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_ TUTORIAL: BitBlt & GDI32 for the Thick-Headed: BitBlt explained, + Load Sprites, Double Buffering!

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Submitted on: 1/26/2015 12:53:00 AM
By: (Tim Miron) yar-interactive software (from psc cd)  
Level: Intermediate
User Rating: By 71 Users
Compatibility: VB 4.0 (32-bit), VB 5.0, VB 6.0, VB Script
Views: 8956
 
     "BitBlt [and GDI32] for the Thick Headed" is an in-depth, plain-english, streight-forward tutorial that teaches everything there is to know about BitBlt, and some related API. It covers everything from loading bitmap files into memory (no more picture boxes) to double-buffering (say goodbye to the SLOW 'AutoRedraw'!) to Explaining the BitBlt API in PLAIN ENGLISH. PLUS the tutorial only takes about 15 to 20 minutes to complete! (A completed version of the example project is also included)

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BitBlt for the Thick-Headed...
    In this tutorial you will learn...

  • Howto load a bitmap file into memory
  • Howto create a back-buffer
  • The BitBlt API explained in plain english
  • Howto use double-buffering techniques to achieve fast, flickerless graphics (faster then AutoRedraw)

    This tutorial should NOT take more then 15 minutes to read - If you read every line and follow every instruction, almost everything you need to know [about BitBlt] is here!


Welcome to my tutorial which I have titled BitBlt for the Thick Headed. If you want to go through this tutorial quickly, all the essential parts are in BOLD. For the record, I mean no offence to anyone on the PSC community, I was going to call it BitBlt for Dummies like the popular For Dummies books, but didn't want to get into copyright complications with book publishers. The goal of this tutorial is to step-by-step explain howto use BitBlt and some other Win32 GDI functions, to accomplish tasks such as double buffering and loading sprites from files - All in a relatively short reading-time (basically i'll try not to ramble on too much) Anyways, let's get started...

The first thing your going to do obviously is create a form (so you can follow along with this tutorial), set the ScaleMode to '3 - Pixel', I suggest you always set the scalemode to Pixels if your going to be using the form with API.

Next Increase the form's size until the ScaleWidth is 320, and the ScaleHeight is 256. We will be using the form as our practice surface, note that the form property called "HasDC" must be set to TRUE. Also, for many of you who fell in love with using AutoRedraw, we will NOT need AutoRedraw because we are going to be using Double Buffering which is ALOT faster, and more professional.

The next step is to declare the API calls that we will need, as shown below. So copy and paste the code below into your form. If you dont know what API is, then you should do some research about it, before you even try to figure this tutorial out! ;-)

'The following API calls are for:

'blitting
Private Declare Function BitBlt Lib "gdi32" (ByVal hDestDC As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal dwRop As Long) As Long

'code timer
Private Declare Function
GetTickCount Lib "kernel32" () As Long

'creating buffers / loading sprites
Private Declare Function CreateCompatibleBitmap Lib "gdi32" (ByVal hdc As Long, ByVal nWidth As Long, ByVal nHeight As Long) As Long
Private Declare Function
CreateCompatibleDC Lib "gdi32" (ByVal hdc As Long) As Long
Private Declare Function
GetDC Lib "user32" (ByVal hwnd As Long) As Long

'loading sprites
Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long

'cleanup
Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
Private Declare Function
DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long

'end of copy-paste here...


Q. What is a DC (also known as: Device Context, hDC)?
A. A Device Context is a number that points to an "address" in memory where data is stored, when using BitBlt, we point to the Address where graphical data is stored in memory.

Next, we need to store the addresses of the DC's that we are creating. DC's addresses are Long values so we will Declare Public Variables to store the DC's memory address as shown below. (copy and paste
'our Buffer's DC
Public myBackBuffer As Long
Public
myBufferBMP As Long

'The DC of our sprite/graphic
Public mySprite As Long

'coordinates of our sprite/graphic on the screen
Public SpriteX As Long
Public
SpriteY As Long

'end of copy-paste here...


Q. Do we have to make all these variable's public?
A. NO, BUT its a good idea to at least make the DC's variables public so that its easier to cleanup the memory after the program is finished.

Now we have the foundation of our code, we have all the API declarations we'll be needing, and all the variables we'll be using in this example. The next thing we're gunna do is create a function that loads graphics into memory, it makes working with the API a bit simpler...
    Device Contexts - TIP: One thing that is important to understand is that a device context alone has no graphical data in it. A device context needs to have a bitmap loaded into it, whether that be a bitmap file, or a blank bitmap to use as a canvas to draw on (which is how you create a back buffer).
Ok, so this is what our function does... It creates a Device Context compatible with the screen, it then loads the specified graphics file into the device context... Copy and paste the function below, but be sure to read all the comments so you understance the concept.
Public Function LoadGraphicDC(sFileName As String) As Long
'cheap error handling
On Error Resume Next

'temp variable to hold our DC address
Dim LoadGraphicDCTEMP As Long

'create the DC address compatible with
'the DC of the screen
LoadGraphicDCTEMP = CreateCompatibleDC(GetDC(0))

'load the graphic file into the DC...
SelectObject LoadGraphicDCTEMP, LoadPicture(sFileName)

'return the address of the file
LoadGraphicDC = LoadGraphicDCTEMP
End Function

'end of copy-paste here...


Q. What is double-buffering?
A. Double Buffering is when you create a graphical surface to paint on (like a canvas) that you blit all of your sprites/graphics/text onto in the memory (offscreen) then blit the final result onto the screen. It prevents flickering (which occurs if multiple sprites are blitted directly onto the screen.) and is much faster then AutoRedraw.

We're gunna be using the function in our example code... but before we go any further with the example project, I'm going to explain the BitBlt API from start to finish.



The BitBlt API...

BitBlt is a function in the DLL "gdi32".
    Technical Deffinition: it performs a bit-block transfer of the color data corresponding to a rectangle of pixels from the specified source device context into a destination device context. In Plain English... This basically means that it copys graphical data from one graphics surface [a bitmap] to another graphics surface [the screen, or a form].
Now lets take a look at the API declaration itself... The API declaration should be placed in the "General Declarations" section of a form or module. Here's what it looks like:
    Declare Function BitBlt Lib "gdi32" Alias "BitBlt" _
    (ByVal hDestDC As Long, _
    ByVal x As Long, _
    ByVal y As Long, _
    ByVal nWidth As Long, _
    ByVal nHeight As Long, _
    ByVal hSrcDC As Long, _
    ByVal xSrc As Long, _
    ByVal ySrc As Long, _
    ByVal dwRop As Long) As Long
The first part of this code, the first line (in this example) says that we're accessing the BitBlt function from the gdi32 DLL. the following lines are parameters that we have to input in order to use the function in our program. Here's a rundown of what each of these parameters is:

  • hDestDC - The hDC of the destination surface (this could be a form.hDC if you want to blit to a form, or it could be the address of a backbuffer that we've created).
  • x - The X (horizontal position) coordinate of where we want the graphic to appear.
  • y - The Y (vertical position) coordinate of where we want the graphic to appear.
  • nWidth - The width of our graphic.
  • nHeight - The height of our graphic.
  • hSrcDC - The hDC of the source graphic, for example the DC address of a sprite that we loaded into memory.
  • xSrc - The X (horizontal) offset, 0 if you want to blit from the very left edge of the source graphic, if you want to start the blit from 18 and over then you would make this value 18, etc.
  • ySrc - The Y (vertical) offset, same idea as xSrc, except vertically
  • dwRop - The drawmode we want to use when blitting our graphic, also known as Raster Operations or ROPs. This parameter is explained below.

    The drawmodes, or Raster Operations/ROPs available are as follows, each of these is a reserved constant in VB, so any one of these words (in italic) can be used in the dwRop parameter to acheive different effects: <-- Blt Modes -->
    • vbSrcCopy - Copy the source image data directly onto the destination, replacing it completely.
    • vbSrcPaint - ORs the source and destination image data, giving a pseudo-alphablending effect.
    • vbSrcAnd - ANDs the source and destination image data, giving a pseudo-gamma effect.
    • vbSrcInvert - XORs the source and destination image data.
    • vbSrcErase - Inverts the destination image data then ANDs with the source image data.
    • vbDstInvert - Inverts the destination image data, and ignores the source image data completely.
    • vbNotSrcCopy - Inverts the source image data and copies directly onto the destination, replacing it completely.
    • vbNotSrcErase - ORs the source and destination image data and inverts the result.

    'An example of using BitBlt
      BitBlt Form1.hDC, PlayerX, PlayerY, 48, 48, picPlayer.hDC, 0, 0, vbSrcCopy


    On with our example project...
    Next in our example project (this is the final part), we're going to use BitBlt in a loop much like you would in a game. Here's what you need to do:
    • Save the project file (and form file) in its own Directory.

    • Create a bitmap (BMP) file, make it 32 X 32 pixels. And save it in the same directory as the project.

    • NAME THE BMP FILE "sprite1.bmp"

    • Create a command button, rename it to cmdTest.

    • Move the command button to the bottom right of the form.

    • Double click on the command button to bring-up its sub in the code-window, so we can enter code to be executed when it is pushed.


    Copy and paste this code into the command button's Click-Event subroutine.
    READ ALL THE COMMENTS, to understand the code...

    '=== THIS CODE GOES IN CMDTEXT_CLICK EVENT ===

    'Timer variables...
    Dim T1 As Long, T2 As Long

    'create a compatable DC for the back buffer..
    myBackBuffer = CreateCompatibleDC(GetDC(0))

    'create a compatible bitmap surface for the DC
    'that is the size of our form.. (320 X 256)
    'NOTE - the bitmap will act as the actual graphics surface inside the DC
    'because without a bitmap in the DC, the DC cannot hold graphical data..

    myBufferBMP = CreateCompatibleBitmap(GetDC(0), 320, 256)

    'final step of making the back buffer...
    'load our created blank bitmap surface into our buffer
    '(this will be used as our canvas to draw-on off screen)
    SelectObject myBackBuffer, myBufferBMP

    'before we can blit to the buffer, we should fill it with black
    BitBlt myBackBuffer, 0, 0, 320, 256, 0, 0, 0, vbWhiteness

    'load our sprite (using the function we made)
    mySprite = LoadGraphicDC(App.Path & "\sprite1.bmp")
    'MsgBox Dir$(App.Path & "\sprite1.bmp")
    'ok now all the graphics are loaded so
    'lets start our main loop..

    'Disable cmdTest, because if the graphics are
    'reloaded there will be memory leaks...
    cmdTest.Enabled = False

    '== START MAIN LOOP ==
    'get current tickcount (this is used as a code timer)
    T2 = GetTickCount
    Do
    DoEvents 'DoEvents makes sure that our mouse and keyboard dont freeze-up
    T1 = GetTickCount

    'if 15MS has gone by, execute our next frame
    If (T1 - T2) >= 15 Then

    'clear the place where the sprite used to be...
    '(we do this by filling in the old sprites place
    'with black... but in games you'll probably have
    'a background tile that you would blit here)

    BitBlt myBackBuffer, SpriteX - 1, SpriteY - 1, _
    32, 32, 0, 0, 0, vbBlackness

    'blit sprites to the back-buffer ***
    'You could blit multiple sprites to the backbuffer,
    'but in our example we only blit on...
    BitBlt myBackBuffer, SpriteX, SpriteY, 32, 32, _
    mySprite, 0, 0, vbSrcPaint

    'now blit the backbuffer to the form...
    BitBlt Me.hdc, 0, 0, 320, 256, myBackBuffer, _
    0, 0, vbSrcCopy

    'move our sprite down on a diagonal...
    'Me.Caption = SpriteX & ", " & SpriteY
    SpriteX = SpriteX + 1
    SpriteY = SpriteY + 1

    'update timer
    T2 = GetTickCount
    End If

    'loop it until our sprite is off the screen...
    Loop Until SpriteX = 320

    'end of copy-paste here...


    DONT RUN THE PROGRAM YET, we need to write the cleanup code...
    The cleanup code is just some code that we add that clears the memory that was occupied by the graphics that we loaded, and the backbuffer that we created (see above code). This code should usually go in the Form_Unload event, so that it is executed when the form unloads...
    Copy and Paste the code below into the form's module
    Private Sub Form_Unload(Cancel As Integer)
    'this clears up the memory we used to hold
    'the graphics and the buffers we made

    'Delete the bitmap surface that was in the backbuffer
    DeleteObject myBufferBMP

    'Delete the backbuffer HDC
    DeleteDC myBackBuffer

    'Delete the Sprite/Graphic HDC
    DeleteDC mySprite

    End
    End Sub


    'end of copy-paste here...


    That's it! We're done! Run the program, click the button and you will see the sprite move from the top-left of the form to the bottom right, without any flickering...

    Example project included.
    yar interactive software - 2002
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