Berkeley CSUA MOTD:Entry 50203
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2024/11/23 [General] UID:1000 Activity:popular
11/23   

2008/6/9-12 [Computer/HW/Memory] UID:50203 Activity:nil
6/9     Video RAM question:
        Let's say I need to display a shape model that was ~3 million
        plates that are each 72 bytes (216MB). Do I have to have a video
        card that has more than 216 MB of RAM or can the system dip into
        main RAM in order to display it? As I understand it, the VRAM is
        a buffer but what happens when the buffer fills? In this instance
        my video card has 64 MB of RAM and the image displays fine. Is
        the rest of the image resident in system RAM? Do people get video
        cards with more VRAM for performance reasons (VRAM is faster and
        dual-ported) or because they can display images they never could
        otherwise? Compare a system with 16GB RAM and 64 MB VRAM to one
        with 1 GB RAM and 2 GB VRAM.
        \_ That wildly depends on how the data is presented to your card.
           \_ Please elaborate.
              \_ If you present the data to your card the right way, it can
                 store the vectors directly.  If you don't, it can't.
                 \_ So what if it can't if I have 16 GB of main RAM? Can I
                    use that or not? Why would I not want to? That is,
                    what does storing directly buy me? Can I display my
                    model in either case but with a performance penalty?
                    Excuse my ignorance, but I am not a gamer or anime
                    freak and never had a graphics class. I realize you
                    touched a little bit below, but I've been reading
                    about rendering pipelines all day and there's not been
                    much that talks about the role of the h/w in this or
                    what happens when you run out of video RAM.
                    \_ If you do "immediate" mode then you're limited by the
                       bus speed to the video card.  If you can cram it all
                       onto the card, you're theoretically maximizing the
                       hardware performance. -emarkp
                       \_ Are there any other penalties? Just the penalty
                          of the speed of the VRAM vs. RAM and the
                          offloading of the main CPU in favor of the GPU?
                          I don't see the bus speed as a limiting factor.
                          You have to load it all into VRAM over some bus
                          anyway, right? I realize say AGP might be
                          faster than PCI but I am more concerned by how
                          the size of VRAM impacts the problem than by
                          issues of throughput, which are clear-cut. If I
                          have 256 MB of VRAM and a 1 GB image then what
                          happens? Does the video card swap to main RAM?
                          \_ At this point, you leave generalities and it
                             depends on the card, and the quality of the
                             driver.  If the card runs low on memory, it can
                             dump whatever it needs to render, and then pull it
                             back from main memory when necessary. -emarkp
                 Typically cards do 16 bits of floating point, but that's
                 moving up to 32.  The rendering pipeline today is basically
                 transforming triangles modelspace->worldspace, lighting,
                 worldspace->cameraspace.  This can be done in "immediate"
                 mode, where each triangle is provided from the program running
                 in the CPU to the video system (D3D, OpenGL, etc.=> video
                 card) or in buffered mode (historically "display lists" on
                 OpenGL though that included much more than just geometry)
                 commonly called "vertex buffer objects".  If there's room on
                 the card, you can push a bunch of geometry into the card, then
                 just tweak the input values and do everything very
                 efficiently.  If there isn't room on the card, then the video
                 memory is mostly 1) frame buffer (what you see) and 2) texture
                 memory.
                 http://en.wikipedia.org/wiki/Rendering_pipeline
                 John Carmack (Id software, etc.) believes he's solved the
                 megatexture problem (imagine a whole world or large
                 environment that can be textured down to tiny detail),
                 with little or no performance issues (implemented in their
                 upcoming game, Rage).  He says his next project is to do the
                 same with geometry.  So things may be changing in the next few
                 years. -emarkp
2024/11/23 [General] UID:1000 Activity:popular
11/23   

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en.wikipedia.org/wiki/Rendering_pipeline
In this stage of the graphics pipeline, geometry is transformed from the eye space of the rendering camera into 2D image space, mapping the 3D scene onto a plane as seen from the virtual camera. Rasterization is the process by which the 2D image space representation of the scene is converted into raster format and the correct resulting pixel values are determined. stream processor since all vertices and fragments can be thought of as independent. This allows all stages of the pipeline to be used simultaneously for different vertices or fragments as they work their way through the pipe.