification, clipping, etc. prior to

displaying text string

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LaMothe Examples

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TextOut Examplemain_window_handle = hwnd; // save main window handleHDC hdc = GetDC(hwnd); // get the dc and hold it

// set the foreground color to randomSetTextColor(hdc, RGB(rand()%256,rand()%256,rand()%256));// set the background color to blackSetBkColor(hdc, RGB(0,0,0));// finally set the transparency mode to transparentSetBkMode(hdc, TRANSPARENT); 

// draw some text at a random locationTextOut(hdc,rand()%400,rand()%400, "GDI Text Demo!", strlen("GDI Text Demo!")); 

ReleaseDC(hwnd,hdc); // release the dc

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What about WM_PAINT?case WM_PAINT:

{

// simply validate the window

hdc = BeginPaint(hwnd,&ps);

// you would do all your painting here

EndPaint(hwnd,&ps);

 

// return success

return(0);

} break;

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Additional Issues

• You should consider saving the old values for foreground, ground, and transparency so that you can restore them after you set them

• Example we might declare the following:COLORREF old_fcolor, // old foreground

old_bcolor; // old background

int old_tmode; // old transparency

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TextOut Example 2main_window_handle = hwnd; // save main window handle

HDC hdc = GetDC(hwnd); // get the dc and hold it

// save old attributes and set new

old_fcolor = SetTextColor(hdc,RGB(255,255,255));

old_bcolor = SetBkColor(hdc, RGB(0,0,0));

old_tmode= SetBkMode(hdc, TRANSPARENT); 

// draw some text at a random location

TextOut(hdc, 120, 200, “Hello", strlen(“Hello")); 

// Restore the old values

SetTextColor(hdc, old_fcolor);

SetBkColor(hdc, old_bcolor);

SetBkMode(hdc, old_tmode);

ReleaseDC(hwnd,hdc); // release the dc

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Text Metrics

• The function GetTextMetrics can be used to query the system for things like– Font height– Average width– Max width– First character defined in font– Last character defined in font

• Set of values returned in fields of a parameter of type tagTEXTMETRIC

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Drawing Shapes

• GDI allows you to draw things like– Points– Lines– Circles and ellipses– Rectangles– Curves– Polygons– Bitmaps– Metafiles (instructions that can be “replayed”)

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Pens

• Pens are GDI objects used to draw lines• Pens have two attributes

– RGB color– Styles like

• PS_DASH - - - -• PS_DOT . . . .• PS_DASHDOT _ . _ . • PS_NULL

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Using a Pen// get the graphics device context hdc = GetDC(hwnd);// create a red colored pen HPEN red_pen = CreatePen(PS_SOLID,1,RGB(255, 0, 0));// selecting pen and saving old value HPEN old_hpen = (HPEN)SelectObject(hdc,red_pen);

// draw with pen

// deleting pen SelectObject(hdc,old_hpen); DeleteObject(red_ pen);// release the device context ReleaseDC(hwnd,hdc)

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Brushes

• Brushes are used to fill in line objects

• Examples// creating a solid brush

HBRUSH blue_brush = CreateSolidBrush(RGB(0,0,255));

// creating a patterned brush

HBRUSH green_brush =

CreateHatchBrush(HS_DIAGCROSS, RGB(0,255,0));

// creating a bitmap brush (8x8)

HBRUSH bitmap_brush =

CreatePatternBrush(hBitMap);

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Selecting and Destroying Brushes

// select brush and save old brush

HBRUSH old_brush = SelectObject(hdc, red_brush);

// use the brush

// restore old brush

SelectObject(hdc, old_brush);

DeleteObject(red_brush);

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Plotting Points

hdc = GetDC(hwnd); // get the dc for the window

// draw 1000 pixelsfor (int index=0; index < 1000; index++) { // get random position int x = rand()%400; int y = rand()%300;

  COLORREF color = RGB(rand()%255,rand()%255,rand()%255);

SetPixel(hdc, x,y, color);} // end for index

ReleaseDC(hwnd, hdc); // release the dc

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Drawing Lines

// create pen

HPEN red_pen = CreatePen(PS_SOLID,1,RGB(255, 0, 0));

// selecting pen and saving old value

SelectObject(hdc,red_pen);

// move to starting point

MoveToEx(hdc,rand()%WINDOW_WIDTH,rand()%WINDOW_HEIGHT,NULL);

// draw the line

LineTo(hdc,rand()%WINDOW_WIDTH,rand()%WINDOW_HEIGHT);

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Drawing Rectangleshdc = GetDC(hwnd); // get the graphics device context  RECT rect; // used to hold rect info// create a random rectanglerect.left = rand()% WINDOW_WIDTH; rect.top = rand()% WINDOW_HEIGHT;rect.right = rand()% WINDOW_WIDTH;rect.bottom = rand()% WINDOW_HEIGHT;// create a random brushHBRUSH hbrush = CreateSolidBrush(RGB(100, 50, 80));// draw either a filled rect or a wireframe rectif ((rand()%2)==1) FillRect(hdc,&rect,hbrush);else FrameRect(hdc,&rect,hbrush);DeleteObject(hbrush); // now delete the brushReleaseDC(hwnd,hdc);// release the device context

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Drawing Circles and Ellipses

• Circle is special case of ellipse• To draw a circle at (100, 100) with diameter 20

Ellipse(hdc, 90, 90,110, 110);

• To draw an ellipse centered at (100, 100) with major axis 100 and minor axis 60Ellipse(hdc, 100-50, 100-30, 100+50, 100+30);

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Bouncing Ball

• An old computer animators trick is to– Draw an object using the foreground color– Erase object by redrawing in the

background color– Change the object screen coordinates and

draw again in the foreground color

• We can make use of this trick and use Ellipse to draw a bouncing ball

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Bouncing Ball

hdc = GetDC(hwnd); // create the pens and brushesHPEN white_pen = CreatePen(PS_SOLID, 1, RGB(255,255,255));HPEN black_pen = CreatePen(PS_SOLID, 1, RGB(0,0,0));HBRUSH green_brush = CreateSolidBrush(RGB(0,255,0));HBRUSH black_brush = CreateSolidBrush(RGB(0,0,0));

// starting position of ballint ball_x = WINDOW_WIDTH/2;int ball_y = WINDOW_HEIGHT/2;

// initial velocity of ballint xv = -4+rand() % 8;int yv = -4+rand() % 8;

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Bouncing Ball// ERASE the last position of the ball// first select the black pen and brush into contextSelectObject(hdc, black_pen);SelectObject(hdc, black_brush);// draw the ballEllipse(hdc, ball_x, ball_y, ball_x + 32, ball_y + 32);// MOVE the ballball_x+=xv;ball_y+=yv;// test for collisions, first x-axisif (ball_x < 0 || ball_x > WINDOW_WIDTH - 32){ // invert x-velocity of ball xv=-xv; // push ball back ball_x+=xv;  } // end if else // test for y-axis collisions if (ball_y < 0 || ball_y > WINDOW_HEIGHT - 32) { // invert y-velocity of ball yv=-yv;  // push ball back ball_y+=yv;  } // end if  // now select the green and white colors for brush and pen SelectObject(hdc, white_pen); SelectObject(hdc, green_brush);  // DRAW the ball Ellipse(hdc, ball_x, ball_y, ball_x + 32, ball_y + 32);  // main game processing goes here if (KEYDOWN(VK_ESCAPE)) SendMessage(hwnd, WM_CLOSE, 0,0);  // slow system down a little Sleep(10);

} // end while // delete all the objectsDeleteObject(white_pen);DeleteObject(black_pen);DeleteObject(green_brush);DeleteObject(black_brush); // release the device contextReleaseDC(hwnd,hdc);

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Bouncing Ball

// ERASE the last position of the ball

// first select the black pen and brush into context

SelectObject(hdc, black_pen);

SelectObject(hdc, black_brush);

// draw the ball

Ellipse(hdc, ball_x, ball_y, ball_x + 32, ball_y + 32);

// MOVE the ball

ball_x+=xv;

ball_y+=yv;

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Bouncing Ball

// test for collisions, first x-axisif (ball_x < 0 || ball_x > WINDOW_WIDTH - 32) { xv=-xv; // invert x-velocity of ball ball_x+=xv; // push ball back } // end ifelse // test for y-axis collisions if (ball_y < 0 || ball_y > WINDOW_HEIGHT - 32) { yv=-yv; // invert y-velocity of ball ball_y+=yv; // push ball back } // end if

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Bouncing Ball

// select the green and white colors for brush and penSelectObject(hdc, white_pen);SelectObject(hdc, green_brush);

// DRAW the ballEllipse(hdc, ball_x, ball_y, ball_x + 32, ball_y + 32);

DeleteObject(white_pen); // delete all the objectsDeleteObject(black_pen); DeleteObject(green_brush);DeleteObject(black_brush); 

ReleaseDC(hwnd,hdc); // release the device context