Basic4Android: Using the new 3D Camera in ABExtDrawing 1.1

I’ve added the Camera object to the ABExtDrawing library for Basic4Android. This object must not be confused with the hardware camera. There are other libraries available for that. The new version 1.1 of the library can be downloaded from the B4X website.

The camera I’m talking about is a nice feature you can use to do 3D effects on the canvas without using OpenGL. The picture above is just a noral B4A canvas where we did some rotations and translations on. It simulates a scrolling list where the listitems rotate around their axe.

Additional, I shows other features of the ABExtDrawing library to do some lighting effects.

Let me show you how this is done in B4A.

We create a new type item3D. This will hold one item in the list.

Sub Process_Globals
	Type item3D (bmp As Bitmap, Top As Int, Left As Int, Width As Int, Height As Int)
End Sub

We also have to declare some variables. Note our mCamera variable which will do the 3D conversions and some constants for our lighting effects. On a Paint object we can set color filters which will affect the color values of what we draw with that Paint object. SetLightingColorFilter takes care of that. A LightingColorFilter takes two colors that are used to modify the colors that we are drawing. The first color will be multiplied with the colors we draw, while the second one will be added to the colors we draw. The multiplication will darken the color and adding will make it brighter so we can use this class to model both shadows and highlights. It would have been even better if instead of adding it would have implemented the screen blend mode, but add works OK.

To actually calculate the light we’ll use a simplified version of Phong lighting.

Sub Globals
	'These global variables will be redeclared each time the activity is created.
	'These variables can only be accessed from this module.
	Dim items As List
	Dim HalfHeight As Float


	' Ambient light intensity
    ' Diffuse light intensity
    ' Specular light intensity
    ' Shininess constant
    Dim SHININESS As Float: SHININESS = 200
    ' The Max intensity of the light

	Dim CurrentRotation As Int
	Dim CurrentTop As Int
	Dim MyCanvas As Canvas

	Dim ScreenTop As Int: ScreenTop = 1
	Dim Panel1 As Panel

	Dim ExDraw As ABExtDrawing
	Dim mCamera As ABCamera
	Dim mMatrix As ABMatrix
	Dim mPaint As ABPaint

	Dim PI As Double: PI= 3.141592653589793238462643383279502884197

	Dim ClearRect As Rect

	Dim CurrY As Int
End Sub

In the Activity_Create sub we initialize mCamera and the other variables like mPaint which we will also need for our lighting. Also our pictures are preloaded.

Sub Activity_Create(FirstTime As Boolean)


	HalfHeight = Activity.Height / 2

	Dim backbmp As Bitmap
	backbmp.Initialize(File.DirAssets, "background.png")
	Dim backbmp2 As Bitmap
	backbmp2.Initialize(File.DirAssets, "background2.png")
	Dim backbmp3 As Bitmap
	backbmp3.Initialize(File.DirAssets, "background3.png")

	Dim conbmp As Bitmap
	conbmp.Initialize(File.DirAssets, "contact_image.png")
	Dim conbmp2 As Bitmap
	conbmp2.Initialize(File.DirAssets, "contact_image2.png")
	Dim conbmp3 As Bitmap
	conbmp3.Initialize(File.DirAssets, "contact_image3.png")

	Dim i As Int
	Dim random As Int
	For i = 0 To 19
		random = Rnd(0,3)
		If random = 0 Then
			items.Add(CreateNewItem(backbmp, conbmp, "Colleague " & i, "Name of colleague " & i, 25, i*175, Activity.Width - 50, 120))
			If random = 1 Then
				items.Add(CreateNewItem(backbmp2, conbmp2, "Friend " & i, "Name of friend " & i, 25, i*175, Activity.Width - 50, 120))
				items.Add(CreateNewItem(backbmp3, conbmp3, "Client " & i, "Name of the client " & i, 25, i*175, Activity.Width - 50, 120))
			End If
		End If

	CurrentRotation = -(DEGREES_PER_SCREEN * ScreenTop) / Activity.Height

	ClearRect.Initialize(0,0,Activity.Width, Activity.Height)


End Sub

The sub CreateNewItem() is used to make one list item. Default B4A canvas drawing functions are used to show how well they work together with the ABExtDrawing functions.

Sub CreateNewItem(Background As Bitmap, icon As Bitmap, Subj As String, desc As String, Left As Int, Top As Int, Width As Int, Height As Int) As item3D
	Dim item As item3D
	Dim c As Canvas
	item.bmp.InitializeMutable(Width, Height)


	' background
	Dim dstR As Rect
	c.DrawBitmap(Background,Null, dstR)

	' draw Icon
	dstR.Initialize(15,15, icon.Width, icon.Height)
	c.DrawBitmap(icon, Null, dstR)

	c.DrawText(Subj, 100, 30,Typeface.DEFAULT_BOLD,16, Colors.White,"LEFT")
	c.DrawText(desc, 100, 60, Typeface.DEFAULT,16, Colors.White,"LEFT")

	item.Top = Top
	item.Left = Left
	item.Width = Width
	item.Height = Height
	Return item
End Sub

In the drawItem() sub all calculations are done for one item in the list. Each item will be a block that will rotate around its X-axis and look like it is rolling on the ground when the list stars to scroll. Each block will be as wide as the item normally is and the depth will be the same as the height. We’ll use the same bitmap for all the sides.

So what do we need to do to achieve this effect? In order to draw the blocks we need to draw the bitmap two times (since we will almost always see two sides of the block). We also need to have some kind of rotation variable to keep track of the main rotation. Since the blocks should rotate when the user scrolls the list and the blocks should have the same rotation (so that they all face up at the same time, see further).

Sub DrawItem(item As item3D)
	Dim CenterX As Float
	Dim CenterY As Float

	' get centerX AND centerY
	CenterX = item.Width / 2
	CenterY = item.Height / 2

    ' get scale
    Dim distFromCenter As Float
	distFromCenter = (item.Top + CenterY - HalfHeight) / HalfHeight
	Dim scale As Float
	scale = (1 - SCALE_DOWN_FACTOR * (1 - Cos(distFromCenter)))

    ' get rotation
    Dim RotationX As Float
	RotationX = CurrentRotation - 20 * distFromCenter
    RotationX = RotationX Mod 90
    If (RotationX < 0) Then
        RotationX = RotationX + 90
    End If	

    ' draw it
    If (RotationX < 45) Then
		drawFace(item, CenterX, CenterY, scale, RotationX - 90)
        drawFace(item, CenterX, CenterY, scale, RotationX)

        drawFace(item, CenterX, CenterY, scale, RotationX)
        drawFace(item, CenterX, CenterY, scale, RotationX - 90)
    End If
End Sub

Finally, DrawFace is called and this is where the magic happens. Worth noting is that the code that will draw one face of the block is the same, it just depends on the rotation, so it’s extracted to a method. To draw a complete block we then simply draw two faces 90 degrees apart at the same place.

To draw a face we first translate the camera so that the face will be drawn closer to us. Then we rotate it and after that we translate it back so we don’t scale it. Keep in mind that the calls to the camera, just like the rotate, translate and scale methods on Canvas, needs to be written in reversed order, so to speak. In the code below, it is the last line that translates the face towards us, then we rotate it, and finally, with the first line, we translate it back.

mCamera.translate(0, 0, centerY);
mCamera.translate(0, 0, -centerY);

The rest of drawFace is not that hard. It gets the matrix from the camera, pre and post translates the matrix and then draws the bitmap with the matrix.

This code will draw each item as if placed in the origin in 3D space and then we move the items to the correct place on the screen using pre and post translate on the matrix. This moves what we draw in 2D space without changing the perspective. We could apply the translation in X and Y on the camera instead, then the translation would be in 3D space and it would affect the perspective. We’re not doing that here because I want the appearance of a larger field of view than the fixed field of view of the camera. Instead, we fake it by slightly rotating and scaling the items depending on the distance from center of the screen.

We calculate the light and create a LightingColorFilter that we can set to our Paint object.

Sub drawFace(item As item3D, CenterX As Float, CenterY As Float, scale As Float, RotationX As Float)
	' save the camera state

    ' translate AND Then rotate the camera
    mCamera.translate(0, 0, CenterY)
	mCamera.translate(0, 0, -CenterY)

    ' get the matrix from the camera AND Then restore the camera

    ' translate AND scale the matrix
    mMatrix.preTranslate(-CenterX, -CenterY)
    mMatrix.postScale(scale, scale)
    mMatrix.postTranslate(item.left + CenterX, + CenterY)

    ' set the light
	Dim cosRotation As Double
	cosRotation = Cos(PI * RotationX / 180)
    Dim intensity As Int
	intensity = AMBIENT_LIGHT + (DIFFUSE_LIGHT * cosRotation)
    Dim highlightIntensity As Int
	highlightIntensity = (SPECULAR_LIGHT * Power(cosRotation,SHININESS))
    If (intensity > MAX_INTENSITY) Then
        intensity = MAX_INTENSITY
    End If
    If (highlightIntensity > MAX_INTENSITY) Then
        highlightIntensity = MAX_INTENSITY
    End If
    Dim light As Int
	light = Colors.rgb(intensity, intensity, intensity)
    Dim highlight As Int
	highlight = Colors.rgb(highlightIntensity, highlightIntensity, highlightIntensity)
    mPaint.SetLightingColorFilter(light, highlight)   

    ' draw the Bitmap
    ExDraw.drawBitmap4(MyCanvas, item.bmp,  mMatrix, mPaint)
End Sub

The DrawMe() sub is the overall function to draw all the items.

Sub DrawMe()
	Dim i As Int
	ExDraw.save2(MyCanvas, ExDraw.MATRIX_SAVE_FLAG)
	MyCanvas.DrawRect(ClearRect, Colors.Black, True, 1dip)
	'MyCanvas.DrawBitmap(FormBack, Null, formR)
	For i = 0 To items.Size - 1

End Sub

And in the Panel1_Touch sub we’ll animate our list. Note that it is here that we make sure all boxes face the same. This is done by the lines:

ScreenTop = ScreenTop + DeltaY
CurrentRotation = -(DEGREES_PER_SCREEN * ScreenTop) / Activity.Height

Doing like this will make the blocks rotate DEGREES_PER_SCREEN degrees when the user scrolls the list an entire screen no matter the pixel-height of the screen.

Here is the full sub:

Sub Panel1_Touch (Action As Int, X As Float, Y As Float) As Boolean 'Return True to consume the event
	Dim DeltaY As Int
	Select Action
        Case Activity.ACTION_DOWN
			CurrY = Y
        Case Activity.ACTION_MOVE
			DeltaY = Y - CurrY
			CurrY = Y
			Dim i As Int
			Dim it As item3D
			For i = 0 To items.Size - 1
				it = items.Get(i)
				it.Top = it.Top + DeltaY
				items.Set(i, it)

			ScreenTop = ScreenTop + DeltaY
			CurrentRotation = -(DEGREES_PER_SCREEN * ScreenTop) / Activity.Height

        Case Activity.ACTION_UP
    End Select
    Return True
End Sub

So this concludes this tutorial. You can download the project from

Note that at the first run, it may not work very smooth on some devices. But after some time it goes very well.

Have fun programming and until next time!

Click here to Donation if you like my work


About Alwaysbusy

My name is Alain Bailleul and I'm the Senior Software Architect/Engineer at One-Two. I like to experiment with new technologies, Computer Vision and A.I. My projects are programmed in B4X , Xojo, C#, java, HTML, CSS and JavaScript. View all posts by Alwaysbusy

7 responses to “Basic4Android: Using the new 3D Camera in ABExtDrawing 1.1

  • SmileCodes

    How to select 3D Menu (List3D) by index or else?

  • GMan

    Very very nice work…how can i trigger a click of one of the Onjects to select one and relaese an action ?

    • Alwaysbusy

      This is a rather old project and was only to show the possibilities of the lib, not to be used as a real list. I think maybe you could check it in the Case Activity.ACTION_UP. Something like this (not tested)

      Dim i As Int
      Dim it As item3D
      For i = 0 To items.Size – 1
      it = items.Get(i)
      if Y >= it.Top and Y <= it.Height then
      ' do something here
      end if

      But I'm guessing here.

  • GMan

    Thx for your reply, but this
    For i = 0 To items.Size – 1
    causes an error of wrong format(s).
    Its to sad that we cant use such a awesome screen not as a clickable menue – its to awesome to stay a simple list 😉

  • GMan

    OK, that was a fault in coping.
    the part
    … items.Size – 1
    has a strange MINUS-sign, i replaced it with a normal MINUS sign an dit works, but: only catches the cube on the very top.
    here is my code so far:
    Case Activity.ACTION_UP
    Dim i As Int
    Dim it As item3D
    For i = 0 To items.Size – 1 ‘Get (i) – 1 ‘ .Size 1
    it = items.Get(i)
    If Y >= it.Top AND Y <= it.Height Then
    ' do something here
    End If

    End Select
    Return True

  • GMan

    But only shows the most top cube when clicking (0)

  • aragar

    I know this post is quite old but just to help someone else that gets here like me looking for a way to detect which item is being clicked.
    There is one correction I made for the code and it works:

    Case Activity.ACTION_UP
    Dim i As Int
    Dim it As item3D
    For i = 0 To items.Size – 1
    it = items.Get(i)
    If Y >= it.Top AND Y <= it.Top+it.Height Then 'Added it.Top+
    ' do something here
    End If

    Have fun!

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