R/trim_transparency.R

In moodymudskipper/tweak: Modifies PNG images

#' transform low brightness and low transparency to full transparency
#'
#' @param img raster object
#' @param alpha_treshold pixels with alpha under this value will have it set to 0
#' @param brightness_treshold pixels with brightness over this value will have alpha set to 0
#' @param reverse if reverse is true the darker pixels will have alpha set to 0
#' @export
make_transparent <- function(img, alpha_treshold=0,brightness_treshold=1,reverse=FALSE){
  assertthat::assert_that(class(img)=="raster")
  lng <- convert(img,to="long")
  bri <- brightness(lng)
  if (reverse)
    lng[bri <= brightness_treshold,4] <- 0 else
      lng[bri >= brightness_treshold,4] <- 0
  lng[lng[,4] <= alpha_treshold,4]        <- 0
  img <- convert(lng,from="long",to="raster")
  img
}

#' trim image
#'
#' @param img raster object
#' @param bottom,left,top,right where to trim
#' @param type rel (0 to 1) or abs (pixels)
#' @export
img_trim <- function(img, bottom=0,left=0,top=0,right=0,type=c("rel","abs")){
  assertthat::assert_that(class(img)=="raster")
  type <- match.arg(type)
  img <- if(
    is.character(img) && length(img) == 1) urlpath2matrix(img) else if(
      length(dim(img))==3)                 array32matrix(img)  else if(
        is.raster(img))                    as.matrix(img)      else if(
          is.matrix(img))                  img

  if(type =="rel"){
    left   <- round(left   * ncol(img))
    right  <- round(right  * ncol(img))
    bottom <- round(bottom * nrow(img))
    top    <- round(top    * nrow(img))
  }
  img <- img[(top+1):(nrow(img)-bottom),(left+1):(ncol(img)-right)]
  img <- as.raster(img)
  img
}

#' Trim automatically transparent area
#'
#' @param img raster object
#' @param alpha_treshold pixels with alpha under this value will have it set to 0
#' @param brightness_treshold pixels with brightness over this value will have alpha set to 0
#' @param reverse if reverse is true the darker pixels will have alpha set to 0
#' @export
auto_trim <- function(img, alpha_treshold=0,brightness_treshold=1,reverse=FALSE){
  assertthat::assert_that(class(img)=="raster")
  img   <- convert(img,to="matrix")
  img0  <- make_transparent(img,alpha_treshold,brightness_treshold,reverse)
  lng   <- rst2lng(img0)
  mat0  <- matrix(lng[,4],nrow=attr(lng,"DIM")[1], attr(lng,"DIM")[2],byrow=FALSE)
  empty_cols <- colSums(mat0) == 0
  keep_cols  <- cumsum(!empty_cols) != 0 & rev(cumsum(rev(!empty_cols) != 0))
  empty_rows <- rowSums(mat0) == 0
  keep_rows  <- cumsum(!empty_rows) != 0 & rev(cumsum(rev(!empty_rows) != 0))
  img <- img[keep_rows,keep_cols]
  img <- as.raster(img)
  img
}

#' Return non transparent area
#'
#' @param img raster object
#' @param type rel (0 to 1) or abs (pixels)
#' @param alpha_treshold pixels with alpha under this value will have it set to 0
#' @param brightness_treshold pixels with brightness over this value will have alpha set to 0
#' @param reverse if reverse is true the darker pixels will have alpha set to 0
#' @export
effective_area <- function(img, type=c("rel","abs"), alpha_treshold=0,brightness_treshold=1,reverse=FALSE){
  assertthat::assert_that(class(img)=="raster")
  type   <- match.arg(type)
  img0   <- make_transparent(img,alpha_treshold,brightness_treshold,reverse)
  lng    <- convert(img0,to="long")
  if (type == "abs")
    area <- sum(lng[,4]==0) else
      area <- sum(lng[,4]==0)/nrow(lng)
  area
}