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cmicrocontrollerfixed-point

Calculating position of element in fixed point for TFT drawing

I'm currently writing the UI for a small uC project. Having some trouble with the calculating the position of a vertical line. The idea is to move the red line along the x - axis towards the end of the rectangle.

The value that is being incremented with an endless rotary encoder and has a range of 0 to 800 with increments of 1. Left side of the rectangle is the beginning of the x - axis aswell with x = 0. The range 0 - 800 represents 0 - 100.00 in Q13.3 fixed point notation with the data type being uint16_t.

The rectangle is currently 300pixel wide, i'm flexible with that but it shouldn't be less than 240pixel. Line is 1pixel wide.

The catch is, that due to performance limitations i can only use fixed point math, there should be no access to the floating point unit at all.

enter image description here

What i'm currently doing is having a counter that increments with every click of the encoder. sth. like:

        if(direction) counter++;        //Running forwards
        if(!direction) counter--;       //Running backwards
        if((counter % 8 ) == 0){
            if(direction) line.x += 3;
            if(!direction) line.x -= 3;
        }

which is less than optimal since the spacing is 3 pixel.

Ideally if float calculation was an option i would simply increment the counter with 0.375

Appreciate any advice on this issue.

cheers

Solution

  • Just take the position of the counter in terms of 0 ... 800 and scale it down to 0 ... width?

    I.e. multiply the value of counter by width and then do an integer division by max where max is the max of your logical range. 

    uint16_t scale(uint16_t value, uint16_t logical_max, uint16_t physical_max)
{
   uint32_t val = value * physical_max; // use a wider type to cope with overflow
   return (uint16_t) ((val / logical_max) & 0xFFFFU);
}