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AssembleFrames

Assemble a sequence of image files into a video file via an intermediate YCbCr stream. The interpretation of the input and output data is precisely defined and is specified in the Video Usability Information (VUI) in the output file.

In particular, the input images (which can be in any format understood by your JVM's Image I/O subsystem) are first converted to the sRGB color space (the conversion is handled by Java's BufferedImage class). The data is then transformed to a linear RGB space in which subsampling is performed. Chroma subsamples are located at the nominal sites for H.264 4:2:0 (code 0). Data at luma and chroma sites is finally decomposed into YCbCr and encoded as 8-bit samples according to BT.709. This raw YCrCb 4:2:0 stream is passed to x264 for encoding as H.264 in an MP4 container.

Dependencies

AssembleFrames is written in Java and therefore requires a Java SE 7 (or later) JVM to run. Video encoding additionally requires that x264 be installed and visible in your $PATH.

Building AssembleFrames requires a Java SE 7 (or later) JDK and an SBT launcher compatible with version 0.13.1.

Usage

To create an assembleframes executable, simply execute sbt universal:package-bin. This will produce a ZIP file at target/universal/assembleframes-<version>.zip. To run AssembleFrames, unzip this archive, add its bin subdirectory to your $PATH, and execute a command of the following form:

assembleframes <output_prefix> <fps> <input_filename>...

Here, <input_filenames>... is a list of image files that should be assembled (in the order listed) into a video. The video file will be written to <output_prefix>.mp4, and the framerate will be <fps> frames per second.

Build-time encoder options are specified in application.conf. These may be overridden at runtime by defining the corresponding properties for the JVM with command line flags of the form -Dcdmuhlb.assembleframes.<property>=<value>.

Technical details

Chroma subsampling is performed using the following filtering and interpolation kernel coefficients:

0.125 0.25 0.125
0.125 0.25 0.125

Reflective boundary conditions are assumed for the first column (that is, when subsampling in column 0, column -1 is assumed to be equal to column 1).

Performance notes

Performance is currently limited by Java's Image I/O library, which is very slow at both reading a PNG file and filling an array with its pixel data.

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Assemble a sequence of image files into a video file via an intermediate YCbCr stream

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