PC Hardware A Beginner’s Guide

So, for a 1024 × 768 resolution using 24-bit color depth, the video card must have at least

2.4MB of video RAM, and more is always better.

Another example: To display a 1600 × 1200 resolution with a 32-bit color depth, the

graphics card needs about 8MB of video RAM:

1600 * 1200 = 1,920,000 (pixels of resolution)

32 / 8 = 4 (bytes of color depth)

1920000 * 4 = 7,680,000 (bytes of video RAM required)

Understand that the above calculations are figuring the video RAM requirements for 2D

images. Table 12-2 shows the amounts of video RAM required by several common

graphics settings.

3D Video Memory

Video cards that support 3D graphics require more video RAM than 2D cards even on the

same resolution and color depth settings. This is because in addition to the 2D (down and

across) a third dimension of depth is added.

Real 3D cards, video cards that truly support three dimensional displays, use three

buffers to hold the graphics data: a front buffer, a back buffer, and a Z-buffer. The addi-

tionoftheZ-bufferconsumesenoughoftheavailablevideoRAMtotypicallyrequirethat

theresolutionbereduced.Forexample,a2Dvideocardwith4MBofvideoRAMcansup-

portdisplaysettingsof1600×1200resolutionand16-bitcolordepthbutcanonlysupport

a 3D game using 800 × 600 resolution and a 16-bit color depth setting.

The front and back buffers are each the size required to hold the color data according to

thecolordepthinuse,suchas24or32bits.TheZ-bufferisusually16bits(or2bytes)insize.

Use this formula to calculate the amount of video RAM needed to support a

3D display:

Resolution * ((Color Depth (in bytes) * 2) + 2) = 3D video RAM requirements

Chapter 12: Video Cards^265

Resolution Color Depth VRAM Required

640 × 480 8-bit 307KB

1,024 × 768 16-bit 1.57MB

1,024 × 768 24-bit 2.36MB

1,600 × 1,200 24-bit 5.76MB

1,600 × 1,200 32-bit 7.68MB

Table 12-2. Common 2D Video RAM Requirements