Seeing graphics outside of screen area? (DDraw)

[Sastraxi]

What I mean is, I want to know how to make DDraw 'clip' the graphics that overlap outside of the screen, not just destroy them.

This is very important as it is for a tile engine!!

[PsychoMark]

Yup, nasty little problem

Assuming you're using Fullscreen Exclusive mode (otherwise you can just use the built-in DirectDraw Clipper object). Clip the graphics yourself, it's the only way:

(msBack = BackBuffer surface, msTile = Graphics surface, mrScreen = Screen sized rectangle)

Dim rSource As RECT
Dim lX As Long
Dim lY As Long

' Calculate the X And Y coordinates of the tile / sprite / graphic here
lX = -5
lY = -5

' Set up the source rectangle As normal
With rSource
    .Left = 0
    .Top = 0
    .Right = 32
    .Height = 32

    ' Now, perform the manual clipping
    ' Left side
    If lX < 0 Then
        .Left = Abs(lX)
        lX = 0
    End If

    ' Right side
    If lX + 32 > mrScreen.Right Then
        .Right = mrScreen.Right - lX
    End If

    ' Top side
    If lY < 0 Then
        .Top = Abs(lY)
        lY = 0
    End If

    ' Bottom side
    If lY + 32 > mrScreen.Bottom Then
        .Bottom = mrScreen.Bottom - lY
    End If
End With

' That's it, now just Blt the tile / sprite / graphic
msBack.BltFast lX, lY, msTile, rSouce, DDBLTFAST_WAIT

It's not that hard, just draw it out on a piece of paper, that's how I got it. It's not tested though, so there might be some small errors in it...

Hope this helps!

[HarryW]

Err... can't you use hardware clipping? Maybe it's a VB thing, cos I'm pretty sure this doesn't pose a problem in C.

[PsychoMark]

"Hardware Clipping", guess that relies on the hardware, or is it widely supported? Anyways, I never heard of hardware clipping in VB (actually, never even heard of hardware clipping in general ), but if that's possible, be sure to let me know too

[HarryW]

Well yeah, I just call it hardware clipping because if the card supports it then it will be done in the HAL (most cards do I think), but it's in the HEL too so you don't have to worry about it being available. In C at least, you have to define a list of clipping regions which is a bit of a pain in the arse data structure to work with, involving alittle dynamic memory allocation... actually it is an incredibly bizarre data structure and I have no idea why they chose it... well anyway I don't know how you'd do it in VB but I'm pretty sure it's possible. Probably a lot simpler too. You define a clip list and then make a call to DDraw->CreateClipper and DDSurface->AttachClipper if my memory serves me correctly.

Hardware clipping is probably the main reason to use Blt instead of BltFast, since BltFast doesn't take any notice of any DirectDraw clippers you have setup.

[PsychoMark]

I don't think it's really possible in VB, the only clippers I know of only work in Windowed mode, so I'm just going to convert my code into a function which handles the clipping...

[Jotaf98]

Hey, PsychoMark, I'm not sure that function will work if the Top/Left is greater than 0. Most DX engines have this problem because they don't need to blt from different parts of one source image, I had to find a really good one that supported it: go to http://www.planet-source-code.com and look for the Grenade Launcher in the VB section.

[PsychoMark]

Oops, yeah, that's right, I got that same problem when programming MouseAttack DX (you can find it at my website, including source code)... here's the clipping code used in that game:

Public Sub Draw(Viewport As RECT)
    Dim rSrc As RECT
    Dim lY As Long
    Dim lX As Long
    
    ' Draw mouse
    With Settings
        rSrc = FillRect(.State * .Width, .AniFrame * .Height, .Width, .Height)
        
        ' Clip!
        lX = .X
        lY = .Y
        
        If .Y < Viewport.Top Then
            rSrc.Top = rSrc.Top + Abs(.Y - Viewport.Top)
            lY = Viewport.Top
        End If
        
        If .X < Viewport.Left Then
            rSrc.Left = rSrc.Left + Abs(.X - Viewport.Left)
            lX = Viewport.Left
        End If
        
        If .Y + .Height > Viewport.Bottom Then
            rSrc.Bottom = rSrc.Bottom - (.Height - (Viewport.Bottom - .Y))
        End If
        
        If .X + .Width > Viewport.Right Then
            rSrc.Right = rSrc.Right - (.Width - (Viewport.Right - .X))
        End If
        
        msBack.BltFast lX, lY, msMouse, rSrc, DDBLTFAST_SRCCOLORKEY
    End With
End Sub

The FillRect is just a simple function which fill a RECT structure (but using Width and Height, so Right becomes Left + Width etc...)

[Stef]

Here's the tile-blitting code from a tile engine I wrote some time ago....

Private Sub BlitTile(X As Integer, Y As Integer, TileNum As Integer, XOffset As Integer, YOffset As Integer)
 Dim rSrc As RECT, rDest As RECT
 If X = 0 Then
  rSrc.Left = TileDesc(TileNum).X + XOffset
  rSrc.Right = TileDesc(TileNum).X + TileSize
  rDest.Left = 0
  rDest.Right = TileSize - XOffset
 ElseIf X >= frmScreen.ScaleWidth Then
  rSrc.Left = TileDesc(TileNum).X
  rSrc.Right = rSrc.Left + XOffset
  rDest.Left = X - XOffset
  rDest.Right = frmScreen.ScaleWidth
 Else
  rSrc.Left = TileDesc(TileNum).X
  rSrc.Right = rSrc.Left + TileSize
  rDest.Left = X - XOffset
  rDest.Right = rDest.Left + TileSize
 End If
 If Y = 0 Then
  rSrc.Top = TileDesc(TileNum).Y + YOffset
  rSrc.Bottom = TileDesc(TileNum).Y + TileSize
  rDest.Top = 0: rDest.Bottom = TileSize - YOffset
 ElseIf Y >= frmScreen.ScaleHeight Then
  rSrc.Top = TileDesc(TileNum).Y
  rSrc.Bottom = TileDesc(TileNum).Y + YOffset
  rDest.Top = Y - YOffset
  rDest.Bottom = frmScreen.ScaleHeight
 Else
  rSrc.Top = TileDesc(TileNum).Y
  rSrc.Bottom = TileDesc(TileNum).Y + TileSize
  rDest.Top = Y - YOffset
  rDest.Bottom = rDest.Top + TileSize
 End If
 srfBackBuf.Blt rDest, srfTiles, rSrc, DDBLT_WAIT
End Sub