SelectAlphaChannel(id[, type, frame])
idas the current output device. This means that all graphics data that are output by Hollywood will be drawn to this alpha channel. Alpha channels are no stand-alone objects in Hollywood; they are always attached to an image object, for example a brush or an animation.
SelectAlphaChannel() always works with the alpha channels of
brushes. However, starting with Hollywood 4.5, you can also use it to
draw to the alpha channel of animations and BGPics. To do this you have
#BGPIC in the optional
type argument. If you use
#ANIM in the
type argument, you have to specify the frame of the anim
that you want to draw to, too. See SelectAnim() for more information.
If you specify
type, note that you can only modify the alpha
channel of BGPics that are currently not associated with a display.
Starting with Hollywood 4.7, you can also pass
#LAYER as the
modify the alpha channel of a layer. Note that if the layer is an anim
layer, you will also have to specify the number of the frame to select.
Alpha channels can be used to give each pixel its own transparency setting.
There are 256 different transparency levels available for each pixel.
An alpha channel value of 0 means that the pixel is fully transparent
and an alpha channel value of 255 means that the pixel is opaque. All
Hollywood graphics functions will render with a static alpha channel
intensity into the alpha channel of the graphics object. You can configure
this intensity using the SetAlphaIntensity() command. Alternatively, you
can set the alpha render mode to a vanilla copy mode. This is done by
calling SetAlphaIntensity() with
#VANILLACOPY as the argument. Then, all
Hollywood graphics commands which output alpha channel pixels will copy
the exact alpha channel data to your brush's alpha channel. This vanilla
copy mode is a new feature of Hollywood 2.5.
The color argument that several Hollywood functions (like Box() or Circle()) expect, is superfluous when rendering to alpha channels. You only need to use SetAlphaIntensity() when rendering to alpha channels.
Alpha channels are usually used for nice background shine-through effects or for anti-aliasing jagged edges. If you do not need different transparency levels but only two choices, namely transparent pixels and opaque pixels, you should use SelectMask() instead because it can be drawn faster. Please note that graphics objects cannot have a mask and an alpha channel. Only one transparency setting is possible. Thus, if you use this command on a graphics object that has a mask, this mask will be deleted first.
If the graphics object you specify in id does not have an alpha channel yet, it will be automatically created when you call a command that wants to draw to the alpha channel. The alpha channel created will be opaque then, i.e. every pixel will have an alpha intensity of 255 which means that it is 0% transparent. To cancel alpha channel rendering mode and return to main display output, just call the EndSelect() function.
If you do not need an alpha channel any longer, you should use the command DeleteAlphaChannel() to remove it from the brush.
You cannot use brush links with this command because the graphics data
of the brush specified by
id will be changed. It is also forbidden to
call commands which change the dimensions of the brush/anim that is
currently used as output device, e.g. you may not call ScaleBrush() or
ScaleAnim() to scale the brush/anim that is currently the output device.
Furthermore, it is not allowed to call
SelectAlphaChannel() for animations
that are loaded from disk. Animations must always reside completely in
memory if you want to draw to their frames using
Only commands that output static graphics can be used when
is active. You may not call animated functions like MoveBrush() or
SelectAlphaChannel() is active.
If you are using type
#LAYER and the specified layer is a vector layer,
SelectAlphaChannel() will rasterize the layer to a brush layer first.
See SelectLayer for details.
#LAYERin case the specified layer is an anim layer (V4.5)
CreateBrush(1, 256, 50, #RED) ; create a brush of size 256x50 SelectAlphaChannel(1) ; alphachannel becomes output device For k = 255 To 0 Step -1 SetAlphaIntensity(k) ; set alpha intensity to k Line(255 - k, 0, 255 - k, 49) ; draw lines with various intensities Next EndSelect ; make display the output device again DisplayBrush(1, #CENTER, #CENTER)This code demonstrates the 256 different transparency levels by creating a brush with the width of 256 pixels and drawing 256 lines with different transparency settings into it. The result will be a red rectangle which smoothly merges with the background picture. Please note that you need a 24-bit screen for the full eye-candy. On 15-bit and 16-bit screens there are not enough colors to display all different levels.