Electrical Engineering

MLM

01:44

I may be overlooking something simple:

B = rgba(0, 0, 0, .5), A = rgba(255, 255, 255, .25)`, A is over B. How do i calculate the resulting rgba color? There is a wiki article but I am still unable to calculate the correct value. I even made a demo to help explain: jsfiddle.net/MadLittleMods/FEy54/4

MLM

02:02

*sorry link broken: jsfiddle.net/MadLittleMods/FEy54

The Photon

(64, 64, 64)?

MLM

@ThePhoton Alpha? This is what I get

The Photon

Or (96, 96, 96)?

MLM

@ThePhoton how did you calculate that value?

The Photon

@MLM I think if you add another layer you'd have to recalculate everything --- but I don't work with this stuff and am just bs'ing

@MLM I noticed that just "B" isn't 0,0,0, so there must be some assumed background color

So I guessed that the background is assumed white and just "B" is giving (128, 128, 128),

MLM

02:08

rgba(96, 96, 96, 0.625) is super close to B = rgba(0, 0, 0, .5), A = rgba(255, 255, 255, .25), A is over B`

The Photon

and averaged that with 0.25 * (white)

@MLM Like I said, just BS'ing and trying to guess what the colors are that actually come up on your applet.

MLM

@ThePhoton It's just chrome

The Photon

@MLM ???

The amount I know about javascript could be summed up by "I saw the extension was .js so I assumed it was javascript"

MLM

I am working with Chrome's rgba blending/compositing. Just want to figure out a single rgba value that will show up the same as multiple blending. jsFiddle is a sandbox to play with html, css, and js

The blending is called source-over which applies to many things

The Photon

Does it not follow the rules given here?

Alpha compositing

In computer graphics, alpha compositing is the process of combining an image with a background to create the appearance of partial or full transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a process called the composite. For example, compositing is used extensively when combining computer-rendered image elements with live footage. In order to combine these image elements correctly, it is necessary to keep an associated matte for each element. This matte contains the coverage info...

C_o = C_a (alpha_a) + C_b (alpha_b)(1-alpha_a)

MLM

02:20

@ThePhoton that looks right (same as other wiki as well). But that givea 64

So am I just missing blending rgba(64, 64, 64, .625) with white?

Actually that would give me a opaque color when it should be trans

The Photon

If you put another layer on top I guess you'd get
C_o = C_c (alpha_c) + C_a (alpha_a)(1-alpha_c) + C_b (alpha_b)(1-alpha_a)(1-alpha_c) ?

(Should say "over" rather than "on top" I guess)

MLM

@ThePhoton the wiki article you linked is ample as the one I did btw.

The Photon

It's not obvious to me whether you can express (A over B) with a single RGBA that would blend the same with any other color you put over it.

Or maybe I should keep reading the wiki where it tells how to calculate the composite alpha.

MLM

02:40

@ThePhoton I think you can I just do not know the right calculation. See image:

Those rgba layers are overtop the image

The Photon

@MLM I think there's a formula in the wiki link, but I'm not 100% clear how it applies.

MLM

I had no trouble calculating the resulting Alpha output using the formula on the wiki but the color is just going over my head.

alpha = .25+.5(1-.25)=.625

MLM

03:07

ahhh I got it!

we were forgetting to divide by output Alpha

so (63.75/.625) = 102

rgba(102, 102, 102, .625) is the correct value

I got that from this SO post but I see now that there the last formula in the description section of the wiki was correct

@ThePhoton Got it ^

The Photon

@MLM good deal

MLM

@ThePhoton Thanks for the help sussing it out

"sussing" urbandictionary.com/define.php?term=sussed

jippie

07:29

morning

abdullah kahraman

09:00

@jippie morning

abdullah kahraman

09:23

@jippie How about it, now?

jippie

just a second, need to start a filesystem formatting job

@abdullahkahraman whazzup?

abdullah kahraman

@jippie I'm fine, how are you?

:P

jippie

I see you ran out of 1k resistors?

abdullah kahraman

lol

jippie

W1-HL is the output of the microcontroller?

abdullah kahraman

09:28

@jippie yes

jippie

so that swings between 0V and 5V at best, right?

abdullah kahraman

@jippie yes

jippie

if W1-HL is 0V then Q4 is .. what's the word again?

abdullah kahraman

@jippie Ooops, I saw the short because of the zener :)

jippie

short? zener?

stay with me here

abdullah kahraman

09:29

@jippie The word is HIGH

jippie

anyways Q4 is not conducting, and Q1 seems conducting.

seems fine

.

abdullah kahraman

@jippie Why? Q4 will have 5.6V on its base when W1-HL is LOW!

jippie

Won't R5 pull it down?

abdullah kahraman

@jippie Will it be enough?

jippie

m

abdullah kahraman

09:31

@jippie If it will pull it down, what is the purpose of the zener? The input will see a high voltage anyways

jippie

ok, so let's say Q4 is conducting when W1-HL is low, right?

what is the state for Q1 in that same condition?

abdullah kahraman

@jippie Conducting

jippie

ok

abdullah kahraman

@jippie Simulation says I'm wrong, too :)

jippie

can you estimate the current through Q1 and Q4

@abdullahkahraman (on a side note, I personally really dislike the configuration with BJTs where the collectors are connected together)

(last time I did that I severely burnt my fingers, many years ago, still remember)

(it is pretty difficult to turn the transistors off quickly enough)

abdullah kahraman

09:39

@jippie lol, sorry

jippie

@abdullahkahraman why sorry? not at all

abdullah kahraman

@jippie you've burnt your fingers!

jippie

@abdullahkahraman ohhhhhhhh

:)

abdullah kahraman

@jippie I hate this thing about English.. When you apologize, you say sorry, when you are sad about something and you share your sadness, you say sorry..

@jippie Ooops, microcontroller won't be able to turn Q1 OFF.. :)

Stupid me..

1

A: Square Wave Voltage Level Shifting (Take 2)

Sorry - rushing - more later if needed. Try this for now. This came from someone (on PICLIST perhaps?) on August 13th - MAY have been Olin. Can check later. I use a different arrangement and will discuss later if needed. This a more clever circuit than may appear, despite its apparent simpli...

jippie

@abdullahkahraman yup

@abdullahkahraman you can replace the top BJT by one configured as a current source. Then you can pull it down and it is reasonably quick discharging the MOSFET too,

/me is out to grab something to eat.

abdullah kahraman

09:55

@jippie bon apetit

abdullah kahraman

10:15

@jippie You've told this to me before, but I still cannot understand it.

jippie

@abdullahkahraman What happens if you replace R15 with in the above diagram with a current source of say 10mA? Consider both situations where the input is low and the input is high.

In the current diagram the base capacitance is charged/discharged through either R14 or R15

abdullah kahraman

@jippie Yes, I see, for example for this one the current source is 2.7mA.

@jippie You mean Q14 and Q15?

jippie

With a current source the charge is pulled out of the transistor capacitance until the current source hits the power rail

@abdullahkahraman yes the lower one

but you can replace R15 (top) with an independent current source, a simple one.

abdullah kahraman

@jippie Oh, I see your point now.. What is your suggestion for that current source?

jippie

a simple bjt configuration, or if you want to go near perfect you could throw in a current mirror :-p

abdullah kahraman

10:26

@jippie Hell no! :)

jippie

@abdullahkahraman you are an analog specialist, or at least you told me once. Current mirrors are simple and cool.

abdullah kahraman

@jippie I am NOT an analog specialist. I just love analog more than digital..

Of course, there is no control, lol

I think simplest one is the above schematic with red text on it.

I've came up with this:

Ah oh, bad news: electronics.stackexchange.com/a/82740/5035

jippie

@abdullahkahraman looks good

@abdullahkahraman Yes I've seen zenervoltage increase too when they get hot.

but you have resistors in there to limit current

abdullah kahraman

@jippie Not for D?. (Ops forgot to re-annotate).

jippie

loose the zeners, keep a spare controller and the .hex file ;o)

abdullah kahraman

10:35

@jippie Hahahahha!

This actually is not my project. It is a friend's friend's graduation project.

I am paid just to write the software, but my heart couldn't resist on giving a schematic. He simply cannot do this.

jippie

so when the software fails it is your fault

and when the hardware fails ... it is your fault too :-p

I hope you can pay your insurance from your pay check :-p

abdullah kahraman

@jippie hahaha, too bad :) That's why I will not demand any money for the schematic, so I will not feel guilty when I no longer help for the schematic and hardware.

jippie

@abdullahkahraman its a good excersise anyway, you learn a lot for it

abdullah kahraman

This is an elevator project. He doesn't even know that a stepper motor will not turn with just connecting the wires to supply.. I really feel sorry for these guys, they obviously picked the wrong department.

@jippie Exactly, and it is fun!

jippie

a stepper motor is just a fancy asynchronous machine

or an asynchronous machine is just a fancy stepper motor

having said that ... a stepper motor probably runs just fine(ish) from a three phase net (better NOT try it though)

abdullah kahraman

10:45

@jippie I don't even know what is an asynchronous machine :)

jippie

a three phase motor

abdullah kahraman

@jippie Yeah, I just know its name now :) And I can guess that probably it runs with three phase AC.

@jippie That edit was necessary lol

jippie

/me is out.

@abdullahkahraman tty later

abdullah kahraman

@jippie thank you for your help :)

David

13:09

I'm using an OPA847 opamp, it has 3.9Ghz gain bandwidth. My input signal is 5Mhz and I'm amplifying it by x50. Would I be fine with any opamp that has 250Mhz (5*50) total GBWP? Or are there other factors I need to consider?

rAsHmI

16:07

is there anyone

jippie

17:30

morning

David

Nobody is here @rAsHmI

angelatlarge

nobody

jippie

17:54

@rAsHmI @Everyone was here before though

hey @angelatlarge and @David

angelatlarge

@jippie hi

jippie

@angelatlarge did I have some good garlic in my dinner ... I'm so thirsty

Everyone

21:13

@jippie And I shall return q+:

Transcript for

Electrical Engineering