Fighting with computers

Computers are not always friendly.

Tuesday, June 28, 2016

Painless transition to El Capitán

My aging desktop computer is a 2011 iMac. When I bought it I loved the concept that would allow me a clean desktop. Truth be told and not iMac's fault, my desktop is almost always a mess despite de computer form factor.

Since I upgraded it to Snow Leopard (mostly for the nee to use a newer version of Java) I have learned about some SMART error on the hard drive. Once I started to feel the pressure of certain application binaries not running because my system libraries were too old, I wanted to upgrade the system but I could not. OSX install would check the hard disk and it will refuse to upgrade if found defective.

Whatever the problem my 1TB is suffering is not killing it for more than two years. And the iMac being the DIY-unfriendly that it is I keep delaying the hard disk replacing. A few months ago I found a spare USB hard disk at home and I used it to install Mavericks on it (yeah, I am not in a hurry to get next memory-hog upgrade). It all worked nicely while I keep on using the internal hard disk too. But one USB less plus another wall wart left me a bit low on available power sockets.

A few days ago I saw a very good offer for 240GB SSD drive and I bite the bullet. Combined with one ElCheapo USB-SATA adapter I got a nice deal. Maybe it is not a top-of-the-line speed-demon but it copies one and a half gigabytes in less than a minute.

I used an old MacBook Pro to download and install El Capitán on the SSD drive. I like being able to use a USB drive as the system unit, a feature I only see on Macs though it might be available in some modern PC motherboards.

But the beauty of it is that I brought the drive home and then use it to boot up my MacBook Air flawlessly too. But that was not the final stop, I just use it to customize the install, add things like Arduino or Chrome. And now, after plugging it in to the iMac and booting from it (OSX uses Cmd key press while booting up to go to boot drive selection) I am writing this entry finally on the iMac. Of course nothing was needed to use the wireless keyboard or mouse that were needing for boot selection or typing the user password. Definitely a much better experience than if I was dealing with other operating system.

And for those Arduino users that like me still bitch about the weirdness of Windows 8.1 Arduino IDE install (to enable non-signed drivers) nothing of that happens here. I even found a signed driver for CH34x USB serial chips found on many Chinese boards. Maybe I will upgrade other systems if the experience continues being positive. I still need to figure out how to get my pictures and music back.

Saturday, June 25, 2016

On placing a tag on an area

The common approach I have used in the past for locating a tag on a given 2D shape has been to use the centroid location. For convex parts there is a very good solution. However, when the shape is not convex the centroid location may be outside of the shape surface.

Whenever the tags are intended to identify a shape it might be a problem is the label falls outside of the shape, even more so when multiple shapes are packed together, as user may not be able to be sure which label belongs to which part.

One idea of fixing that is to make sure the tag location is always inside the part, and for that purpose I have evolved through for different algorithms, trying to find the best result.

Algorithm 1

If centroid is within the shape area, then just use that. When it is outside (concave shape) then an horizontal sweep is done in 10% increments, at the centroid height, looking for a spot within the shape area. If that is not found, then same approach is repeated with a vertical sweep at the centroid width too.  It appears as a black box in the video.

Algorithm 2

At the centroid, one horizontal line is traced and shape is explored for the longest area intersection with this line. The middle point of this line is now used for performing a similar sweep but this time done vertically. The tag location will be at the middle point of the longest vertical intersection. It appears a in blue color in the video.

Algorithm 3

Similar to algorithm 2, but adding a second horizontal sweep trying to get a better centered result. It appears as a pink box in the video.

Algorithm 4

It follows a topological approach, looking for the point that it is furthest from the shape perimeters. To do so the shape is painted as a bitmap and a dilate operation is done repeatedly till the last pixels are removed from the image. It is the location of that last pixel the desired tag location. It appears in red color in the video.
Usually, the black box is hiding the centroid that appears as a small circle, but on a few cases that can be seen as the black box is moved away from the centroid.

If you have another way of solving the problem, please let me know in the comments below.

Algorithm 5

Actually, similar to number 4: Instead of using a bitmap, I use the vector representation of the perimeter as a polygon. Then I perform, repeatedly, negative polygon buffer operations [on the larger block] until polygon area reaches a certain threshold. Then I use the centroid of that remaining polygon as the location for the label. It turns out much more efficient than its cousin Algorithm 4 (provided you have a decent polygon offset implementation).

Tuesday, June 21, 2016

A cheap idea for thermal imaging

Sometimes I needed to check how heat was distributed on a surface. A cool but expensive way is to use a thermographic camera. I do not have one at hand.

But an ongoing project uses thermochromic ink. That is an ink that becomes transparent once a temperature threshold is reached. It goes from a certain color to no color at all. So if you paint a piece of cloth and place it on a given surface you can do the measurement of temperature at each point.

The following pictures show the heating process of a certain aluminium heated bed. My sample cloth was not large enough to cover the whole bet but you get the idea.

 Heat sources start to show as whiter areas. 

 Now heat spreads a bit more.

 Reaching the temperature threshold at many points

For best results a glass on top would make sure the cloth is making contact with the whole surface evenly (top left corner was not having a good contact which explains the apparent colder temperature).