Video Card Memory and Monitor Resolution Hardware Guide

Video Card Memory and Monitor Resolution Hardware Guide

With the new video cards coming out with ever increasing graphic memory sizes, choosing a video card is no longer dependant upon just how fast the GPU runs or how many stream processors are available.  These differing amounts of graphic memory can greatly affect the performance of gamming, especially at very high resolutions.  Without the proper amount of graphic memory, your gaming could suffer and make certain games almost unplayable except at lowered settings.

All About Monitor Resolutions and Settings

Modern monitors since the early 1990s used an RGB (red green blue) system for showing objects on the monitor.  Each pixel on the monitor is made up of three small light sources, each having red, green, or blue lighting.  To create a certain color at that pixel, each light source is given a certain value or luminosity to achieve the final pixel color.  There are two common ways to encode the pixel data:  16-bit or 32-bit.

16-bit High Color Mode

A 16-bit mode known as Highcolor, in which there are either 5 bits per color, called 555 mode (32,768 total colors), or the same with an extra bit for green (because the green component contributes most to the brightness of a color in the human eye), called 565 mode (65,535 colors).This was the high-end for some display adapters for personal computers during the 1990's, but today is considered slightly obsolete in favour of the 24 or 32 bpp graphic modes. It is still in use in many devices with color screens as cell phones, digital cameras, personal digital assistants (PDA) and portable videogame consoles.

32-bit True Color Mode

The so-called 32 bpp display graphic mode is identical in precision to the 24 bpp mode; there are still only eight bits per component, and the eight extra bits are often not used at all. The reason for the existence of the 32 bpp mode is the higher speed at which most modern 32-bit (and better) hardware can access data that is aligned to byte addresses evenly divisible by a power of two, compared to data not so aligned.

With the need for compositing images came a variant of 24-bit RGB which includes an extra 8-bit channel for transparency, thus resulting also in a 32-bit format. The transparency channel is commonly known as the alpha channel, so the format is named RGBA. Note again that since it does not change anything in the RGB model, RGBA is not a distinct color model, it is only a representation that integrates transparency information along with the color information. This extra channel allows for alpha blending of the image over another, and is a feature of the PNG format.

Now each pixel on the monitor needs to have 32 bits (4 bytes) of data.  As current monitor resolutions keep on rising, so does the amount of data needed to display.  Below I will list the current display resolutions and how much data is needed to drive them.  As you can see, each frame of monitor data can reach very large sizes in the very large resolutions.

Computer Standard	Resolution	Aspect Ratio	Pixels	16-bit Size	32-bit Size
VGA	640×480	4:3	307,200	.6 MB	1.2 MB
WGA or WVGA	800×480	5:3	384,000	.73 MB	1.46 MB
SVGA	800×600	4:3	480,000	.91 MB	1.83 MB
XGA	1024×768	4:3	786,432	1.5 MB	3.0 MB
XGA+	1152×864	4:3	995,328	1.90 MB	3.79 MB
SXGA	1280×1024	5:4	1,310,720	2.5 MB	5 MB
WXGA	1280×800	16:10	1,024,000	1.95 MB	3.9 MB
WSXGA or WXGA+	1440×900	16:10	1,296,000	2.47 MB	4.94 MB
SXGA+	1400×1050	4:3	1,470,000	2.8 MB	5.6 MB
WSXGA	1600×1024	25:16	1,638,400	3.12 MB	6.25 MB
WSXGA+	1680×1050	16:10	1,764,000	3.36 MB	6.72 MB
UXGA	1600×1200	4:3	1,920,000	3.66 MB	7.32 MB
WUXGA	1920×1200	16:10	2,304,000	4.4 MB	8.79 MB
QXGA	2048×1536	4:3	3,145,728	6 MB	12 MB
WQXGA	2560×1600	16:10	4,096,000	7.81 MB	15.62 MB

Graphic Memory Data Storage

Besides the pixel data representing that image frame, there are other elements which must reside within the frame buffer (graphic memory) which can also use more memory then you realize.

Textures

Textures are graphical images that are mapped onto the 3D object's faces.  The textures are loaded into the graphical memory ahead of time for quick access later by the GPU.  The amount of texture data can vary widely from game to game, but a major portion of a game's total file size can be in the textures alone.  First person shooters and other close action games usually have the highest amount of texture files.  However, depending upon the scene and complexity, the texture data can easily be larger than all the other data on the video card.

Anti-Aliasing

While you may not think that this would take up much memory, it is in fact one of the biggest memory users.  Many forms of anti-aliasing involves creating a pixel image bigger than the final pixel image and down-sampling the pixels to fit inside a single pixel.  2x AA requires an image 2x as big as the display resolution while 4x requires an image 4x as big.  With some cards allowing up to 16x AA, the image file require for AA can become quite large indeed.  A common display resolution 1600 x 1200 requires 7.32 MB for the final pixel image data.  At 4x AA, the require data file for the AA needs to almost 30 MB large.

Depth Buffer

As 3D objects are converted to pixels, each pixel is given a depth or z-value stored inside the Z or Depth Buffer.  As a new object is drawn within the same X and Y coordinates, the new Z values of the pixels are compared against the old ones.  If the pixel is nearing, the new depth is written over the old one.  If the object is further, no change is made to the Z value of that pixel.  Each value in the Z Buffer is a 32-bit floating point number.  Thus the Z Buffer is roughly close to the same size as the pixel data.

Double Buffering

Most modern video cards employ a technique called double buffering.  In essence, there are two pixel data files.  One is in use by the video card to fill with pixel data while the other one is used for rendering the image through the RAMDAC and onto your monitor.  Once the first buffer is full, the two buffers switch place and the whole operation is restarted.  With double buffering enabled, it doubles the space needed over a single pixel data file.

Pixel Maps

To achieve more realistic scenes within 3D applications, the pixels of a given 3D object may be mapped to a height file (bump mapping, parallax mapping, normal mapping, etc.)  While the 3D object may have a limited number of polygons, the height files creates the illusion of many more smaller polygons giving the object a more realistic look.  Depending upon the height mapping employed and how many objects, the height map files can also be quick large in file size.

Vertex Data

All of the in game objects are created using an array of 3D points called vertex data.  The vertex data has the 3D coordinates of each corner of the 3D object.

Other Data

There are many other smaller files and data which also resides in the graphic memory as well.  These include but are not limited to polygon meshes, lighting maps, and etc.

Graphic Memory Transfer Rates

Besides being able to support all the data files within the memory of the card, the memory must also be fast enough to push the final image data through to RAMDAC and onto the monitor.  Frame rates can fall if the memory transfer rate is not high enough to support the resolution selected.

The RAMDAC

The Random Access Memory Digital Analog Convertor is used to convert the digital pixels into analog signals for usage in older analog monitors.  It is also used for controlling the digital output as well including the DVI and HDMI interfaces now commonly found.  Almost all current RAMDACs on video cards currently run at 400 MHz.  This means that the largest common display setting (2560 x 1600) can easily be display at 60 Hz.  It also means that the even more common resolution (1600 x 1024) can be run at 170 Hz.  Although impractical and impossible to run a monitor at that rate, almost all current video cards can display any resolution you want if not limited by memory bandwidth or monitor limits.

Memory Bandwidth

Depending upon the refresh rating you have the monitor set to, your video card may not be able to support the transfer rates needed.  Most users run their monitors at 60 Hz, but higher-end monitors support higher refresh rates which can make gameplay smoother if your video card can support it.  Please note that the transfer rates quoted below are the theoretical minimum as the RAMDAC and other hardware may impede the total bandwidth of the memory.  Please allow at least a 10% to 20% buffer based upon this error.

Resolution	Bit Depth	60 Hz	72 Hz	80 Hz	85 Hz	90 Hz	100 Hz
800x600	16	54.9	65.9	73.2	77.8	82.4	91.6
	32	109	132	146	156	165	183
1024x768	16	90	108	120	128	135	150
	32	180	216	240	255	270	300
1280x1024	16	150	180	200	213	225	250
	32	300	360	400	425	450	500
1600x1200	16	220	264	263	311	330	366
	32	440	257	586	623	659	732
2560x1600	16	469	562	625	664	703	781
	32	938	1125	1250	1328	1406	1562

All transfer rates are quoted in MB/s.

 

Conclusion

Hopefully you have learned how much graphic memory and the memory bandwidth needed for the exact resolution you plan to game at.  You may have noticed that many video cards loose performance once they reach a certain resolution.  Looking at the tables above, you can tell whether or not if the card was memory or bandwidth restricted.

 

You may have also noticed that some lower-end video cards like the NVidia 9600 GT comes in variants with 1 GB of GDDR3 memory.  With a maximum bandwidth of only 56 GB/s it may seem like it could handle any resolution above.  However, the graphic memory also needs to carry data from the CPU and system memory and also be constantly be written to from the GPU.  This will limit the bandwidth left over for transmitting the final image data to the RAMDAC.  The fact of the matter is, the difference between the 512 MB version and 1 GB version is negligible.  This is due in part to bandwidth limitations and also GPU limitations.

 

In short, the following resolutions and graphic memory suggestions are recommended.

 

64 MB	Non 3D applications below 1600x1200
128 MB	3D applications at or below 1024 x 768
256 MB	3D application at or below 1280 x 1024
512 MB	3D applications at or below 1600 x 1200
1 GB+	3D application at or above 2560 x 1600