1.8" display and drawChar function
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1.8" display and drawChar function

by Andy_123 on Wed Feb 13, 2013 11:07 am

I am not sure if this is a right place to post, if not point me to the correct place please

it appears that default GFX library function drawChar updates one pixel at the time, setting window for each pixel individulally in drawPixel routine.

this causes a lot of overhead, as it is setting "window" 48 times instead of 1 for each character (size 1)
Set Window takes 7 SPI writes for each pixel, that we can avoid
Overhead is even greater for the font sizes 2 or larger.

I think it would be more efficient to set window for whole character once (6x8 size) and then push all pixels directly writing spidata
Hardware will take care of shifting to the next pixel and setting next row in this case.

But this will require to read all five bytes of each character before displaying and as a result 6 bytes of RAM instead of 1 for the "line" variable.

I did not write whole new routine, just made quick test for font size 1 and it works much faster, making text appearance almost instant.

Any comments or ideas?
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Re: 1.8" display and drawChar function

by thekitty on Thu Feb 14, 2013 1:46 pm

COuld you put your test code snippet in a code window on a reply post? I'd be interested in your technique.

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Re: 1.8" display and drawChar function

by Andy_123 on Thu Feb 14, 2013 2:18 pm

Code that I am using right now is implemented in my library conversion from Arduino to PIC18 and PIC32
Last night I got fully finctional code for any size not just size=1 that I initially did.
if you want to see PIC code - I can post it tonight

LAter on I am planning to go back to Arduino ST7735R libraries and add my drawChar code using low level direct SPI write routines setting Window only once per char

Once changed, I will post it here
Last edited by Andy_123 on Thu Feb 14, 2013 10:18 pm, edited 1 time in total.
Posts: 11
Joined: Sun Jan 27, 2013 2:29 pm

Re: 1.8" display and drawChar function

by Andy_123 on Thu Feb 14, 2013 10:16 pm

Here is updated Arduino code

But, I guess my code works only if text background color is set, because it writes all pixels of the char
Original code only sets active pixels, alowing transaprent background

As a result, code below requires use of tft.setTextColor(color, bg); instead of tft.setTextColor(color);

Code: Select all | TOGGLE FULL SIZE
// draw a character fast
void Adafruit_ST7735::drawChar(int16_t x, int16_t y, unsigned char c,
             uint16_t color, uint16_t bg, uint8_t size) {
 uint8_t line[6], i,ii,j,jj;
  if((x >= _width)            ||               // Clip right
     (y >= _height)           ||               // Clip bottom
     ((x + 5 * size - 1) < 0) ||               // Clip left
     ((y + 8 * size - 1) < 0))                  // Clip top
                                        //read 6 columns first: 5 from ROM, last=0
  for (i=0; i<5; i++ ){
     line[i] = pgm_read_byte(font+(c*5)+i);
   line[5]=0;                               // blank column
                                       // prepare colors
   uint8_t fHi =color>>8;
   uint8_t fLo =color;
   uint8_t bHi =bg>>8;
   uint8_t bLo =bg;

    setAddrWindow(x, y, x+6*size-1, y+8*size-1);   // set window for the full character
   *rsport |=  rspinmask;                     // prepare data connection
   *csport &= ~cspinmask;                     // set CS low, since setAddrWindow will set it high

   for (j = 0; j<8; j++) {               // row in character
      for (jj = 0; jj<size; jj++) {         // pixel row in each row
         for (i = 0; i<6; i++) {               // column in character
            for (ii = 0; ii<size; ii++) {// pixel column
              if (line[i] & 0x1)  {            // write color pixel
              } else {         
                 spiwrite(bHi);            // or write background color
            }                           // ii - pixel of a column done
            if(jj==(size-1)) line[i] >>= 1;      // shift source only if we did all pixel rows
         }                              // i - all columns in the row done
      }                                 // jj - all pixel in the row done
   }                                    // j - all rows done
   *csport |= cspinmask;                     // set cs high
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Joined: Sun Jan 27, 2013 2:29 pm