Python Get Screen Pixel Value in OS X

A small improvement, but using the TIFF compression option for screencapture is a bit quicker:

$ time screencapture -t png /tmp/test.png
real        0m0.235s
user        0m0.191s
sys         0m0.016s
$ time screencapture -t tiff /tmp/test.tiff
real        0m0.079s
user        0m0.028s
sys         0m0.026s

This does have a lot of overhead, as you say (the subprocess creation, writing/reading from disc, compressing/decompressing).

Instead, you could use PyObjC to capture the screen using CGWindowListCreateImage. I found it took about 70ms (~14fps) to capture a 1680x1050 pixel screen, and have the values accessible in memory

A few random notes:

• Importing the Quartz.CoreGraphics module is the slowest part, about 1 second. Same is true for importing most of the PyObjC modules. Unlikely to matter in this case, but for short-lived processes you might be better writing the tool in ObjC
• Specifying a smaller region is a bit quicker, but not hugely (~40ms for a 100x100px block, ~70ms for 1680x1050). Most of the time seems to be spent in just the CGDataProviderCopyData call - I wonder if there's a way to access the data directly, since we dont need to modify it?
• The ScreenPixel.pixel function is pretty quick, but accessing large numbers of pixels is still slow (since 0.01ms * 1650*1050 is about 17 seconds) - if you need to access lots of pixels, probably quicker to struct.unpack_from them all in one go.

Here's the code:

import time
import struct

import Quartz.CoreGraphics as CG


class ScreenPixel(object):
    """Captures the screen using CoreGraphics, and provides access to
    the pixel values.
    """

    def capture(self, region = None):
        """region should be a CGRect, something like:

        >>> import Quartz.CoreGraphics as CG
        >>> region = CG.CGRectMake(0, 0, 100, 100)
        >>> sp = ScreenPixel()
        >>> sp.capture(region=region)

        The default region is CG.CGRectInfinite (captures the full screen)
        """

        if region is None:
            region = CG.CGRectInfinite
        else:
            # TODO: Odd widths cause the image to warp. This is likely
            # caused by offset calculation in ScreenPixel.pixel, and
            # could could modified to allow odd-widths
            if region.size.width % 2 > 0:
                emsg = "Capture region width should be even (was %s)" % (
                    region.size.width)
                raise ValueError(emsg)

        # Create screenshot as CGImage
        image = CG.CGWindowListCreateImage(
            region,
            CG.kCGWindowListOptionOnScreenOnly,
            CG.kCGNullWindowID,
            CG.kCGWindowImageDefault)

        # Intermediate step, get pixel data as CGDataProvider
        prov = CG.CGImageGetDataProvider(image)

        # Copy data out of CGDataProvider, becomes string of bytes
        self._data = CG.CGDataProviderCopyData(prov)

        # Get width/height of image
        self.width = CG.CGImageGetWidth(image)
        self.height = CG.CGImageGetHeight(image)

    def pixel(self, x, y):
        """Get pixel value at given (x,y) screen coordinates

        Must call capture first.
        """

        # Pixel data is unsigned char (8bit unsigned integer),
        # and there are for (blue,green,red,alpha)
        data_format = "BBBB"

        # Calculate offset, based on
        # http://www.markj.net/iphone-uiimage-pixel-color/
        offset = 4 * ((self.width*int(round(y))) + int(round(x)))

        # Unpack data from string into Python'y integers
        b, g, r, a = struct.unpack_from(data_format, self._data, offset=offset)

        # Return BGRA as RGBA
        return (r, g, b, a)


if __name__ == '__main__':
    # Timer helper-function
    import contextlib

    @contextlib.contextmanager
    def timer(msg):
        start = time.time()
        yield
        end = time.time()
        print "%s: %.02fms" % (msg, (end-start)*1000)


    # Example usage
    sp = ScreenPixel()

    with timer("Capture"):
        # Take screenshot (takes about 70ms for me)
        sp.capture()

    with timer("Query"):
        # Get pixel value (takes about 0.01ms)
        print sp.width, sp.height
        print sp.pixel(0, 0)


    # To verify screen-cap code is correct, save all pixels to PNG,
    # using http://the.taoofmac.com/space/projects/PNGCanvas

    from pngcanvas import PNGCanvas
    c = PNGCanvas(sp.width, sp.height)
    for x in range(sp.width):
        for y in range(sp.height):
            c.point(x, y, color = sp.pixel(x, y))

    with open("test.png", "wb") as f:
        f.write(c.dump())

I came across this post while searching for a solution to get screenshot in Mac OS X used for real-time processing. I have tried using ImageGrab from PIL as suggested in some other posts but couldn't get the data fast enough (with only about 0.5 fps).

The answer https://stackoverflow.com/a/13024603/3322123 in this post to use PyObjC saved my day! Thanks @dbr!

However, my task requires to get all pixel values rather than just a single pixel, and also to comment on the third note by @dbr, I added a new method in this class to get a full image, in case anyone else might need it.

The image data are returned as a numpy array with dimension of (height, width, 3), which can be directly used for post-processing in numpy or opencv etc… getting individual pixel values from it also becomes pretty trivial using numpy indexing.

I tested the code with a 1600 x 1000 screenshot - getting the data using capture() took ~30 ms and converting it to a np array getimage() takes only ~50 ms on my Macbook. So now I have >10 fps and even faster for smaller regions.

import numpy as np

def getimage(self):
    imgdata=np.fromstring(self._data,dtype=np.uint8).reshape(len(self._data)/4,4)
    return imgdata[:self.width*self.height,:-1].reshape(self.height,self.width,3)

note I throw away the “alpha” channel from the BGRA 4 channel.