videofig

Lightweight image sequence visualization utility based on matplotlib

Features

• play matplotlib plots in real time (35 fps for 640x360 images)
• press Enter to toggle pause, ➡️ to next frame, ⬅️ to previous frame, etc
• pause then drag the scroll bar for navigation
• draw multiple plots with grid specification
• save plots in a directory
• support windows, linux, osx

Installation

pip install videofig

Examples

Example 1: Plot a dynamic sine wave

Example 2: Show images together with object bounding boxes

Note that the FPS is low in the gif since saving images takes time. Without saving images (by setting SAVE_PLOTS = False in example scripts), the FPS is about 35.

Example 3: Show multiple plots

To generate these gifs, follow these steps:

1. change SAVE_PLOTS to True in respective example scripts
2. python example1.py
3. ffmpeg -ss 0 -i example1_save/%04d.jpg -vf "setpts=1.0*PTS,fps=25,scale=480:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" -loop 0 example1_save.gif

Introduction

Python is an elegant programming language with rich add-on libraries to meet various needs. For scientific computation, it has numpy; for plotting, it has matplotlib. Personally, I use Python for video analysis and it works like a charm except for one thing: visualize image sequences for detailed inspection.

When visualizing image sequences for detailed inspection, it is desirable to have play, pause, forward by one frame, backward by one frame, etc utilities. Moreover, we may add some custom plots(bounding box, for example) and graphics on top. In Matlab, we have VideoPlayer in Computer Vision System Toolbox, but to the best of my knowledge, there isn't any tools in Python that provides similar functionality. Let me know, if there are any : )

Accidentally, I came across the excellent script of João Filipe Henriques, which provides utilities for detailed image sequence inspection before VideoPlayer is available in Matlab. Inspired by this, I decided to write a similar function in Python before more sophisticated tools come out.

Dependency

This tool is specifically designed to be minimal, lightweight and readable such that anyone can easily modify it to suit different needs. The only dependency is Matplotlib. I have tested it in Python 2.7, but it should also work in Python 3.5.

• matplotlib >= 2.0.0

Basic Usage

videofig(NUM_FRAMES, REDRAW_FUNC)

Creates a figure with a horizontal scrollbar and shortcuts to scroll automatically. The scroll range is 0 to NUM_FRAMES - 1. The function REDRAW_FUN(F, AXES) is called to redraw at scroll position F (for example, REDRAW_FUNC can show the frame F of a video) using AXES for drawing. F is an integer, AXES is a instance of Axes class

This can be used not only to play and analyze standard videos, but it also lets you place any custom Matplotlib plots and graphics on top.

The keyboard shortcuts are:

• Enter(Return) -- play/pause video (25 frames-per-second default).
• Backspace -- play/pause video 5 times slower.
• Right/left arrow keys -- advance/go back one frame.
• Page down/page up -- advance/go back 30 frames.
• Home/end -- go to first/last frame of video.

Advanced Usage

Please see example scripts on how to achieve more advanced effects.