wro4j-core/src/main/java/ro/isdc/wro/model/resource/processor/support/ImageUrlRewriter.java
private String computeNewImageLocation(final String cssUri, final String imageUrl) {
    LOG.debug("cssUri: {}, imageUrl {}", cssUri, imageUrl);
    final String cleanImageUrl = cleanImageUrl(imageUrl);
    // TODO move to ServletContextUriLocator as a helper method?
    // for the following input: /a/b/c/1.css => /a/b/c/
    int idxLastSeparator = cssUri.lastIndexOf(ServletContextUriLocator.PREFIX);
    if (idxLastSeparator == -1) {
      if (ClasspathUriLocator.isValid(cssUri)) {
        idxLastSeparator = cssUri.lastIndexOf(ClasspathUriLocator.PREFIX);
        // find the index of ':' character used by classpath prefix
        if (idxLastSeparator >= 0) {
          idxLastSeparator += ClasspathUriLocator.PREFIX.length() - 1;
        }
      }
      if (idxLastSeparator < 0) {
        throw new IllegalStateException("Invalid cssUri: " + cssUri + ". Should contain at least one '/' character!");
      }
    }
    final String cssUriFolder = cssUri.substring(0, idxLastSeparator + 1);
    // remove '/' from imageUrl if it starts with one.
    final String processedImageUrl = cleanImageUrl.startsWith(ServletContextUriLocator.PREFIX) ? cleanImageUrl.substring(1)
        : cleanImageUrl;
    final String computedImageLocation = cleanPath(cssUriFolder + processedImageUrl);
    LOG.debug("computedImageLocation: {}", computedImageLocation);
    return computedImageLocation;
  }
private String computeNewImageLocation(final String cssUri, final String imageUrl) {
    LOG.debug("cssUri: {}, imageUrl {}", cssUri, imageUrl);
    final String cleanImageUrl = cleanImageUrl(imageUrl);
    // TODO move to ServletContextUriLocator as a helper method?
    // for the following input: /a/b/c/1.css => /a/b/c/
    int idxLastSeparator = cssUri.lastIndexOf(ServletContextUriLocator.PREFIX);
    if (idxLastSeparator == -1) {
      if (ClasspathUriLocator.isValid(cssUri)) {
        idxLastSeparator = cssUri.lastIndexOf(ClasspathUriLocator.PREFIX);
        // find the index of ':' character used by classpath prefix
        if (idxLastSeparator >= 0) {
          idxLastSeparator += ClasspathUriLocator.PREFIX.length() - 1;
        }
      }
      if (idxLastSeparator < 0) {
        throw new IllegalStateException("Invalid cssUri: " + cssUri + ". Should contain at least one '/' character!");
      }
    }
    final String cssUriFolder = cssUri.substring(0, idxLastSeparator + 1);
    // remove '/' from imageUrl if it starts with one.
    final String processedImageUrl = cleanImageUrl.startsWith(ServletContextUriLocator.PREFIX) ? cleanImageUrl.substring(1)
        : cleanImageUrl;
    final String computedImageLocation = cleanPath(cssUriFolder + processedImageUrl);
    LOG.debug("computedImageLocation: {}", computedImageLocation);
    return computedImageLocation;
  }
Concatenates cssUri and imageUrl after few changes are applied to both input parameters.

@param cssUri
the URI of css resource.
@param imageUrl
the URL of image referred in css.
@return processed new location of image url.