diff --git a/libffmpegthumbnailer/moviedecoder.cpp b/libffmpegthumbnailer/moviedecoder.cpp
index f45eb99..34ef917 100644
--- a/libffmpegthumbnailer/moviedecoder.cpp
+++ b/libffmpegthumbnailer/moviedecoder.cpp
@@ -106,12 +106,12 @@ void MovieDecoder::destroy()
     avformat_network_deinit();
 }
 
-bool MovieDecoder::embeddedMetaDataIsAvailable()
+bool MovieDecoder::embeddedMetaDataIsAvailable() const
 {
     return m_UseEmbeddedData;
 }
 
-string MovieDecoder::getCodec()
+string MovieDecoder::getCodec() const
 {
     if (m_pVideoCodec) {
         return m_pVideoCodec->name;
@@ -374,7 +374,7 @@ void MovieDecoder::initializeFilterGraph(const AVRational& timeBase, const std::
     checkRc(avfilter_graph_config(m_pFilterGraph, nullptr), "Failed to configure filter graph");
 }
 
-int MovieDecoder::getWidth()
+int MovieDecoder::getWidth() const
 {
     if (m_pVideoCodecContext) {
         return m_pVideoCodecContext->width;
@@ -383,7 +383,7 @@ int MovieDecoder::getWidth()
     return -1;
 }
 
-int MovieDecoder::getHeight()
+int MovieDecoder::getHeight() const
 {
     if (m_pVideoCodecContext) {
         return m_pVideoCodecContext->height;
@@ -392,7 +392,7 @@ int MovieDecoder::getHeight()
     return -1;
 }
 
-int MovieDecoder::getDuration()
+int MovieDecoder::getDuration() const
 {
     if (m_pFormatContext) {
         return static_cast<int>(m_pFormatContext->duration / AV_TIME_BASE);
@@ -540,7 +540,7 @@ void MovieDecoder::getScaledVideoFrame(const std::string& scaledSize, bool maint
     }
 }
 
-void MovieDecoder::checkRc(int ret, const std::string& message)
+void MovieDecoder::checkRc(int ret, const std::string& message) const
 {
     if (ret < 0) {
         char buf[256];
@@ -550,7 +550,7 @@ void MovieDecoder::checkRc(int ret, const std::string& message)
     }
 }
 
-int32_t MovieDecoder::getStreamRotation()
+int32_t MovieDecoder::getStreamRotation() const
 {
     if (!m_pVideoStream || !m_pVideoStream->codecpar) {
         return -1;
diff --git a/libffmpegthumbnailer/moviedecoder.h b/libffmpegthumbnailer/moviedecoder.h
index 22113d4..b0436bb 100644
--- a/libffmpegthumbnailer/moviedecoder.h
+++ b/libffmpegthumbnailer/moviedecoder.h
@@ -46,19 +46,19 @@ class MovieDecoder
     MovieDecoder(AVFormatContext* pavContext = nullptr);
     ~MovieDecoder();
 
-    std::string getCodec();
+    std::string getCodec() const;
     void seek(int timeInSeconds);
     void decodeVideoFrame();
     void getScaledVideoFrame(const std::string& scaledSize, bool maintainAspectRatio, VideoFrame& videoFrame);
 
-    int getWidth();
-    int getHeight();
-    int getDuration();
+    int getWidth() const;
+    int getHeight() const;
+    int getDuration() const;
 
     void initialize(const std::string& filename, bool preferEmbeddedMetadata);
     void destroy();
 
-    bool embeddedMetaDataIsAvailable();
+    bool embeddedMetaDataIsAvailable() const;
 
 private:
     int32_t findPreferredVideoStream(bool preferEmbeddedMetadata);
@@ -68,10 +68,10 @@ class MovieDecoder
 
     bool decodeVideoPacket();
     bool getVideoPacket();
-    int32_t getStreamRotation();
+    int32_t getStreamRotation() const;
     std::string createScaleString(const std::string& size, bool maintainAspectRatio);
 
-    void checkRc(int ret, const std::string& message);
+    void checkRc(int ret, const std::string& message) const;
 
 private:
     int m_VideoStream;
diff --git a/libffmpegthumbnailer/videothumbnailer.cpp b/libffmpegthumbnailer/videothumbnailer.cpp
index 6e2c8da..178d3df 100644
--- a/libffmpegthumbnailer/videothumbnailer.cpp
+++ b/libffmpegthumbnailer/videothumbnailer.cpp
@@ -201,7 +201,7 @@ VideoFrameInfo VideoThumbnailer::generateThumbnail(const string& videoFile, Imag
     return info;
 }
 
-void VideoThumbnailer::generateSmartThumbnail(MovieDecoder& movieDecoder, VideoFrame& videoFrame)
+void VideoThumbnailer::generateSmartThumbnail(MovieDecoder& movieDecoder, VideoFrame& videoFrame) const
 {
     vector<VideoFrame> videoFrames(SMART_FRAME_ATTEMPTS);
     vector<Histogram<int>> histograms(SMART_FRAME_ATTEMPTS);
@@ -262,7 +262,7 @@ void VideoThumbnailer::writeImage(const string& videoFile, ImageWriter& imageWri
     imageWriter.writeFrame(&(rowPointers.front()), videoFrame.width, videoFrame.height, m_ImageQuality);
 }
 
-string VideoThumbnailer::getMimeType(const string& videoFile)
+string VideoThumbnailer::getMimeType(const string& videoFile) const
 {
     string extension = getExtension(videoFile);
 
@@ -289,7 +289,7 @@ string VideoThumbnailer::getMimeType(const string& videoFile)
     }
 }
 
-string VideoThumbnailer::getExtension(const string& videoFilename)
+string VideoThumbnailer::getExtension(const string& videoFilename) const
 {
     string extension;
     auto pos = videoFilename.rfind('.');
@@ -328,7 +328,7 @@ void VideoThumbnailer::applyFilters(VideoFrame& frameData)
     }
 }
 
-int VideoThumbnailer::getBestThumbnailIndex(vector<VideoFrame>& videoFrames, const vector<Histogram<int>>& histograms)
+int VideoThumbnailer::getBestThumbnailIndex(vector<VideoFrame>& videoFrames, const vector<Histogram<int>>& histograms) const
 {
     Histogram<float> avgHistogram;
     for (auto&& histogram : histograms) {
diff --git a/libffmpegthumbnailer/videothumbnailer.h b/libffmpegthumbnailer/videothumbnailer.h
index 7de942a..ce277a6 100644
--- a/libffmpegthumbnailer/videothumbnailer.h
+++ b/libffmpegthumbnailer/videothumbnailer.h
@@ -70,13 +70,13 @@ class VideoThumbnailer
 
 private:
     VideoFrameInfo generateThumbnail(const std::string& videoFile, ImageWriter& imageWriter, AVFormatContext* pAvContext = nullptr);
-    void generateSmartThumbnail(MovieDecoder& movieDecoder, VideoFrame& videoFrame);
+    void generateSmartThumbnail(MovieDecoder& movieDecoder, VideoFrame& videoFrame) const;
     void writeImage(const std::string& videoFile, ImageWriter& imageWriter, const VideoFrame& videoFrame, int duration, std::vector<uint8_t*>& rowPointers);
 
-    std::string getMimeType(const std::string& videoFile);
-    std::string getExtension(const std::string& videoFilename);
+    std::string getMimeType(const std::string& videoFile) const;
+    std::string getExtension(const std::string& videoFilename) const;
 
-    int getBestThumbnailIndex(std::vector<VideoFrame>& videoFrames, const std::vector<Histogram<int>>& histograms);
+    int getBestThumbnailIndex(std::vector<VideoFrame>& videoFrames, const std::vector<Histogram<int>>& histograms) const;
     void applyFilters(VideoFrame& frameData);
 
     void TraceMessage(ThumbnailerLogLevel lvl, const std::string& msg);