Re: [pygame] VideoCapture, PIL, and PyGame

[Nirav Patel <olpc@xxxxxxxxxxxxxx>]

I don't know exactly how VideoCapture is implemented, but the camera
buffer is apparently in BGR color order and the pixels are in reverse
order.

Thus, you can't use it directly as a Surface.  You could fix the
upsidedown image by using pygame.image.fromstring() with the last
argument as True.  It would probably be faster than going through PIL,
but then once you switch R and B with Numpy, it'll be slower again.
It would only be worth it if you don't want PIL as a dependancy.

If you want speed, the best way would be to write a C function to
write to a new Surface, reading from the end of the image to the
beginning.

A little psuedocode, without all the python API and SDL stuff.

Uint8* src = end_of_camera_buffer;
Uint8* dst = start_of_pygame_surface;
for (int i = 0; i < width*height*byte_depth; i++) {
    *src-- = *dst++;
}

Nirav

On Sun, Jun 14, 2009 at 12:35 AM, Ian Mallett<geometrian@xxxxxxxxx> wrote:
> Hi,
>
> So, I'm trying to get VideoCapture working with PyGame.  I've modified the
> standard VideoCapture.py file to make it simpler and easier to read.  I've
> attached it as well as my program, main.py.
>
> VideoCapture.py defines a class Device.  Device has two functions that I'm
> concerned with: .getBuffer(), which returns (string_data,width,height), and
> .getImage(), which returns a PIL image made with .getBuffer().
>
> I'm trying to get a PyGame surface from all this.  Every code sample I've
> seen converts the PIL image (returned from .getImage()) back into a string,
> and then makes a PyGame image with pygame.image.frombuffer(...):
> im = Camera.getImage()
> image = pygame.image.frombuffer(im.tostring(), im.size, im.mode)
>
> Though this works nicely, it seems terribly inefficient--VideoCapture.py
> takes the data as a string from .getBuffer(), and makes a PIL image.  The
> main, main.py, must then change this PIL image back into a string again, and
> then make a PyGame surface from that.  (Basically:
> STRING->PIL->STRING->PYGAME).  This setup doesn't fly realtime.  Moreover,
> it requires an extra dependency, PIL.
>
> So, I tried just converting the original string from .getBuffer() directly
> into a PyGame surface.  (Basically: STRING->PYGAME):
> data = Camera.getBuffer()
> image = pygame.image.frombuffer(data[0],(data[1],data[2]),"RGB")
>
> But this has problems.  It doesn't crash, but the image is upsidedown and
> the red and blue color channels appear to be flopped.  I tried fixing it
> with surfarray, which didn't work.  Then I tried finding what was wrong, and
> got inconsistent results.
>
> How can I get the second method (method 2: STRING->PYGAME) to look like the
> first method (method 1: STRING->PIL->STRING->PYGAME)?  main.py lets you
> switch back and forth (press 1 for method 1, 2 for method 2).
>
> Ian
>