Fade an image to black instead of inverting colors

Coding
,
10

My suggestion is to draw the stimulus, change the blend mode, draw the filter, and then reset the blend mode. If you change the blend mode to GL_DST_COLOR, GL_ZERO, then the filter will be multiplied with the stimulus to produce the output (see this great blend mode helper site).

Here is an example:

from __future__ import division
from psychopy import core, visual, event
import numpy as np

import pyglet.gl

CS = 'rgb'  # ColorSpace
WHITE = [1, 1, 1]
LIGHT_GREY = [0.5, 0.5, 0.5]
GREY = [0, 0, 0]
BLACK = [-1, -1, -1]
RED = [1, -1, -1]
BLUE = [-1, -1, 1]
GREEN = [-1, 1, -1]

def color_filter(img, rgb_filter):
    """ Filter a greyscale image through an RGB color filter

    img: a 2D numpy array
    filter: a 3-number RGB sequence (-1)-(1)
    """
    img_rgb = np.empty([img.shape[0], img.shape[1], 3])
    rgb_filter = rescale_zero_min(rgb_filter)
    img = rescale_zero_min(img)
    for n_channel in range(3): # For each color channel
        img_rgb[:,:,n_channel] = img * rgb_filter[n_channel]
    return img_rgb

def rescale_zero_min(x):
    """ Rescale a color from (-1)-(1) to 0-1.
    """
    return (np.array(x) + 1) / 2

img = np.array([[-1, 0], [0, 1]]) # Image bitmap

win = visual.Window([400, 400], monitor='testMonitor',
                    color=LIGHT_GREY, colorSpace=CS,
                    useFBO=True,
                    units='pix')

stimulus = visual.ImageStim(win=win, image=img,
                     colorSpace=CS,
                     pos=(0, -100),
                     size=(100, 100),
                     units='pix')

new_stimulus = visual.ImageStim(win=win, image=img,
                     colorSpace=CS,
                     pos=(0, 100),
                     size=(100, 100),
                     units='pix')

img_filter = visual.ImageStim(win=win, image=np.ones((100, 100)),
                     colorSpace=CS,
                     pos=(0, 100),
                     size=(100, 100),
                     units='pix')

# Show the example stimuli
for color in (WHITE, LIGHT_GREY, GREY, BLACK, RED, BLUE, GREEN):
    filtered_image = color_filter(img, color)
    stimulus.setImage(filtered_image) ## <---- This is too slow

    stimulus.draw()

    new_stimulus.draw()

    pyglet.gl.glBlendFunc(pyglet.gl.GL_DST_COLOR, pyglet.gl.GL_ZERO)

    img_filter.color = color
    img_filter.draw()

    # reset the blend mode
    pyglet.gl.glBlendFunc(pyglet.gl.GL_SRC_ALPHA, pyglet.gl.GL_ONE_MINUS_SRC_ALPHA)

    win.flip()

    frame = np.array(win.getMovieFrame())

    assert np.all(frame[:200, :] == frame[200:, :])

    event.waitKeys()
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