## The closest that I can come, to making Maxima On Android display an animation.

Maxima On Android really wasn’t designed, either to allow interactive rotation of 3D Plots, or, to allow animated, 3D Plots. But let us put ourselves in the position that it is the chosen Computer Algebra System, and that the user wants ‘interactive 3D Plots’.

What Maxima On Android has to offer natively, is a 3D Plot, but displayed from a static perspective. In fact, I’ve read that the way it works is, that Maxima On Android plots to a PNG File, and then displays the PNG File in a secondary window, from which the user may switch back to the CAS window / worksheet.

Various efforts I’ve made have failed, to get the viewing window for the plots to animate. One possible reason could be, the possibility that Maxima On Android actually waits, until the entire worksheet has finished executing, before allowing the viewing window to be opened. Repeated, iterative plotting locks up the app, until the loop has finished, at which point, at best, the last version of the plot can be viewed.

The following was the best that I could do, to get this port of Maxima to plot interactively:


i: 0$Frame():= block ( plot3d(sin(%pi*(x+(i/4)))*cos(%pi*y), [x, -1, 1], [y, -1, 1]), i: i + 1 )$

Frame()\$



This Maxima Batch File / Script requires some participation by the user, to work. The user may Load it from within the Maxima On Android GUI, after which the first iteration of the plot will display. After that, the user needs to tap on the ‘Back’ arrow, to get back to the worksheet. Then, tapping on the last ‘Frame()’ command, will cause it to display in the command field. Then, tapping ‘Enter’ will cause the next iteration of the plot to appear.

From the second point on, in the process, that the user has tapped on the ‘Back’ arrow, the command ‘Frame()’ should still be in the command field. Therefore, ‘Enter’ can just be tapped again, and the process repeated as often as desired.

(Updated 7/05/2020, 10h50… )

## A Method for Obtaining Signatures for Photogrammetry

I have posed myself the question in the past, in which a number of photos is each subdivided into a grid of rectangles, how a signature can be derived from each rectangle, which leads to some sort of identifier, so that between photos, these identifiers will either match or not, even though there are inherent mismatches in the photos, to decide whether a rectangle in one photo corresponds to the same subject-feature, as a different rectangle in the other photo.

Eventually, one would want this information in order to compute a 3D scene-description – a 3D Mesh, with a level of detail equal to how finely the photos were subdivided into rectangles.

Since exact pixels will not be equal, I have thought of somewhat improbable schemes in the past, of just how to compute such a signature. These schemes once went so far, as first to compute a 2D Fourier Transform of each rectangle @ 1 coefficient /octave, to quantize those into 1s and 0s, to ignore the F=(0,0) bit, and then to hash the results.

But just recently I have come to the conclusion that a much simpler method should work.

At full resolution, the photos can be analyzed as though they formed a single image, in the ways already established for computing an 8-bit color palette, i.e. a 256-color palette, like the palettes once used in GIF Images, and for other images that only had 8-bit colors.

The index-number of this palette can be used as an identifier.

After the palette has been established, each rectangle of each photo can be assigned an index number, depending on which color of the palette it best matches. It would be important that this assignment not take place, as though we were just averaging the colors of each rectangle. Instead, the strongest basis of this assignment would need to be, how many pixels in the rectangle match one color in the palette. (*)

After that, each rectangle will be associated with this identifier, and for each one the most important result will become, at what distances from its camera-position the greatest number of other cameras confirm its 3D position, according to matching identifiers.

Dirk