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311 lines
12 KiB
C++
311 lines
12 KiB
C++
/**********************************************************************
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* File: degradeimage.cpp
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* Description: Function to degrade an image (usually of text) as if it
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* has been printed and then scanned.
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* Authors: Ray Smith
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* Created: Tue Nov 19 2013
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*
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* (C) Copyright 2013, Google Inc.
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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**********************************************************************/
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#include "degradeimage.h"
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#include <cstdlib>
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#include "allheaders.h" // from leptonica
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#include "genericvector.h"
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#include "helpers.h" // For TRand.
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#include "rect.h"
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namespace tesseract {
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// A randomized perspective distortion can be applied to synthetic input.
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// The perspective distortion comes from leptonica, which uses 2 sets of 4
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// corners to determine the distortion. There are random values for each of
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// the x numbers x0..x3 and y0..y3, except for x2 and x3 which are instead
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// defined in terms of a single shear value. This reduces the degrees of
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// freedom enough to make the distortion more realistic than it would otherwise
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// be if all 8 coordinates could move independently.
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// One additional factor is used for the color of the pixels that don't exist
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// in the source image.
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// Name for each of the randomizing factors.
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enum FactorNames {
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FN_INCOLOR,
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FN_Y0,
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FN_Y1,
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FN_Y2,
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FN_Y3,
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FN_X0,
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FN_X1,
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FN_SHEAR,
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// x2 = x1 - shear
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// x3 = x0 + shear
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FN_NUM_FACTORS
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};
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// Rotation is +/- kRotationRange radians.
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const float kRotationRange = 0.02f;
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// Number of grey levels to shift by for each exposure step.
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const int kExposureFactor = 16;
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// Salt and pepper noise is +/- kSaltnPepper.
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const int kSaltnPepper = 5;
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// Min sum of width + height on which to operate the ramp.
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const int kMinRampSize = 1000;
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// Degrade the pix as if by a print/copy/scan cycle with exposure > 0
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// corresponding to darkening on the copier and <0 lighter and 0 not copied.
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// Exposures in [-2,2] are most useful, with -3 and 3 being extreme.
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// If rotation is nullptr, rotation is skipped. If *rotation is non-zero, the
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// pix is rotated by *rotation else it is randomly rotated and *rotation is
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// modified.
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//
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// HOW IT WORKS:
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// Most of the process is really dictated by the fact that the minimum
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// available convolution is 3X3, which is too big really to simulate a
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// good quality print/scan process. (2X2 would be better.)
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// 1 pixel wide inputs are heavily smeared by the 3X3 convolution, making the
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// images generally biased to being too light, so most of the work is to make
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// them darker. 3 levels of thickening/darkening are achieved with 2 dilations,
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// (using a greyscale erosion) one heavy (by being before convolution) and one
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// light (after convolution).
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// With no dilation, after covolution, the images are so light that a heavy
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// constant offset is required to make the 0 image look reasonable. A simple
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// constant offset multiple of exposure to undo this value is enough to achieve
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// all the required lightening. This gives the advantage that exposure level 1
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// with a single dilation gives a good impression of the broken-yet-too-dark
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// problem that is often seen in scans.
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// A small random rotation gives some varying greyscale values on the edges,
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// and some random salt and pepper noise on top helps to realistically jaggy-up
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// the edges.
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// Finally a greyscale ramp provides a continuum of effects between exposure
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// levels.
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Pix* DegradeImage(Pix* input, int exposure, TRand* randomizer,
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float* rotation) {
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Pix* pix = pixConvertTo8(input, false);
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pixDestroy(&input);
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input = pix;
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int width = pixGetWidth(input);
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int height = pixGetHeight(input);
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if (exposure >= 2) {
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// An erosion simulates the spreading darkening of a dark copy.
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// This is backwards to binary morphology,
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// see http://www.leptonica.com/grayscale-morphology.html
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pix = input;
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input = pixErodeGray(pix, 3, 3);
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pixDestroy(&pix);
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}
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// A convolution is essential to any mode as no scanner produces an
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// image as sharp as the electronic image.
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pix = pixBlockconv(input, 1, 1);
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pixDestroy(&input);
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// A small random rotation helps to make the edges jaggy in a realistic way.
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if (rotation != nullptr) {
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float radians_clockwise = 0.0f;
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if (*rotation) {
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radians_clockwise = *rotation;
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} else if (randomizer != nullptr) {
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radians_clockwise = randomizer->SignedRand(kRotationRange);
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}
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input = pixRotate(pix, radians_clockwise,
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L_ROTATE_AREA_MAP, L_BRING_IN_WHITE,
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0, 0);
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// Rotate the boxes to match.
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*rotation = radians_clockwise;
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pixDestroy(&pix);
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} else {
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input = pix;
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}
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if (exposure >= 3 || exposure == 1) {
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// Erosion after the convolution is not as heavy as before, so it is
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// good for level 1 and in addition as a level 3.
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// This is backwards to binary morphology,
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// see http://www.leptonica.com/grayscale-morphology.html
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pix = input;
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input = pixErodeGray(pix, 3, 3);
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pixDestroy(&pix);
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}
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// The convolution really needed to be 2x2 to be realistic enough, but
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// we only have 3x3, so we have to bias the image darker or lose thin
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// strokes.
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int erosion_offset = 0;
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// For light and 0 exposure, there is no dilation, so compensate for the
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// convolution with a big darkening bias which is undone for lighter
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// exposures.
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if (exposure <= 0)
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erosion_offset = -3 * kExposureFactor;
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// Add in a general offset of the greyscales for the exposure level so
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// a threshold of 128 gives a reasonable binary result.
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erosion_offset -= exposure * kExposureFactor;
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// Add a gradual fade over the page and a small amount of salt and pepper
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// noise to simulate noise in the sensor/paper fibres and varying
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// illumination.
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l_uint32* data = pixGetData(input);
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for (int y = 0; y < height; ++y) {
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for (int x = 0; x < width; ++x) {
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int pixel = GET_DATA_BYTE(data, x);
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if (randomizer != nullptr)
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pixel += randomizer->IntRand() % (kSaltnPepper*2 + 1) - kSaltnPepper;
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if (height + width > kMinRampSize)
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pixel -= (2*x + y) * 32 / (height + width);
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pixel += erosion_offset;
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if (pixel < 0)
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pixel = 0;
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if (pixel > 255)
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pixel = 255;
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SET_DATA_BYTE(data, x, pixel);
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}
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data += input->wpl;
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}
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return input;
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}
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// Creates and returns a Pix distorted by various means according to the bool
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// flags. If boxes is not nullptr, the boxes are resized/positioned according to
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// any spatial distortion and also by the integer reduction factor box_scale
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// so they will match what the network will output.
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// Returns nullptr on error. The returned Pix must be pixDestroyed.
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Pix* PrepareDistortedPix(const Pix* pix, bool perspective, bool invert,
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bool white_noise, bool smooth_noise, bool blur,
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int box_reduction, TRand* randomizer,
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GenericVector<TBOX>* boxes) {
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Pix* distorted = pixCopy(nullptr, const_cast<Pix*>(pix));
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// Things to do to synthetic training data.
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if (invert && randomizer->SignedRand(1.0) < 0)
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pixInvert(distorted, distorted);
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if ((white_noise || smooth_noise) && randomizer->SignedRand(1.0) > 0.0) {
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// TODO(rays) Cook noise in a more thread-safe manner than rand().
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// Attempt to make the sequences reproducible.
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srand(randomizer->IntRand());
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Pix* pixn = pixAddGaussianNoise(distorted, 8.0);
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pixDestroy(&distorted);
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if (smooth_noise) {
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distorted = pixBlockconv(pixn, 1, 1);
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pixDestroy(&pixn);
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} else {
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distorted = pixn;
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}
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}
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if (blur && randomizer->SignedRand(1.0) > 0.0) {
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Pix* blurred = pixBlockconv(distorted, 1, 1);
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pixDestroy(&distorted);
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distorted = blurred;
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}
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if (perspective)
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GeneratePerspectiveDistortion(0, 0, randomizer, &distorted, boxes);
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if (boxes != nullptr) {
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for (int b = 0; b < boxes->size(); ++b) {
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(*boxes)[b].scale(1.0f / box_reduction);
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if ((*boxes)[b].width() <= 0)
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(*boxes)[b].set_right((*boxes)[b].left() + 1);
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}
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}
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return distorted;
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}
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// Distorts anything that has a non-null pointer with the same pseudo-random
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// perspective distortion. Width and height only need to be set if there
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// is no pix. If there is a pix, then they will be taken from there.
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void GeneratePerspectiveDistortion(int width, int height, TRand* randomizer,
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Pix** pix, GenericVector<TBOX>* boxes) {
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if (pix != nullptr && *pix != nullptr) {
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width = pixGetWidth(*pix);
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height = pixGetHeight(*pix);
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}
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float* im_coeffs = nullptr;
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float* box_coeffs = nullptr;
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l_int32 incolor =
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ProjectiveCoeffs(width, height, randomizer, &im_coeffs, &box_coeffs);
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if (pix != nullptr && *pix != nullptr) {
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// Transform the image.
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Pix* transformed = pixProjective(*pix, im_coeffs, incolor);
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if (transformed == nullptr) {
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tprintf("Projective transformation failed!!\n");
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return;
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}
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pixDestroy(pix);
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*pix = transformed;
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}
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if (boxes != nullptr) {
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// Transform the boxes.
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for (int b = 0; b < boxes->size(); ++b) {
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int x1, y1, x2, y2;
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const TBOX& box = (*boxes)[b];
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projectiveXformSampledPt(box_coeffs, box.left(), height - box.top(), &x1,
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&y1);
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projectiveXformSampledPt(box_coeffs, box.right(), height - box.bottom(),
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&x2, &y2);
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TBOX new_box1(x1, height - y2, x2, height - y1);
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projectiveXformSampledPt(box_coeffs, box.left(), height - box.bottom(),
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&x1, &y1);
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projectiveXformSampledPt(box_coeffs, box.right(), height - box.top(), &x2,
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&y2);
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TBOX new_box2(x1, height - y1, x2, height - y2);
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(*boxes)[b] = new_box1.bounding_union(new_box2);
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}
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}
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free(im_coeffs);
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free(box_coeffs);
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}
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// Computes the coefficients of a randomized projective transformation.
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// The image transform requires backward transformation coefficient, and the
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// box transform the forward coefficients.
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// Returns the incolor arg to pixProjective.
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int ProjectiveCoeffs(int width, int height, TRand* randomizer,
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float** im_coeffs, float** box_coeffs) {
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// Setup "from" points.
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Pta* src_pts = ptaCreate(4);
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ptaAddPt(src_pts, 0.0f, 0.0f);
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ptaAddPt(src_pts, width, 0.0f);
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ptaAddPt(src_pts, width, height);
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ptaAddPt(src_pts, 0.0f, height);
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// Extract factors from pseudo-random sequence.
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float factors[FN_NUM_FACTORS];
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float shear = 0.0f; // Shear is signed.
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for (int i = 0; i < FN_NUM_FACTORS; ++i) {
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// Everything is squared to make wild values rarer.
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if (i == FN_SHEAR) {
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// Shear is signed.
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shear = randomizer->SignedRand(0.5 / 3.0);
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shear = shear >= 0.0 ? shear * shear : -shear * shear;
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// Keep the sheared points within the original rectangle.
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if (shear < -factors[FN_X0]) shear = -factors[FN_X0];
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if (shear > factors[FN_X1]) shear = factors[FN_X1];
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factors[i] = shear;
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} else if (i != FN_INCOLOR) {
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factors[i] = fabs(randomizer->SignedRand(1.0));
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if (i <= FN_Y3)
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factors[i] *= 5.0 / 8.0;
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else
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factors[i] *= 0.5;
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factors[i] *= factors[i];
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}
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}
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// Setup "to" points.
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Pta* dest_pts = ptaCreate(4);
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ptaAddPt(dest_pts, factors[FN_X0] * width, factors[FN_Y0] * height);
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ptaAddPt(dest_pts, (1.0f - factors[FN_X1]) * width, factors[FN_Y1] * height);
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ptaAddPt(dest_pts, (1.0f - factors[FN_X1] + shear) * width,
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(1 - factors[FN_Y2]) * height);
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ptaAddPt(dest_pts, (factors[FN_X0] + shear) * width,
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(1 - factors[FN_Y3]) * height);
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getProjectiveXformCoeffs(dest_pts, src_pts, im_coeffs);
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getProjectiveXformCoeffs(src_pts, dest_pts, box_coeffs);
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ptaDestroy(&src_pts);
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ptaDestroy(&dest_pts);
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return factors[FN_INCOLOR] > 0.5f ? L_BRING_IN_WHITE : L_BRING_IN_BLACK;
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}
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} // namespace tesseract
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