Floating Point
-
I would also like you to think about a fixed point format (http://en.wikipedia.org/wiki/Fixed-point_arithmetic).
A lot of sensor values can be represented in e.g. 8.8 or even 16.16. Think of e.g. temperatures (range -127..+127 is usually sufficient and I've yet to see an affordable sensor reporting in more than 256 steps per degree ), humidity (0..100%, 256 steps/degree), battery level (0..100%) etc.
Fixed point arithmetic requires very little resources compared to floating point and has no trouble converting between different architectures (apart from endianness).
For full efficiency sensor libraries should also support this format, as it doesn't make much sense to have a sensor library report in floating point and convert this to fixed point by MySensors.
A good Arduino-style fixed point library would help IMHO. Did a quick google on the subject, but didn't find much (apart from https://code.google.com/p/libfixmath/)I think support for real floating point values should still be possible, but this could be an interesting addition.
What do you think?
I would also like you to think about a fixed point format
See my suggestion above about defining an optional scaling factor for each variable, as we can define a precision for each floating variable now.
Simple decade scaling: integer factors of ten, using the same configured integer that would define precision for floats.
or
Flexible scaling: configure a floating point factor by which the integer OTA value would be multiplied to give real world units.
So a temperature of 29.7 could be sent as a 16 bit integer 297 with the gateway knowing (from configuratiion) that it must be multiplied by 0.1 before being converted to a string. Either approach could handle that.
The flexible version could also handles things like angles that are reported in degrees, or 0..1023 or radians or whatever, with the full accuracy of the sensor.
I think you are suggesting that we adopt one or more standard fixed point formats as additional OTA variable formats (as well as supporting the math function in the node library). I can certainly see value in that, as well. Like floating point, you would need to configure a precision when converting to decimal values (since 0.1 is not an accurate binary fraction in fixed point either). So 29.7 degrees would be encoded as 29*256 + ((256 * 7) / 10). = 7603, and converted to a float that would be 29.69921875. If like a float this fixed point value was configured with one digit of decimal accuracy, we'd get the 29.7 value again.
Either of these might sometimes let a node avoid the floating point libraries entirely in many cases, even when it wants to report non-integer values.
Could be handy if we ever get ATtiny nodes.
-
I would also like you to think about a fixed point format (http://en.wikipedia.org/wiki/Fixed-point_arithmetic).
A lot of sensor values can be represented in e.g. 8.8 or even 16.16. Think of e.g. temperatures (range -127..+127 is usually sufficient and I've yet to see an affordable sensor reporting in more than 256 steps per degree ), humidity (0..100%, 256 steps/degree), battery level (0..100%) etc.
Fixed point arithmetic requires very little resources compared to floating point and has no trouble converting between different architectures (apart from endianness).
For full efficiency sensor libraries should also support this format, as it doesn't make much sense to have a sensor library report in floating point and convert this to fixed point by MySensors.
A good Arduino-style fixed point library would help IMHO. Did a quick google on the subject, but didn't find much (apart from https://code.google.com/p/libfixmath/)I think support for real floating point values should still be possible, but this could be an interesting addition.
What do you think?
@Yveaux said:
Yep, would also be a good addition. Floating points is really crazy inefficient to use on an Arduino.
I will look at sending floats binary now... my secret knock sensor will have to wait :(.
Does anyone want to help on the fixed point stuff? -
@Yveaux said:
Yep, would also be a good addition. Floating points is really crazy inefficient to use on an Arduino.
I will look at sending floats binary now... my secret knock sensor will have to wait :(.
Does anyone want to help on the fixed point stuff? -
@hek said:
Does anyone want to help on the fixed point stuff?
I can try to put some skeleton together to get the interface right, as this is probably the hardest part.
-
@hek @Yveaux
Let me see if I am understanding.The payload types would be enhanced.
typedef enum { P_STRING, P_BYTE, P_INT16, P_UINT16, P_LONG32, P_ULONG32, P_CUSTOM } payload;to add an 8:8 and/or 16:16 fixed point formats, eg: P_FIX8P8 or P_FIX16P16. I'm guessing only signed fixed point, no unsigned?
And to add a 4 byte binary floating point P_FLOAT32?
One small suggestion: put P_CUSTOM first, so its numeric code doesn't change when you add additional formats. Or if it's too late for that, we can skip over P_CUSTOM for these new formats.
And also the payload setters:
MyMessage& set(const char* value); MyMessage& set(uint8_t value); MyMessage& set(double value, uint8_t decimals); MyMessage& set(unsigned long value); MyMessage& set(long value); MyMessage& set(unsigned int value); MyMessage& set(int value);would be enhanced to support these types.
MyMessage& set(double value, uint8_t decimals);could be unchanged as seen by the user even if the underlying OTA representation became binary. But we might add something like:
MyMessage& set(fix8p8 value, uint8_t: decimals); MyMessage& set(fix16p16 value, uint8_t: decimals);(the decimals parameter is needed for this like for floats, as described earlier)
This implies creating new C++ types, in this example "fix8p8", which is basically a int16_t with an implicit radix point in the middle.
Adding is simple. Multiply of fix8p8 is easy because you can use a long as temp before renormalizing, but multiply of fix16p16 gets trickier, of course.
Another discussion to have before it's set in stone...
Adding fixed point support both OTA and within library code has value, so I'm not against it. Getting the library right and educating users is going to be some work, tho. I use fixed point math fairly often, but it definitely has some gotchas that we are biting off.
The concept of having a scaling factor (see a few messages back) may be an easier step to implement and educate. It's an easy concept: every integer step represents X units, so the integer value must be multiplied by the scaling factor to get the real value in units. Default = 1.0 with both the same, as now.
The simplest version just scales by factors of 10, and could be implemented by adding a second integer to the set() function. In this version to send -12.5 you use set(-125,1), or to send 0.14 you use set(14,2). This can be interpreted into a string without even using floating point math, you just adjust where to insert a decimal point in a printed integer.
(The slightly more complex version would use an arbitrary floating point factor as the scale, so you could use 0.1 or 3.56 or whatever).
(Aside: from the viewpoint of the gateway, the fixed point enhancement is equivalent to having a fixed scaling factor of 2*-8 or 2^-16)
These enhancements are not mutually exclusive, but I would think that one of the scaled integer version might be easier to implement and understand as a first step.
-
@Zeph said:
(the decimals parameter is needed for this like for floats, as described earlier)
I don't see why the decimals parameter is needed. Currently it is used for the amount of decimals converted to textual presentation. This is not required for fixed point presentation (unless you want a scaling factor).
IMHO scaling just complicates things too much -- you also need to exchange the scaling factor with the gateway.This implies creating new C++ types, in this example "fix8p8", which is basically a int16_t with an implicit radix point in the middle.
Adding is simple. Multiply of fix8p8 is easy because you can use a long as temp before renormalizing, but multiply of fix16p16 gets trickier, of course.
My idea is to just wrap the new types in a class library, which allows for easy conversion and maths with these new fixedpt types.
Getting the library right and educating users is going to be some work, tho. I use fixed point math fairly often, but it definitely has some gotchas that we are biting off.
The library should shield regular users from the internals and pitfalls of fixed point. Most sketches just get a value from a sensor library and pass it on to MySensors, without modifying the value.
As part of this exercise we also have to modify these libraries which return their values in float-format, as it doesn't make sense to keeps floats in partly... -
@hek @Yveaux
Let me see if I am understanding.The payload types would be enhanced.
typedef enum { P_STRING, P_BYTE, P_INT16, P_UINT16, P_LONG32, P_ULONG32, P_CUSTOM } payload;to add an 8:8 and/or 16:16 fixed point formats, eg: P_FIX8P8 or P_FIX16P16. I'm guessing only signed fixed point, no unsigned?
And to add a 4 byte binary floating point P_FLOAT32?
One small suggestion: put P_CUSTOM first, so its numeric code doesn't change when you add additional formats. Or if it's too late for that, we can skip over P_CUSTOM for these new formats.
And also the payload setters:
MyMessage& set(const char* value); MyMessage& set(uint8_t value); MyMessage& set(double value, uint8_t decimals); MyMessage& set(unsigned long value); MyMessage& set(long value); MyMessage& set(unsigned int value); MyMessage& set(int value);would be enhanced to support these types.
MyMessage& set(double value, uint8_t decimals);could be unchanged as seen by the user even if the underlying OTA representation became binary. But we might add something like:
MyMessage& set(fix8p8 value, uint8_t: decimals); MyMessage& set(fix16p16 value, uint8_t: decimals);(the decimals parameter is needed for this like for floats, as described earlier)
This implies creating new C++ types, in this example "fix8p8", which is basically a int16_t with an implicit radix point in the middle.
Adding is simple. Multiply of fix8p8 is easy because you can use a long as temp before renormalizing, but multiply of fix16p16 gets trickier, of course.
Another discussion to have before it's set in stone...
Adding fixed point support both OTA and within library code has value, so I'm not against it. Getting the library right and educating users is going to be some work, tho. I use fixed point math fairly often, but it definitely has some gotchas that we are biting off.
The concept of having a scaling factor (see a few messages back) may be an easier step to implement and educate. It's an easy concept: every integer step represents X units, so the integer value must be multiplied by the scaling factor to get the real value in units. Default = 1.0 with both the same, as now.
The simplest version just scales by factors of 10, and could be implemented by adding a second integer to the set() function. In this version to send -12.5 you use set(-125,1), or to send 0.14 you use set(14,2). This can be interpreted into a string without even using floating point math, you just adjust where to insert a decimal point in a printed integer.
(The slightly more complex version would use an arbitrary floating point factor as the scale, so you could use 0.1 or 3.56 or whatever).
(Aside: from the viewpoint of the gateway, the fixed point enhancement is equivalent to having a fixed scaling factor of 2*-8 or 2^-16)
These enhancements are not mutually exclusive, but I would think that one of the scaled integer version might be easier to implement and understand as a first step.
@Zeph said:
The payload types would be enhanced.
typedef enum { P_STRING, P_BYTE, P_INT16, P_UINT16, P_LONG32, P_ULONG32, P_CUSTOM } payload;Darn, just realized we only got 3 bits to describe payload type. We need another one to fit the new ones.
MyMessage& set(double value, uint8_t decimals);Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
-
@Zeph said:
The payload types would be enhanced.
typedef enum { P_STRING, P_BYTE, P_INT16, P_UINT16, P_LONG32, P_ULONG32, P_CUSTOM } payload;Darn, just realized we only got 3 bits to describe payload type. We need another one to fit the new ones.
MyMessage& set(double value, uint8_t decimals);Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
-
@Zeph said:
The payload types would be enhanced.
typedef enum { P_STRING, P_BYTE, P_INT16, P_UINT16, P_LONG32, P_ULONG32, P_CUSTOM } payload;Darn, just realized we only got 3 bits to describe payload type. We need another one to fit the new ones.
MyMessage& set(double value, uint8_t decimals);Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
Also the Atmega based boards do not support double, well they do in naming but are float precisions
My Domotica project: http://www.pidome.org
-
Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
Also the Atmega based boards do not support double, well they do in naming but are float precisions
-
Just pushed the float changes.
To free up some bits in header for the new fixed point types (and simplify things) I'm considering reducing the commandTypes to just 3 values (SET, REQ, INTERNAL) the rest (PRESENTATION, STREAM) will be moved to be INTERNAL messages.
I could make serial interface unaffected by this change. But I'd rather remove it there as well.@Yveaux . Regarding remove the unsigned variant (e.g. ULONG). It is actually good to keep this. As there actually are some sensors reporting large numbers like meter-ticks which can be huge.
-
Just pushed the float changes.
To free up some bits in header for the new fixed point types (and simplify things) I'm considering reducing the commandTypes to just 3 values (SET, REQ, INTERNAL) the rest (PRESENTATION, STREAM) will be moved to be INTERNAL messages.
I could make serial interface unaffected by this change. But I'd rather remove it there as well.@Yveaux . Regarding remove the unsigned variant (e.g. ULONG). It is actually good to keep this. As there actually are some sensors reporting large numbers like meter-ticks which can be huge.
@hek Maybe not for 1.4, but you should consider removing a lot of the data from the header and leave only the routing info & message type in.
Depending on message type you then get a 'nested' header which tells you about the message-type specifics.
This also will help in the struggle to store data format types, for which you've now reserved 3 bits. They are always sent, also when there's no SET/GET data present in the message. Then you simply reserve e.g. a byte which will go a long way...http://yveaux.blogspot.nl
-
@hek Maybe not for 1.4, but you should consider removing a lot of the data from the header and leave only the routing info & message type in.
Depending on message type you then get a 'nested' header which tells you about the message-type specifics.
This also will help in the struggle to store data format types, for which you've now reserved 3 bits. They are always sent, also when there's no SET/GET data present in the message. Then you simply reserve e.g. a byte which will go a long way...@Yveaux said:
This also will help in the struggle to store data format types, for which you've now reserved 3 bits. They are always sent, also when there's no SET/GET data present in the message. Then you simply reserve e.g. a byte which will go a long way...
Darn... you are so right..
-
@Yveaux said:
This also will help in the struggle to store data format types, for which you've now reserved 3 bits. They are always sent, also when there's no SET/GET data present in the message. Then you simply reserve e.g. a byte which will go a long way...
Darn... you are so right..
-
@Zeph said:
(the decimals parameter is needed for this like for floats, as described earlier)
I don't see why the decimals parameter is needed. Currently it is used for the amount of decimals converted to textual presentation. This is not required for fixed point presentation (unless you want a scaling factor).
IMHO scaling just complicates things too much -- you also need to exchange the scaling factor with the gateway.This implies creating new C++ types, in this example "fix8p8", which is basically a int16_t with an implicit radix point in the middle.
Adding is simple. Multiply of fix8p8 is easy because you can use a long as temp before renormalizing, but multiply of fix16p16 gets trickier, of course.
My idea is to just wrap the new types in a class library, which allows for easy conversion and maths with these new fixedpt types.
Getting the library right and educating users is going to be some work, tho. I use fixed point math fairly often, but it definitely has some gotchas that we are biting off.
The library should shield regular users from the internals and pitfalls of fixed point. Most sketches just get a value from a sensor library and pass it on to MySensors, without modifying the value.
As part of this exercise we also have to modify these libraries which return their values in float-format, as it doesn't make sense to keeps floats in partly...@Yveaux said:
@Zeph said:
(the decimals parameter is needed for this like for floats, as described earlier)
I don't see why the decimals parameter is needed. Currently it is used for the amount of decimals converted to textual presentation.
I was think of when packets are converted to the comma separated textual representation for the API.
Convert 25.7 degrees to fixed point at the node, then at the gateway convert the fixed point it to a text string, and you'll see what I mean. It's not for the scaling option.
IMHO scaling just complicates things too much -- you also need to exchange the scaling factor with the gateway.
There are tradeoffs either way. In the current architecture, to support scaling you'd need to at least tell the gateway the scaling factor as part of the one-time presentation configuration. (Alternately, the gateway could retrieve the scaling factor along with type and name from local configuration, rather than receiving all of those OTA, but that's another discussion)
Beyond that there's no need for new libraries, and it's easy to explain.
However, I understand that you are excited by the fixed point functionality (which could be useful for more than just OTA encoding). I don't want to discourage that exploration. I look forward to some examples of encoding at the node end, and decoding at the gateway end, for some sensors like the DHT-22 or 18B20.
This implies creating new C++ types, in this example "fix8p8", which is basically a int16_t with an implicit radix point in the middle.
Adding is simple. Multiply of fix8p8 is easy because you can use a long as temp before renormalizing, but multiply of fix16p16 gets trickier, of course.
My idea is to just wrap the new types in a class library, which allows for easy conversion and maths with these new fixedpt types.
Yes, that was what I was guessing. Go for it!
As part of this exercise we also have to modify these libraries which return their values in float-format, as it doesn't make sense to keeps floats in partly...
Agreed. It would be nice if we rarely needed to even link the floating point library in nodes. (And it would make an ATtiny based node more feasible someday).
-
@Zeph said:
The payload types would be enhanced.
typedef enum { P_STRING, P_BYTE, P_INT16, P_UINT16, P_LONG32, P_ULONG32, P_CUSTOM } payload;Darn, just realized we only got 3 bits to describe payload type. We need another one to fit the new ones.
MyMessage& set(double value, uint8_t decimals);Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
@hek said:
MyMessage& set(double value, uint8_t decimals);Shouldn't this be set(float, uint8_t). Wouldn't it be confusing to have double-argument when only sending 32-bit float?
I was quoting an excerpt of the current system. I would tend to agree with changing that to float, just for clarity of intent, even though they are the same in GCC for the AVR
-
@Yveaux said:
@Zeph said:
(the decimals parameter is needed for this like for floats, as described earlier)
I don't see why the decimals parameter is needed. Currently it is used for the amount of decimals converted to textual presentation.
I was think of when packets are converted to the comma separated textual representation for the API.
Convert 25.7 degrees to fixed point at the node, then at the gateway convert the fixed point it to a text string, and you'll see what I mean. It's not for the scaling option.
IMHO scaling just complicates things too much -- you also need to exchange the scaling factor with the gateway.
There are tradeoffs either way. In the current architecture, to support scaling you'd need to at least tell the gateway the scaling factor as part of the one-time presentation configuration. (Alternately, the gateway could retrieve the scaling factor along with type and name from local configuration, rather than receiving all of those OTA, but that's another discussion)
Beyond that there's no need for new libraries, and it's easy to explain.
However, I understand that you are excited by the fixed point functionality (which could be useful for more than just OTA encoding). I don't want to discourage that exploration. I look forward to some examples of encoding at the node end, and decoding at the gateway end, for some sensors like the DHT-22 or 18B20.
This implies creating new C++ types, in this example "fix8p8", which is basically a int16_t with an implicit radix point in the middle.
Adding is simple. Multiply of fix8p8 is easy because you can use a long as temp before renormalizing, but multiply of fix16p16 gets trickier, of course.
My idea is to just wrap the new types in a class library, which allows for easy conversion and maths with these new fixedpt types.
Yes, that was what I was guessing. Go for it!
As part of this exercise we also have to modify these libraries which return their values in float-format, as it doesn't make sense to keeps floats in partly...
Agreed. It would be nice if we rarely needed to even link the floating point library in nodes. (And it would make an ATtiny based node more feasible someday).
@Zeph said:
to support scaling you'd need to at least tell the gateway the scaling factor as part of the one-time presentation configuration
This method (and the same holds for the decimals-parameter) seems attrictive, but has a few drawbacks:
- The presentation message currently has no 'guaranteed' delivery; we would need to change that as without this info the gateway cannot interpret the incoming data
- It also has to be sent to the sensor (actuator actually) nodes from the gateway when data goes the other way. No 'presentation' mechanism from gateway to sensor currently exists.
- It places an administration burden on the gateway, and possibly on actuators
Beyond that there's no need for new libraries, and it's easy to explain.
Possibly, but when you start mixing up values with different scaling factors or want to do (simple) maths on them the story changes completely...
-
I've been google'ing around looking for existing fixed point c++ libraries which fit the ATMega and tend to try out the following: https://code.google.com/p/libfixmath
It has regular updates, a unit test suite, impressive performance advantages (especially addition/substract, see https://code.google.com/p/libfixmath/wiki/Benchmarks), has been tested on ATMega and uses an MIT license.
It only supports 16.16, but other derivates like 8.8 seems doable.Rolling my own from start is too much work for me, as implementation is tricky at some points (unit tests are a requirement IMHO)
Anyone has a better suggestion?
http://yveaux.blogspot.nl
-
I've been google'ing around looking for existing fixed point c++ libraries which fit the ATMega and tend to try out the following: https://code.google.com/p/libfixmath
It has regular updates, a unit test suite, impressive performance advantages (especially addition/substract, see https://code.google.com/p/libfixmath/wiki/Benchmarks), has been tested on ATMega and uses an MIT license.
It only supports 16.16, but other derivates like 8.8 seems doable.Rolling my own from start is too much work for me, as implementation is tricky at some points (unit tests are a requirement IMHO)
Anyone has a better suggestion?
