Caveats when using ‘avidemux’ (under Linux).

One of the applications available under Linux, that can help edit video / audio streams, and that has been popular for many years – if not decades – is called ‘avidemux’. But in recent years the subject has become questionable, of whether this GUI- and command-line- based application is still useful.

One of the rather opaque questions surrounding its main use, is simply highlighted in The official usage WiKi for Avidemux. The task can befall a Linux user, that he either wants to split off the audio track from a video / audio stream, or that he wants to give the stream a new audio track. What the user is expected to do is to navigate to the Menu entries ‘Audio -> Save Audio’, or to ‘Audio -> Select Track’, respectively.

Screenshot_20190314_131933

Screenshot_20190314_132014

 

 

What makes the usage of the GUI not straightforward is what the manual entries next state, and what my personal experiments confirm:

  • External Audio can only be added from ‘AC3′, ‘MP3′, or ‘WAV’ streams by default,
  • The audio track that gets Saved cannot be played back, if In the format of an ‘OGG Vorbis’, an ‘OGG Opus’, or an ‘AAC’ track, as such exported audio tracks lack any header information, which playback apps would need, to be able to play them. In those cases specifically, only the raw bit-stream is saved.

The first problem with this sort of application is that the user needs to perform a memorization exercise, about which non-matching formats he may or may not, Export To or Import From. I don’t like to have to memorize meaningless details, about every GUI-application I have, and in this case the details can only be read through detailed research on the Web. They are not hinted at anywhere within the application.

(Updated 3/23/2019, 15h35 … )

Continue reading Caveats when using ‘avidemux’ (under Linux).

Multisampling

One of the problems with bit-mapped graphics is “aliasing”. This is the phenomenon by which pixels along the edge of a pure shape will seem either to belong to that shape or not so, resulting in an edge which has rectangular errors. Even at fairly high resolutions, this can lead to a low-quality experience. And so schemes have been devised since the beginning of digital graphics, to make this effect less pronounced, even if we do choose raster-graphics.

3D has not been left out. One of the strategies which has existed for some time, is to Super-Sample each screen pixel, let us say by subdividing it by a fixed factor, such as 4×4 sub-pixels. This is also known as “Full-Screen Anti-Aliasing”, or ‘FSAA’. The output of the sub-pixels can be mixed in various ways, to result in a blended color-value for the resulting screen pixel.

But one problem with FSAA has been from the start, that it slows down rendering a whole lot. And so an alternative was devised, called Multi-Sampling.

The main idea behind Multi-Sampling is, that only the screen-pixels that span a triangle-edge, are objectionable in the degree with which they suffer from aliasing. Therefore, most of the screen-pixels are not super-sampled. And, the limited logic of the GPU has a hard time trying to distinguish, which triangle-edges are also model / entity -edges, where aliasing does the most damage. But, because the GPU has specialized logic circuits, which are referred to somewhat incorrectly as one render-output generator, that rasterize a given triangle, those circuits can be expanded somewhat feasibly into also being able to detect which screen-pixels do straddle the edge between two triangles. And then, for the sake of argument, only those may be subdivided into 4×4 sub-samples, each of which is Fragment-Shaded once.

But the logic gets just a bit more complicated. There is no simple way, in which the render-output generator can know, which other triangle a current triangle borders on. This is because in general, once each triangle has been processed, it is forgotten. Once each Geometry Shader input-topology has been processed, it too is forgotten, and the GS proceeds to process the next input-topology with complete amnesia…

Continue reading Multisampling