Introduction 

In the articles that follow over the coming months I hope that the journey that takes us to the "other side" will prove to be both educational, and more importantly, enjoyable. My plan is to build a prototype isometric engine ( used in such games as Command and Conquer, Dark Reign, StarCraft & Diablo ) using DelphiX  (a free set of DirectX components), that will eventually incorporate some 3D elements as well. While building this prototype engine I intend to cover such topics as collision detection, shortest path algorithms across a map, unit formations/groupings using boids and animating your characters. 

Our first order of  business is to get an overview of DirectX , followed by some basic terminology and quickly followed by a DelphiX overview closely followed by a simple example in using some DelphiX components to create a worm-hole effect that  looks similar to this... 

DirectX Overview

It was created to solve the problem of having umpteen hardware manufactures all coming out with the latest and greatest 2D/3D accelerator, advanced sound synthesis hardware, networking hardware and strange input devices. Under Dos gaming programming you would have had to probably write your own device drivers for these new pieces of hardware. Using Windows 95/98/NT, and soon to be, Windows 2000, most ( if not all ) of the compatibility issues are supposed to be solved.

Also when using DirectX if you happen to have hardware acceleration ( such as 3Dfx, Nvidia and the like ) it will use that instead, thus freeing up the CPU to do other things. So, the person who spends $500 on their super duper 128 bit video card gets their money's worth and the game looks better and runs faster without additional code. Just FYI ( for your information ), DirectX 6.1 works with Windows 95 and 98.  DirectX 3 works with NT 4 but only with service pack 3.  There is an un-official/un-supported DirectX 5 patch for NT 4 that can be found here.  I have also got DirectX 6.1 running on NT 5 beta 2, though Microsoft have said that when NT 5/2000 is officially released it will support DirectX 7.0. 

DirectX is currently made up of the following 7 major components :- 

• DirectDraw - Allows direct access to display memory and hardware features, while maintaining compatibility with Windows based applications.
• DirectInput - Interface for Input Devices ( Mouse, Keyboards, Joysticks, Headgear etc... ).
• DirectPlay - Interface for multi-player type options ( TCP/IP, Direct Dial, Local Networking ).
• DirectSound - Interface for sound hardware ( WAV, MIDI files and the like ).
• DirectSound3D - Interface to allow you to position sounds anywhere in three-dimensional space, thus giving you that Surround Sound feel.
• DirectMusic - Interface for creating an array of original musical patterns and instructions that respond to user events.
• Direct3D - Interface for 3-D hardware ( includes Immediate and Retained Modes ).

[ Figure 1 - DirectX architecture ]

Terminology 

Surfaces - Video memory that is used to store various bitmaps and offscreen video butters are all referred to as surfaces in DirectX. The video memory or surface that is currently displayed on the screen is referred to as the primary surface.  This surface is as big as the amount of memory needed for the current display mode. If the display mode is 640 x 480 x 256 colours ( 8 bit per pixel ), then the primary surface would use 307,200 bytes of video memory.  You usually also create at least one offscreen surface that is the same size as the primary surface this offscreen surface is used for flipping  ( this will be covered shortly ).  This means that you need 614,400 bytes of video memory just to get started, without even loading any bit maps.  When you run out of video memory, your direct Draw surfaces will be created in system memory ( normal RAM ) thereby losing the benefits of hardware acceleration.  Currently a 2MB video card should be considered the absolute minimum for getting a fairly simple game up and running. 

Double Buffering - Is a technique used to achieve fast, smooth animation. It is usually a separate image buffer that is used to compose your scene on, before it is visible. This is usually used with page flipping ( see next explanation ) . 

Page Flipping animation - This term probably goes back to your school days when you used to entertain yourself by drawing scenes on consecutive pages at the bottom of thick books so  you could flip the pages and get an animation effect.  If you never did this as a child, give it a try now, it will help crystalise the effect we are looking for.  Scenes are usually composed by copying ( also known as blitting )  bitmaps or sprites to an offscreen surface ( or buffer ), and when the scene is complete the video hardware has the ability to flip to the offscreen surface, thus making the newly composed scene visible.  Page flipping is extremely fast because there is no memory copying involved.  It is simply a matter of setting a register on the video card that tells it where to start displaying  from in video memory. 
There are  basically 3 steps in creating page flipping animations. 

1. Compose the scene on the offscreen surface.
2. Flip to the offscreen surface to display it, which also makes the previously displayed surface the offscreen surface now.
3. Repeat step 1.

Tearing - Most monitors try and "refresh" the display about 70 times per second ( 70 Hz ), and it normally refreshes from top to bottom. The problem occurs when you try to draw a new image on the screen when the monitor is in the middle of redrawing/refreshing the current display. When you do this the bottom half of the screen is drawn using the new image while the top half still has the old image. This visual effect is called tearing or shearing. In the dark days of DOS you would have had to check for a vertical retrace to determine when the screen was refreshing so that you could avoiding this tearing from happening. With DirectX this check is taken care of for you. 

Bitmaps and Sprites - Bitmaps are basically images. Sprites are the same as bitmaps but usually have transparent areas. The term Sprites, is also used for animated images within a game. Sprites are usually made up of several frames of animation, Such as a man walking or as in this example a space ship flying... 

Clipping - Is a technique used to ensure that drawing routines are only able to draw within a specified ( usually ) rectangular area or region, such as a window. If you used  DirectX without clipping you would be able to draw anywhere on the screen. This is fine if you are in full screen mode, but is definitely unacceptable if your application is windowed. 

Colour theory - Colours under Windows is usually represented, by the RGB (Red, Green, Blue)  colour model. Using the 3 primary colours red, green and blue, nearly all possible visible colours can be produced by combining various proportions of  the three . These are commonly stored as three bytes - each byte represents the relative intensity of each primary colour as a value from 0 to 255 inclusive, thus giving us 256 possible colours. 0 being of lowest intensity and obviously 255 being the highest. 

In Delphi, colours are represented by the TColor class that is defined within the Graphics unit. 
You can specify colours either using the pre-defined constants like clBlue, clLime etc. or for  greater accuracy, specifying the TColor as a specific 4-byte hexadecimal number where the low three bytes represent RGB colour intensities for blue, green, and red, respectively. The value $00FF0000 represents full intensity, pure blue, $0000FF00 is pure green, and $000000FF is pure red, $00000000 is black, $00FFFFFF is white and  $00FF00FF is purple. I'm sure you get the idea. A little extract from the VCL help file also tells us that "If the highest-order byte is zero ($00), the color obtained is the closest matching color in the system palette. If the highest-order byte is one ($01), the color obtained is the closest matching color in the currently realized palette. If the highest-order byte is two ($02), the value is matched with the nearest color in the logical palette of the current device context."

There are other colour models, such as HSV (Hue, Saturation, Luminance), but I will not be covering them in these articles. The book "Computer Graphics, principles and practise" by Foley & van Dam ( often referred to as The Computer Graphics Bible ) explains colour models in some detail, and how to convert between colour models.