inst/notes.md

In rsimon64/qrdecode: Read QR Codes

https://github.com/IdreesInc/QR-Code-Extractor

Step One: Remove noise from sample image

Convert original image to greyscale imager::grayscale Apply Gaussian blur to reduce noise imager::blur_anisotropic Apply Canny edge filter to eliminate distractions imager::cannyEdges

Step Two: Narrow down search to only QR code locators

Find all remaining contours imager::contour Filter all contours with approximate vertex count of four -> inspect list result from contour Filter all quadrilaterals that are approximately squares and have a certain number of children

Step Three: Locate the locators

Loop through all squares Find all squares that are similar in size to the current square If the closest two squares to the current square are similar distances away, estimate that this square is the upper-left locator With the other squares, find the angle from the upper-left vertex to determine the orientation of the QR code

Step Four: Find the alignment pattern square (colloquially called "tiny square" in my code)

While searching through squares in previous step, store any squares that are less than half the size of the locator squares After determining locator orientations, calculate the midpoint of the QR code Select from the possible alignment pattern squares the one closest to the midpoint that is also within the bounds of the code itself If alignment pattern is found: Determine fourth corner of the QR code to be a rational distance from the alignment pattern (in a direction opposite of the midpoint) If no alignment pattern is found (smaller QR codes lack this, or the camera may not detect it): Determine edges of the locators that are along the edges of the entire QR code that would intersect to form the fourth corner Find line intersection and determine that point to be the fourth corner

Step Five: Compensate for perspective warping and extract the code

For each code, warp the vertices into a square to fix alignment Scale down with cubic interpolation into 29x29 pixel square (change dimensions for different types of codes) Convert to one bit black and white by thresholding the image Return formatted codes in list

rsimon64/qrdecode documentation built on May 30, 2019, 4:09 p.m.