3.2. GRAPHICS IAlpha blending within games

ALPHA BLENDING An exploration of the use of alpha blending within games

How do we want to combine images?In games there are two common ways in which images are combined...

‘Normal’ alpha blending, i.e. drawing a transparent or translucent image so that it shows what is underneath (e.g. drawing the pipes so that the background is seen)

Additive blending, i.e. adding some image to the existing image (e.g. adding an effect image so that it brightens some region of the image)

Alpha blending (or more generally Alpha Compositing)Alpha blending is typically done in

hardware and controls how two images are combined to produce a single composite image (one image is termed the source, the other the destination – the source image is added to the destination image).The most common blend function is:

OutputColour = SourceColour * SourceBlend

+ DestinationColour * DesinationBlend

Aside: Each channel (i.e. red, green, blue, alpha) is separately

blended based on the blend function)

Alpha blending (blend functions)A wide range of different blend functions are defined, including those shown:

• Each component of the colour is multiplied by (0, 0, 0, 0). Zero

• Each component of the colour is multiplied by (1, 1, 1, 1). One

• Each component of the colour is multiplied by the source colour, i.e. (Rs, Gs, Bs, As).

SourceColor

• Each component of the colour is multiplied by the alpha value of the source, i.e. (As, As, As, As).

SourceAlpha

• Each component colour is multiplied by the destination colour, i.e. (Rd, Gd, Bd, Ad).

DestinationColor

• Each component of the colour is multiplied by the alpha value of the destination, i.e. (Ad, Ad, Ad, Ad)

DestinationAlpha

Inverse functions are also defined, e.g. InverseSourceAlpha

Alpha blending (normal alpha blending)The combination of blend functions that provides ‘normal’ alpha blending, i.e. enabling translucency, is shown below:

As SourceAlpha increases, SourceColour dominates, and DestinationColour reduces.

OutputColour = SourceColour * SourceAlpha + DestinationColour * InverseSourceAlpha

Source Alpha

Src Dest Output

1.0 S * 1.0 D * 0.0 S0.0 S * 0.0 D * 1.0 D0.5 S * 0.5 D * 0.5 (S+D)*

0.5

Alpha blending (getting the right order)The order in which images are combined is important if using this form of blending, i.e. Images that are ‘behind’ a transparent image must be drawn first before the transparent image is drawn

Aside: In XNA use SpriteBatch

SpriteSortMode BackToFront to draw

back to front.

Pillar (50% transparency)

Player (transparent background)

If player is to be drawn behind pillar, player must be drawn before pillar

Alpha blending (additive blending)A less common, but popular, means of alpha blending is additive blending:

OutputColour = SourceColour * SourceAlpha

+ DestinationColour * One

Additive blending is commutative, i.e. It does not matter the order in which images are combined, the end result is the same.

Using this form of blending there is no need to sort by depth order when combining images (although it can be more difficult to ‘visualise’ the output)

Alpha blending (in Java)

AlphaComposite alpha = AlphaComposite.getInstance( AlphaComposite.SRC_OVER,0.5f);

graphics2d.setComposite(alpha); graphics2d.drawImage(image,…);

The java.awt.AlphaComposite class uses what are called the Porter-Duff compositing rules to implement alpha compositing.Mostly commonly the SRC_OVER rule is used (equivalent to normal alpha blending).

Aside: The code repository uses normal alpha blending by default.

Alpha blending (in XNA)

BlendState blendState = new BlendState();

blendState.ColorSourceBlend =

Blend.DestinationColor;blendState.ColorDestinationBlend =

Blend.SourceColor;

GraphicsDevice.BlendState = blendState;

SpriteBatch supports both normal alpha blending and additive blending, via the BlendState (which can be AlphaBend, Additive, or None).Other forms of additive blending can be directly specified by changing the render state within the sprite batch. Example: for 2x multiplicative blending (which lightens/ darkens the destination based on the source colour – good for applying lighting effects)

Aside: For more info see: http://msdn.microsoft.com/en-us/library/bb976070.aspx

SUMMARY: STEPS IN YOUR GAME DESIGNImage loading, management and rendering summary

Summary: Image loading and managementIn your game design you should consider the following separate-but-dependent processes.

Load graphics

Game objects

Asset Manager

Q1: What graphics need to be loaded? In what order?

All at once?

To do:Develop design

Q2: How are assets managed?

(The default managers will likely suffice)

Q3: Which graphics are used by each game object? How/when do game objects

obtain graphics from the manager.

Request / receive graphics

Summary: Image drawingTo do:

Develop design

Game objects

Q1: Which layers should be drawn? In

what order?

Q2: Which layer objects are visible and should

be drawn?Q3: In what order

should visible objects be drawn?

Q4: How will each object be drawn?

Drawing a layer

Visible objects

Layers

Draw ordering

Drawing

DEVELOPING YOUR GAME DESIGNHaving another stab at producing your own game design

Graphics in your game…Using your own game idea, take a game screen (I’d suggest the main game layer) and:• Briefly identify key game

objects within the layer• Decide if any visibility culling is

possible, and if so, what.• Decide the order in which the

game objects should be drawn.

To do:Develop design

Start10 mins9 mins8 mins7 mins6 mins5 mins4 mins3 mins2 mins 1 min 30 secFinished

Question Clinic : This process should result in questions. Feel free to ask (and/or include in

the Question Clinic)

EXTRACTING IMAGESMaking a transparent / translucent image

Extracting imagesMost popular image editing programs, e.g. GIMP, will enable you to make/edit transparent/ translucent images.To do this you may need to make sure the image has an alpha channelThe ‘magic wand’ selection tool can be used to select regions (it may be necessary to play with the selection threshold). Once selected, unwanted regions can be cut.

It may be necessary to use the selection tool a number of times to refine the cut. Translucency can often be varied in a number of different ways. Simply changing the layer opacity is often easiest.Images shown below can be used to practice (easy/medium/hard cut)

Summary

To do:Complete Question Clinic

Complete/iterate the design for loading, selecting, ordering and drawing images in your game.

Write some code that loads and displays images.

Today we explored:

The theory and uses of alpha blending

Summary of key questions when using graphics

Started to plan how graphics will be selected, drawn in your game.