Saturday, 26 October 2013

Pi Eyes Stage 6

Right, we've got all kinds of bits working but there's another ingredient I need before the system is just about 'functional'. For image processing I need the camera feed at multiple resolutions, so I can do cheap processing operations on high res feeds, and expensive ones on low res feeds. To do this I use the video splitter component, and have reworked my camera api to:
  • Create 4 separate outputs, each with its own resizer that does the RGB conversion but generates a different resolution.
  • Output 0 = full res, output 1 = half res etc
  • You still use ReadFrame or Begin/EndReadFrame, but now you pass in a 'level' as well
  • Internally the camera code has become a bit more complex to handle this multi output system but it's mostly just rearranging code.
I won't go into the code here as it was lots of tweaks all over the place and not easy to write. Here is a nice image of 2 of the outputs to make it more clear:

As you can see, in the top image I am at full resolution, however in the lower one it's displaying me at (in this case) 1/8th of the upper resolution. Just to demonstrate it is actually getting all the feeds (and the above isn't just from running the app twice!), this video shows it flicking between them live:

Here's the actual application code for the program above:

//entry point
int main(int argc, const char **argv)
    printf("PI Cam api tester\n");
    printf("Starting camera\n");
    CCamera* cam = StartCamera(MAIN_TEXTURE_WIDTH, MAIN_TEXTURE_HEIGHT,15);

    //create 4 textures of decreasing size
    GfxTexture textures[4];
    for(int texidx = 0; texidx < 4; texidx++)
        textures[texidx].Create(MAIN_TEXTURE_WIDTH >> texidx, MAIN_TEXTURE_HEIGHT >> texidx);

    printf("Running frame loop\n");
    for(int i = 0; i < 3000; i++)
        //pick a level to read based on current frame (flicking through them every second)
        int texidx = (i / 30)%4;

        //lock the chosen frame buffer, and copy it directly into the corresponding open gl texture
        const void* frame_data; int frame_sz;

        //begin frame, draw the texture then end frame


Note the really crucial point is that my app above is just reading 1 of the levels each frame, however they are all available every frame, so if I chose (and the cpu was available) I could do something with every level. That's really key and undoubtedly what I'll need going forwards. Code for the whole thing is here:

In terms of frame rate it has suffered unfortunately. That image resizer really seems to chew through frame time for some reason. Maybe there's lots of copies going on or something else funky, but it is going disappointingly slow. At 1280x720 the frame rate is probably worse than 10hz when reading the hi res data.

Next up I reckon I'll clean up the api a little - give the user options as to what to enable/disable and make sure everything shuts down right and add a post with a little tutorial on its use. Once that's done, onto gpu acceleration land....

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