protected function registerFill($style)
    {
        $styleId = $style->getId();

        // Currently - only solid backgrounds are supported
        // so $backgroundColor is a scalar value (RGB Color)
        $backgroundColor = $style->getBackgroundColor();

        if ($backgroundColor) {
            $isBackgroundColorRegistered = isset($this->registeredFills[$backgroundColor]);

            // We need to track the already registered background definitions
            if ($isBackgroundColorRegistered) {
                $registeredStyleId = $this->registeredFills[$backgroundColor];
                $registeredFillId = $this->styleIdToFillMappingTable[$registeredStyleId];
                $this->styleIdToFillMappingTable[$styleId] = $registeredFillId;
            } else {
                $this->registeredFills[$backgroundColor] = $styleId;
                $this->styleIdToFillMappingTable[$styleId] = $this->fillIndex++;
            }

        } else {
            // The fillId maps a style to a fill declaration
            // When there is no background color definition - we default to 0
            $this->styleIdToFillMappingTable[$styleId] = 0;
        }
    }