client/cros/camera/mtf_calculator.py

# Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

# Import guard for OpenCV.
try:
    import cv
    import cv2
except ImportError:
    pass

import math
import numpy as np


def _ExtractPatch(sample, edge_start, edge_end, desired_width, crop_ratio):
    '''Crop a patch from the test target.'''
    # Identify the edge direction.
    vec = edge_end - edge_start

    # Discard both ends so that we MIGHT not include other edges
    # in the resulting patch.
    # TODO: Auto-detect if the patch covers more than one edge!
    safe_start = (1 - crop_ratio) * edge_start + crop_ratio * edge_end
    safe_end = crop_ratio * edge_start + (1 - crop_ratio) * edge_end
    minx = int(round(min(safe_start[0], safe_end[0])))
    miny = int(round(min(safe_start[1], safe_end[1])))
    maxx = int(round(max(safe_start[0], safe_end[0])))
    maxy = int(round(max(safe_start[1], safe_end[1])))
    if abs(vec[0]) > abs(vec[1]): # near-horizontal edge
        ylb = max(0, miny - desired_width)
        yub = min(sample.shape[0], maxy + desired_width + 1)
        patch = np.transpose(sample[ylb:yub, minx:(maxx + 1)])
    else:  # near-vertical edge
        xlb = max(0, minx - desired_width)
        xub = min(sample.shape[1], maxx + desired_width + 1)
        patch = sample[miny:(maxy + 1), xlb:xub]

    # Make sure white is on the left
    if patch[0, 0] < patch[-1, -1]:
        patch = np.fliplr(patch)
    # Make a floating point copy.
    patch = np.asfarray(patch)

    # TODO(sheckylin) Correct for vignetting.
    return patch


def _FindEdgeSubPix(patch, desired_width):
    '''Locate the edge position for each scanline with subpixel precision.'''
    ph = patch.shape[0]
    pw = patch.shape[1]

    # Get the gradient magnitude along the x direction.
    k_gauss = np.transpose(cv2.getGaussianKernel(7, 1.0))
    temp = cv2.filter2D(patch, -1, k_gauss, borderType=cv2.BORDER_REFLECT)
    k_diff = np.array([[-1, 1.0]])
    grad = abs(cv2.filter2D(temp, -1, k_diff,
                            borderType=cv2.BORDER_REPLICATE))

    # Estimate subpixel edge position for each scanline.
    ys = np.arange(ph, dtype=np.float64)
    xs = np.empty(ph, dtype=np.float64)
    x_dummy = np.arange(pw, dtype=np.float64)
    for y in range(ph):
        # 1st iteration.
        b = np.sum(x_dummy * grad[y])
        a = np.sum(grad[y])
        c = int(round(b / a))

        # 2nd iteration due to bias of different num of black and white pixels.
        dw = min(min(c, desired_width), pw - c - 1)
        b = np.sum(x_dummy[(c - dw):(c + dw + 1)] *
                   grad[y, (c - dw):(c + dw + 1)])
        a = np.sum(grad[y, (c - dw):(c + dw + 1)])
        xs[y] = int(round(b / a))

    # Fit a second-order polyline for subpixel accuracy.
    fitted_line = np.polyfit(ys, xs, 1)
    fitted_parabola = np.polyfit(ys, xs, 2)
    angle = math.atan(fitted_line[0])
    pb = np.poly1d(fitted_parabola)
    centers = [pb(y) for y in range(ph)]

    return angle, centers


def _AccumulateLine(patch, centers):
    '''Add up the scanlines along the edge direction.'''
    ph = patch.shape[0]
    pw = patch.shape[1]

    # Determine the final line length.
    w = min(int(round(np.min(centers))), pw - int(round(np.max(centers))) - 1)
    w4 = 2 * w + 1

    # Accumulate a 4x-oversampled line.
    psf4x = np.zeros((4, w4), dtype=np.float64)
    counts = np.zeros(4, dtype=np.float64)
    for y in range(ph):
        ci = int(round(centers[y]))
        idx = 3 + 4 * ci - int(4 * centers[y] + 2)
        psf4x[idx] += patch[y, (ci - w):(ci + w + 1)]
        counts[idx] += 1

    counts = np.expand_dims(counts, axis=1)
    psf4x /= counts
    psf = np.diff(psf4x.transpose().flatten())
    return psf


def _GetResponse(psf, angle):
    '''Compose the MTF curve.'''
    w = psf.shape[0]

    # Compute FFT.
    magnitude = abs(np.fft.fft(psf))

    # Smooth the result a little bit.
    # This is equivalent to applying window in the spatial domain
    # as enforced in the Imatest's algorithm.
    k_gauss = np.transpose(cv2.getGaussianKernel(7, 1.0))
    cv2.filter2D(magnitude, -1, k_gauss, magnitude,
                 borderType=cv2.BORDER_REFLECT)

    # Slant correction factor.
    slant_correction = math.cos(angle)

    # Compose MTF curve.
    # Normalize the low frequency response to 1 and compensate for the
    # finite difference.
    rw = w / 4 + 1
    magnitude = magnitude[0:rw] / magnitude[0]

    freqs = np.arange(rw, dtype=np.float64) * 4 / w / slant_correction
    attns = magnitude / (np.sinc(np.arange(rw, dtype=np.float64) / w) ** 2)
    return freqs, attns


def _FindMTF50P(freqs, attns, use_50p):
    '''Locate the MTF50P given the MTF curve.'''
    peak50 = (attns.max() if use_50p else 1.0) / 2.0
    idx = np.nonzero(attns < peak50)[0]
    if idx.shape[0] == 0:
        return freqs[-1]
    idx = idx[0]

    # Linear interpolation.
    ratio = (peak50 - attns[idx - 1]) / (attns[idx] - attns[idx - 1])
    return freqs[idx - 1] + (freqs[idx] - freqs[idx - 1]) * ratio


def Compute(sample, edge_start, edge_end, desired_width, crop_ratio,
            use_50p=True):
    '''Compute the MTF50P value of an edge.

    This function implements the slanted-edge MTF calculation method as used in
    the Imatest software. For more information, please visit
    http://www.imatest.com/docs/sharpness/ . The function in default setting
    computes the so-called MTF50P value instead of the traditional MTF50 in
    order to better cope with the in-camera post-sharpening. The result quality
    improves with longer edges and wider margin widths.

    Args:
        sample: The test target image. It needs to be single-channel.
        edge_start: The (rough) start point of the edge.
        edge_end: The (rough) end point of the edge.
        desired_width: Desired margin width on both sides of the edge.
        crop_ratio: The truncation ratio at the both ends of the edge.
        use_50p: Compute whether the MTF50P value or the MTF50 value.

    Returns:
        1: The MTF50P value.
        2, 3: The MTF curve, represented by lists of frequencies and
              attenuation values.
    '''
    patch = _ExtractPatch(sample, edge_start, edge_end, desired_width,
                          crop_ratio)
    angle, centers = _FindEdgeSubPix(patch, desired_width)
    psf = _AccumulateLine(patch, centers)
    freqs, attns = _GetResponse(psf, angle)
    return _FindMTF50P(freqs, attns, use_50p), freqs, attns