Cenci's Home > SiteMap > My Articles > Direct3D

Direct3D

Pengenalan

Microsoft telah merekabentuk dan melancarkan game development library called DirectX (DX). Sebahagian daripada perpustakaan ini ialah 3D graphics immediate mode rendering API called Direct3D.

DirectX merupakan set API yang dibangunkan oleh Microsoft Corporation untuk pembangunan aplikasi Windows masa-nyata yang bermutu. DirectX terdiri dari DirectDraw, DirectSound, DirectPlay, DirectInput dan Direct3D. Setiap satu dari komponen-komponen ini menjurus kepada kawasan atau bahagian berlainan yang diperlukan dalam pembangunan aplikasi permainan dan multimedia.

• DirectDraw - (DD) utk interaksi-interaksi 2D, like fast 2D blitting (bit block transfers), overlays, dll.

• DirectSound - utk menggabungkan bunyi ke dalam aplikasi.
  

• DirectPlay - utk menggabungkan berbilang pengguna ke dalam aplikasi aplikasi, menggunakan rangkaian utk komunikasi antara pengguna.
  

• Direct3D - (D3D) utk menggabungkan kebolehan 3D ke dalam aplikasi.
  

• DirectInput - utk menggabungkan kebolehan menyokong penggunaan alatan-alatan lain, seperti joysticks, ke dalam aplikasi.

Application Programming Interfaces (API)?

First is the notion of an Application Programming Interface, or API. An API is essentially a library of small functions that a software developer pieces together to form a complete computer program. For example, a 3D graphics API might include functions that draw objects, simulate the effects of lights, and determine which objects are visible from a given point of view. A software developer could use these functions to build a flight simulation game: drawing mountains and buildings that would be visible through the cockpit canopy of an airplane, and lighting them as they would be lit by the sun.

The design of a 3D graphics API determines what can be drawn, how quickly it can be drawn, and how easy it is for a software developer to achieve a desired effect. Thus the API is of fundamental technical and business importance to the software developers who use it.

Direct3D adalah sebahagian dari DirectDraw (API for accessing video hardware). Direct3D tak boleh wujud tanpa DirectDraw, selagi ia adalah sebahagian antara-muka DirectDraw. Oleh itu, ini bermakna kita perlu mengetahui asas DirectDraw sebelum kita boleh menggunakan Direct3D.

DirectDraw

a) Overview

As mentioned above DirectDraw has an advantage over the standard windows Graphics Device Interface (GDI). Whereas the GDI has to go through windows to draw, Directdraw talks directly to the video memory. Besides faster video access DirectDraw also provides automatic video configuration, the automatic use of graphics acceleration hardware, and a set of standard graphics functions.

b) Overall architecture - HEL/HAL

DirectDraw's Hardware Abstraction Layer (HAL) in combination with its Hardware Emulation Layer allows applications to automatically make the best use out of the graphics card that they own. If there is a computer that has the appropriate hardware a request from Directdraw will be sent directly to the HAL. If on the otherhand the hardware does not support a particular operation the request will go to the HEL were the appropriate command is emulated (albeit slower) in software. This makes the process totally transparent to the application.

c) Direct draw objects

DirectDraw is composed of 4 main object classes, DirectDraw itself, IDirectDrawSurface IDirectDrawPalette and IDirectDrawClipper.

e) DirectDraw

This object that needs to be created for starting off DirectDraw. The Directdraw object represents the display device, and is responsible for setting up the properties of the display, such as resolution and how the Directdraw window will interact in the windows environment. In a system with multiple display devices one might have one DirectDraw object for each one. This object also provides the means for creating the other 3 types of objects. The DirectDraw object does all this through member functions that are contained in its class.

e) Surfaces

DirectDraw surfaces are the pieces of memory that are used for drawing on. A surface contains the image data, sprites, bitmaps, the actual screen etc. A surface is created through a member function of the DirectDraw object.

• Simple Surfaces
  At least one surface must be created, namely that belonging to the main display image. This Surface is referred to as the Primary Surface. In order to draw onto this surface you can use windows GDI functions. Or if you wanted to draw a bitmap you could create another Surface place the bitmap into that surface and then blit it from that surface onto your primary surface. It is also possible to write to surface memory directly.

• Complex Surfaces
  A complex surface is a surface that has multiple buffers, where one buffer would represent the active display and the others would be hidden. This is especially useful for generating smooth animation because you can make all your changes to a scene hidden from view and then blit it all to the screen at once. A complex surface consists of a Front Buffer which is the buffer corresponding to the display, and one or more Back buffers. The Front and back buffers can be thought of as a FIFO stack, with the first buffer begin the front buffer. A member function is provided to flip buffers in which case the front buffer gets moved to the end of the stack and becomes a back buffer. And the next buffer on the stack becomes the Front buffer.
  

• Color Keying
  In addition to providing the basic drawing framework a surface also provides some color keying schemes in its Bliting functions. Color keying is a way of implementing transparency. DirectDraw supports two types of transparency: Source color keying, destination color keying. For source color keying, a surface can be defined to have a certain color or range of colors defined as transparent. When this surface is Blited onto another the colors within that range do not get copied thus giving transparency. Destination color keying is a property that comes into play on the surface that is being blitted to. If enabled when a bitmap is being blitted to this surface only those color ranges specified will be overwritten by the incoming data. This might be useful if you had a surface with a window and wanted to show something moving behind the window. The destination color keys would be set to the colors of the glass. Anything blitted onto this picture would only show up if it were located on the glass pixels.
  

• Overlays
  Another capability is useing overlays. With overlays each surface has a Z-order associated with it. When two surfaces are blited onto a screen and they coincide with each other in location, the Z-order determines which one comes out on top.
  

f) Clippers

Clippers are another object created by the DirectDraw object. Clippers can be used to control where in a window Directdraw is allowed to draw. This is particularly useful in a windowing type environment because it easily allows you to make sure your application doesn't keep writing to portions of the screen that have been covered by by another window. The boundaries of a clipper are defined by giving the clipper a clip-list, which is a list of rectangles that the application is allowed to draw in. After creation a clipper is attached a surface by passing it to one of the member functions of that surface.

g) Pallettes

The remaining object type that can be created is the a Pallete object. This object is obviously responsible for the management of the colors for a particular surface. The pallete entries are specified in the standard RGB triplets, and each pallette object is associated with a surface in a similar way to how the clippers are. Using the pallete objects member function you can do all the neccassary manipulations or a palette.

f) Directly Drawing on the Surface

As mentioned before you can draw on a surface either by using the GDI commands of by bliting from surface to surface. However if neither of those suit your needs you also have the option of getting at the surface memory directly. By retrieving the memory pointer from a surface method you can then access the surface memory much in the same way you would access video memory directly in a Dos session. This allows you to implement your own drawing/bliting functions if you wish to. In order to ensure that the surface can only be acceseed by one object at a time whenever you wish to modify a surface you can Lock it.

Direct3D digunakan untuk membangunkan aplikasi/program 3-dimensi,masa-sebenar(real-time) dan interaktif. Bagi menghasilkan aplikasi seperti yang tersebut, DirectX menyediakan fungsi/kemudahan berikut:

Ketidakbergantungan terhadap peralatan - aplikasi tidak bergantung kepada peralatan (device) dan menjadi lebih mudah-alih(portable)

Mudah melakukan penambahan 3D ke aplikasi - oleh kerana Direct3D menyediakan mekanisma piawai dan set algorithma piawai, aplikasi yang memerlukan kemudahan berikut dapat dibangunkan dengan lebih cepat.

Alternatif capaian ke peralatan - satu daripada fungsi kemudahan Direct3D iaitu, ia menggunakan sokongan peralatan, jika boleh. Dalam keadaan dimana peralatan tidak menyokong fungsi tertentu Direct3D, Direct3D menyediakan alternatif lain dimana fungsi tersebut akan dijalankan dan diimplimentasikan menggunakan perisian(software). Dalam masa perlaksanaan, perisian tersebut boleh mengesan kebolehan peralatan dan menggunakan peralatan tersebut jika boleh.

Common driver model to hardware - guarantees to applications, that all the drivers supporting Direct3D will support a defined minimal set of features and capabilities. Dengan ini, aplikasi-aplikasi yang dibangunkan menggunakannya boleh dijalankan pada semua jenis peralatan. Sebagai tambahan, Direct3D meAdditionally, Direct3D provides a specification to all hardware developers, which help their cards support the various Direct3D features. Applications using these features will see a boost in performance.

Pemacu-pemacu peralatan (Device Drivers)?

Second is the notion of a device driver. A device driver (driver for short) is a piece of software that implements an API on a given hardware device. For example, the functions of a 3D graphics API might be implemented differently on the video card in a Compaq PC than they would be implemented on another video card in a Dell PC. In such a case, the two hardware vendors (Compaq and Dell) would each write drivers for the 3D graphics API on their respective video cards. Note that the API would be used in the same way on both machines; only its implementation details (the drivers) would differ.

A driver is key to the viability of its associated API and all the application programs that use it. The existence of a driver determines whether a given API is even available on a particular machine. The quality of the driver (its completeness, performance, and reliability) in large measure determines the quality of the software that uses it on that machine. A driver typically requires a significant amount of effort for development and testing, and is therefore of major importance to the hardware vendor who must supply it as well as the software developers and consumers who will use it.

Direct3D terdiri dari dua distinct APIs iaitu higher-level Retained-Mode API (offers scene- and object-management services and a built-in geometry engine, freeing application developers from having to maintain and manipulate object databases) dan lower-level Immediate-Mode API (provides direct access to the hardware and allows experienced 3-D programmers to implement their own rendering and scene management. This supplies both flexibility and performance advantages over Retained Mode). Kebanyakan documentation untuk Direct3D menyatakan dua APIs ini secara berasingan. Dan ramai yang menganggap that the APIs are mutually exclusive. In fact, there are several instances in which we may wish to use functionality dari kedua-dua API dalam satu aplikasi.

Apa sebenarnya RetainedMode dan Immediate Mode?

Perbezaan ketara antara retained mode dengan immediate mode that has lead to this classification is the ability to address the data even after it has been rendered. If an application has access to the data being displayed, after it is rendered, without change in its basic structure, then the application is said to have the ability to retain the object information and hence an application working in this mode or using such a facility is working in the retained mode. For example, if the application is able to address the data points of a cube even after it has been rendered, then the application is said to retain the data being rendered and hence is working in the retained mode. As compared to this, if the application cannot access the data that was rendered, after rendering its, then the application is said to be working in the immediate mode. In the immediate mode, once the data is rendered, it is lost for ever to the developer. Data yang telah dirender masih available, tetapi data tersebut bukan lagi data 3D, tapi pixel-pixel 2D.

Kedudukan dan hubungan

Gambarajah 1 menunjukkan perbezaan bahagian-bahagian Direct3D, dalam hubungan dengan modul-modul lain system Win32.

Gambarajah 1: Kedudukan Direct3D.

Daripada gambarajah 1, it is clear that the retained mode uses the immediate mode, transparent to the developer using the retained mode. The developer is not made aware of this usage. From the figure, it is also clear that the retained mode also uses some features of DirectDraw. Retained mode, immediate mode and the Direct3D HAL, bersama, membentuk Direct3d komponen DirectX. Though many of the existing programs for 3D graphics on the Windows platform talk to the different parts directly, it is envisaged that the DirectDraw dan Direct3D components of DirectX will be incorporated into future versions of Win32 systems. Any system providing 3D, will have to use Direct3D to provide its own features.

Lapisan (Layers)

Direct3D menggunakan dua lapisan, dipanggil

• Hardware Abstraction Layer (HAL)

• Hardware Emulation Layer (HEL)
  

All of the features of Direct3D are built on top of the HAL, which provides hardware independence and makes applications portable. The HEL is a companion of Direct3D and provides software emulation for the features of the 3D rendering pipeline, not supported by the hardware. This layer is tightly integrated with the DirectDraw HAL and the Graphics Device Interface (GDI) driver of the Win32 system. This layer helps provide a unified driver model for accelerated 3D.

Enjin Merender (Rendering Engine)

The rendering engine forms an important part of Direct3D. It is responsible for taking a scene definition in terms of points in 3D, the different texture specifications, the lights and the camera specifications, and rendering ready, so that it can be displayed on the display device.

The functionality of the rendering engine is provided using three modules, namely

• transformation module,
  

• lighting module dan
  

• rasterization module.
  

Each of these modules can be hardware accelerated, transparent to the user of the application. The application developer only has to put the detection facility into the application, which will allow it to query the hardware to find and use its capabilities, if present.

Gambarajah 2 shows the three modules of the rendering engine and their interactions with the Direct3D API, before displaying the results on the rendering target, which is the 2D display surface. The diagram shows the sequence of operations performed on the data, before it is displayed.

Gambarajah 2: Rendering Engine Modules

The 3D data to be displayed, is given to the transformation module, which maps the 3D data onto its equivalent 2D data. This 2D data is then given to the lighting module, which calculates the light received by the data, considering the lights in the scene. The lit data is then given to the rasterization module, which calculates the transparency and applies the texture to the data. After rasterization, the data is 2D, lit using the different lights in the scene and may also have the specified texture maps applied to them.

Let us now consider each of the modules in a bit more detail.

1. Transformation Module

The transformation module is the first of the three modules of the rendering engine. This module handles the geometry transformations in the rendering engine. To do this, it uses three four-by-four (4x4) matrices, namely the view transformation matrix, the world transformation matrix and the projection matrix. For an explanation of these three matrices, refer [5], [6], [11], [12], [13] and [14]. The three matrices are maintained in three state registers, namely the viewing matrix, the world matrix and the projection matrix, repectively. This module uses one more state register, the viewport, for holding the dimensions of the 2D display area.

The transformation module combines all the matrices into one composite matrix and uses this for computations, as using only one matrix, as opposed to four, speeds up the calculations in the application.

It is possible to set the states of the state registers separately, in addition to setting the value of the composite matrix. But, it is advisible to let Direct3D calculate the composite matrix, as the matrix multiplication operations required, have been specially optimized in Direct3D. Additionally, the newer versions of DirectX make use of MMX technology.

Gambarajah 3 menunjukkan a diagrammatic representation of the transformation module.

Gambarajah 3: Transformation Module

2. Lighting Module

The lighting module of the rendering engine is the second of the three modules. It uses the data provided by the transformation module and calculates the lighting information for the received data.

This module maintains a stack of the current lights and the ambient light level and the different material properties of the data. All this information is used while calculating the light falling at a particular point in the scene. Gambarajah 4 shows a diagrammatic representation of the lighting module.

Gambarajah 4: Lighting Module

The lighting module can be operated in any one of the two lighting models it supports. The two models supported are:

• Monochromatic model
  - also known as the ramp model. This model uses only the gray component of each light source specified in the scene, to calculate a single shade value. This shade value is the diffuse component. Though this model supports multiple light sources, the color components of the lights are ignored.
  - A restriction of this model is that only gray shades can be displayed and the textures used have to be of 8-bit depth. An advantage of this model over the RGB model is that it gives better performance than the RGB model.
  

• RGB model
  - is the other model supported by the lighting module of the rendering engine. This model helps produce more realistic effects of the given scene, as it uses the full color content of the light sources and the material of the object being lit. This model supports multiple colored light sources.
  - A limitation of this model is that it may produce a banding effect if the color depth available is not very good. In the banding effect, the transition from one color to another, is not smooth. It is as if two different colored bands have been placed side-by-side. This effect is produced when the number of pixels available for representing the colors is far less than the number of colors actually required by the application to display the data properly. Also, its performance as compared to the monochromatic model may be less

3. Rasterization Module

The rasterization module is the last of the three modules of the rendering engine. This module takes only execute calls and the data and displays it onto the display surface.

On being given the execute call, the module goes through the list of vertices to be displayed and generates the transfomed vertices to be rendered. Clipping parameters can also be specified in this module. The module also culls back-facing triangles, viz. the triangles whose surface normals face away from the camera. An important point about this module is that it renders only clockwise oriented triangles.

Gambarajah 5 shows a diagrammatic representation of the rasterization module.

Gambarajah 5: Rasterization Module

Singkatan

DD - DirectDraw
DX - DirectX
D3D - Direct3D
API - Application Programming Interfaces
HAL - Hardware Abstraction Layer
HEL - Hardware Emulation Layer
Direct3D IM - Direct3D Immediate Mode
Direct3D RM - Direct3D Retained Mode

AuthorCENCI LastUpdate October 21, 2001