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We also wrote a similar feature highlighting all of the PlayStation 4's apps and functionality. Really exciting stuff, it must be said. I can't wait to get Hulu and Netflix on PS4, so I no longer have to worry about my iPhone, iPad, Tivo, PS3, Xbox 360, desktop PC, laptop PC, Mac and connected TV all breaking down and not being able to watch the next episode of Melrose Place. I know I'm sounding crabby about all this, but I do understand the concept of an all-in-one entertainment and games system. It's just that I simply don't find this stuff exciting. This aspect of the machine makes it an appliance, and as with every appliance, all I care about is that it works.

Small computers are emerging as important components of the contemporary computing scene. Their operating systems vary from specialized software for an embedded system to the same style of OS used on a generic desktop or server computer. This article describes a course in which systems are classified by their hardware capability and the traditional OS topic areas are then dissected, augmented, reassembled, and reorganized to illustrate the aspects of each topic as applicable to each class of small computers. Ultimately, the course covers the same material as a conventional OS course, but from a new perspective.

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The X Window System allows multiple windows to occupy the samecoordinates on the screen. The core protocol defines which portions of eachwindow are visible and which are occluded by overlapping windows, but theoverlapping windows are always completely opaque. Various techniques can be used to simulate non-opaque windows in controlledenvironments. The Shape Extension can be used to make areas of the windowtransparent. A background of ``None'' can be used to inherit the contents ofthe screen in the region occupied by the window when it is first mapped.Where available, hardware overlays can be used which expose a transparentpixel value.None of these techniques can be used for translucency in a general way;hardware overlays and the Shape Extension can only provide transparency andcannot blend the pixel colors together. A background of ``None'' cannot beused when the occluding windows are to be reconfigured or when the occludedregion contents are expected to change.The X Translucent Window Extension is described which solves the generaltranslucency problem by assigning alpha values for pixels in occludingwindows. These values are used to blend the occluding window contents withthe occluded region for display. The details of managing translucent windowhierarchies, re-parenting translucent windows and X visual differencesbetween blended pixels are discussed.TranslucencyPhysically, translucent objects absorb some, but not all, of the lightpassing through them. The color of the object affects which wavelengths aremost strongly absorbed. Visually, translucent objects appear to affect thecolor and brightness of objects beyond them.The effect of light on a translucent object can be simulated by blending thecolor of the translucent object with that of objects beyond it. Whendealing with computer images, translucency can be described as amathematical operation on the color data of a collection of images. Thismathematical composition of images was formally defined by Thomas Porter andTom Duff in 1984 [PD84]. They define formulae which usecolor data in conjunction with a per-pixel opacity value called ``alpha''.With these formulae many intuitive image manipulations can be performed.Image Compositing OperatorsEach of the operators defined by Porter and Duff operate independently oneach of the color channels in each pixel. The equations are abbreviated toshow the operation on a single channel of a single pixel.A common compositing operation is to place one image over another.Transparent areas of the overlying image allow the underlying image to showthrough. Opaque areas hide the underlying image while translucent areasblend the two images together. By defining the ``alpha'' of a pixel as anumber from 0 to 1 measuring its opacity, a simple equation combines twopixel colors together:

Uses For Translucent WindowsAs user interface design moves forward with increasing graphics performance,things formerly passed over as computationally intractable are now quitereasonable. Here are a couple of ideas how translucent windows could beused.Window Management EffectsIt is possible to simulate translucency during some window manipulationoperations to provide additional feedback for the user. This can beimplemented by capturing a static image of the window and the desktop,blending them, and updating the display.This means that any changes that occur to the window contents during theoperation cannot be reflected dynamically on the screen, limiting this towindow movement operations. Moving the blending operations into the windowsystem allows for the dynamic composition of application and underlyingimages.Applications should not be required to cooperate to provide these effects.This requires an external control over window hierarchy translucence.Transient DataUser interface elements which appear transiently over application windowssuch as menus and dialog boxes often occlude information useful in theoperation of the transient action. By making the background of thetransient window transparent and using translucency to highlight dialogelements, an effective user interface element can be usable and yet stillallow interpretation of occluded application data.While this can be effected by eliminating windowing for the transientelements and rendering them directly to the application window, the abilityto continue to use window management and other windowing metaphors forthese elements provides a strong incentive to incorporate these semanticsinto the window system.Overlays and AnnotationApplications with complex image displays frequently present the ability forusers to overlay associated information or annotate the image withoutaffecting the underlying image data. These needs are satisfied today onlywith graphics hardware that supports overlays. But even where supported,the annotations are usually limited to fewer color planes than the mainimage and cannot incorporate translucence in the image, only opacity andtransparency.Again, these effects can be implemented by the application, however, theunderlying graphics are often expensive to redisplay. Moving theseoperations into the window system provides the ability to use overlayhardware where available and gracefully fallback to software when necessary.Both transient windows and annotations require per-pixel translucency thattracks rendering operations.The X Rendering ExtensionThe X Window System [SG92] is a networked, extensible window systemproviding hierarchical windows. It was designed to operate on a widevariety of graphics hardware, from simple frame buffers to high-end graphicssystems with overlays, underlays, accumulation buffers, z-buffers, doublebuffering, etc. Over the last 12 years, it has become the standard windowsystem for Unix and Unix-like systems.The X Rendering Extension [Pac00] provides a new renderingarchitecture for X applications including image compositing, sub-pixelpositioned geometric primitives and application management of glyphs. The XTranslucent Window Extension builds on the image composition ideas andmechanisms provided by the X Rendering Extension.New ObjectsThe ``PictFormat'' object holds information needed to translate X pixelvalues into color and alpha data. For TrueColor visuals, the color data areextracted directly from the pixel while pseudo color visuals use a separateColormap. The PictFormat references the appropriate Colormap in that case.It also indicates the portion of the pixel which contains alpha information(if any).To encapsulate rendering state and color information, X Drawables (pixmapsand windows) are wrapped inside a new ``Picture'' object. An externalpixmap Picture containing alpha data can be associated with the Picture.This external alpha data overrides any embedded alpha data.Rendering OperatorsThe X Rendering Extension uses a modified version of the Plan 9 renderingprimitive [Pik00] as the basis for image composition:

Using this basic rendering primitive, the extension defines geometricoperations by specifying the construction of an implicit mask which is thenused in the general primitive above. Anti-aliased graphics can be simulatedby generating implicit masks with partial opacity along the edges.Color ManagementThe core X protocol defines all rendering primitives in terms of pixelvalues. While this works when the rendering is done with booleanoperations, it's color-based rendering must have a color interpretation foreach pixel value.The PictFormat object contains the information necessary to translate apixel value into a color. The converted color can then be composited withother colors to generate the displayed color. Once a final color iscomputed, the extension converts it back to a pixel value using a fixedpalette with optional dithering to improve color fidelity. Pseudo colordisplays use a fixed palette generated by the server for all images; theflexibility of a dynamic palette was discarded in favor of reducedcolormap flashing and simpler code.Windowing SemanticsX provides a hierarchical window system. Windows provide a view onto adocument or scene. Windows are stacked over or under their peers andcontain subwindows. The top of the window hierarchy is called theroot and is distinguished from other windows by being contained in nowindow.The visible area of a subwindow is confined to the visible area of thecontaining window. The visible area of a window is occluded by allsubwindows. A window along with all subwindows stack together with respectto that windows peers.Given the above semantics, the visible portion of a window can be found bycomputing the portion of the window within the visible portion of itsincluding window and subtracting the areas of any subwindows and overlyingpeer windows. The visible portions of each window are combined to formthe final displayed image. As the visible portions of each window form apartition of the root window area, each pixel on the screen belongs tothe visible portion of precisely one window.Transparent WindowsGraphics hardware frequently provides the ability to ``color-key'' one framebuffer over another frame buffer. This compositing is done in hardware,and allows either the underlay or overlay to be display, but not acombination of the two.For hardware with this capability, X lists pairs of visuals for each screen,one as the underlay and another as the overlay. Transparent areas inwindows created in the overlay visual show through contents of windows inthe underlay. The transparent areas are typically specified with a specialpixel value.The semantics of this mechanism are meant to expose the underlying hardwareabilities, rather than match the windowing model and can generate surprisingresults. One such surprise was that transparent pixels in the overlayvisual would unconditionally show through to the nearest occluded window inthe underlay, even if intervening windows existed in the overlay.The X Shape Extension [Pac89] provides a mechanism for altering thevisible region of a window. This affects graphical output as well as pointerinput: areas outside of the shape do not receive pointer events. The ShapeExtension can be used to implement partial window transparency, but theeffect on input is usually not desirable. Additionally, the regular Xsemantics fail to ensure that the occluded window contents will be preservedby the X server for redisplay. A large class of applications fail toperform acceptably when their contents are damaged, making shaped windowsunsuitable.The final problem is that the Shape Extension is implemented by modifying thewindow clipping regions within the server. These regions are represented aslists of rectangles. Complex bitmap shapes generate clip lists of thousandsof rectangles, slowing the server unacceptably.Windowing as CompositingAn informal description of typical window system semantics is that ofoverlapping pieces of paper on a desktop. This characterizes the originalintent as developed at Xerox, but seems far removed from the formalX semantics described above.Reinterpreting those in terms of image composition provides a moretransparent description.A window is composed of an image and zero or more stacked subwindows. Thevisible image of a window is generated by composing the window with thevisible image of its subwindows using the operator. The alpha valueof a window is 1 inside the shape of the window and 0 outside.Translucent Windowing SemanticsExisting systems provide two interpretations for translucency. A reasonablesystem will provide mechanisms for both and also allow hardware accelerationwhen possible.Systems based on hardware overlays provide a special pixel value thatexposes data in the underlying window. These embed transparencyin the pixel value itself. Each pixel can be either opaque or transparent,but not translucent.Systems designed to animate window operations provide an external opacityvalue that controls the blending of a window to the desktop. The datawithin the window needn't contain opacity information, rather that isapplied by the external window management agent to affect the display of theresulting image.PrototypeTo provide a framework for exploring window compositing, a simple prototypewas constructed that allows for arbitrary compositing between windows.As seen above, windowing can be described in terms of compositing images inlayers. The displayed image of a window is formed by compositing the windowimage data along with the displayed images of each inferior window.This suggests a relatively straightforward, if somewhat inefficient,implementation of windowing. Instead of providing window layering byclipping each rendering operation to a single shared image, window layeringcan be implemented by compositing separately rendered images.Prototype ImplementationThe image data for each window is kept in an off-screen image buffer. Thecomplete displayed image is formed by recursively compositing these imagestogether.A separate displayed image buffer is used to render the composite image ofthe window and subwindows. The window image data are copied to thatdisplayed image buffer. Finally, the displayed image for each inferior isgenerated and composited to the displayed image buffer. The root windowuses the frame buffer itself for the displayed image.As all rendering operations now occur off screen, the X server must updatethe displayed image for the root window whenever visible changes occur. Theprototype keeps a single region which encloses any damage. Each renderingcommand updates that region. When the server is about to wait foradditional X requests, it recurses through the window hierarchy updating thedamaged areas of each displayed image. Finally, the damaged region isemptied.Prototype ResultsThe initial prototype provides an alternate implementation of X windowing.By itself, that is not terribly useful. To demonstrate the capabilities ofthe architecture, the server was modified to mask each ``override redirect''window with a constant alpha value of 2/3. This makes most menus appeartranslucent.The prototype is minimally functional. Performance is poor, enough for ademonstration but not for real applications. Nonetheless, it is interestingto see the effect of translucency on real applications and gauge theusability of various user interface ideas. For example, making menusentirely translucent leads to readability problems. The text should probablybe opaque and outlined in a contrasting color while the background of themenu remains translucent. The Phillips TiVo, a Linux-based video storagedevice, displays text in this manner.The prototype can be extended to support the Translucent Window Extension;adding semantics for different compositing operators is quite easy once theentire contents of every window is available.Improving the DesignWhile the prototype demonstrates an easy intuitive architecture for windowcompositing, it uses memory for window image data which is not used togenerate the final display. It also recomposites window images at eachlevel of the hierarchy, making deep window trees perform poorly. Theprototype has been useful, but architectural changes are needed for aproduction system.The final design should be equivalent to the existing X windowing system inthe absence of translucency.Opaque WindowsWindows with a constant alpha value of 1 (those without an alpha channel ormask) can be directly rendered to the enclosing window's image buffer. Thisis a generalization of the standard X rendering model in which all windowsshare the frame buffer for image data. Similar rules apply here: theenclosing window and any occluded windows must clip rendering out of thatarea. When compositing the displayed image, any occluded areas from otherwindows must not touch the occluding pixels.Occluded Window RegionsSections of windows occluded by opaque windows are not needed to generatethe final displayed image on the screen and so need not be contained in anyimage. Existing X clipping operations can be used to modify renderingoperations in this case.For the root window, this will allow direct rendering to the frame buffer,entirely avoiding the cost of compositing windows while ensuringacceleration of rendering operations with any display hardware.Translucent SubwindowsThe prototype stores the entire window image off screen so that the imagecan be composited with subwindows. Instead, only the portion of a windowcovered by translucent windows need be stored in a separate buffer with theremainder rendered directly to the displayed image buffer.This gives each window two regions, one containing the area rendereddirectly to the displayed image buffer and a second containing the areacovered by translucent subwindows which must be rendered to an off-screenimage buffer.Multiple Frame BuffersThe prototype works only for a homogeneous display; all windows must be truecolor at the same depth. To extend this for hardware with overlays, theimplementation must allow for windows of different depths and visual classes.For opaque windows, providing separate displayed image buffers where neededin the hierarchy is sufficient. Translucent windows must be blended togetherto be displayed. For windows not using a true color visual, the pixelvalues must be converted to color values, blended together then convertedback to pixel values and stored in the displayed image buffer of anappropriate format. The semantics for this conversion are described indetail by the Rendering Extension.X Translucent Window ExtensionThe X Translucent Window Extension exposes semantics for window translucencyto applications, allowing them to manipulate the composition of window datato the screen. The Translucent Window Extension uses the RenderingExtension compositing primitive with a fixed OVER operator.