Bitmap output from class field

Projects Programming
22

Have you cracked open the library's source code yet as I suggested WAY back in Post #6??? It seems there there must be a difference in the data passed to the function.

23

Yes, I opened the source code of the library, but still can't figure out what's wrong

24

What did the serial debug prints that you put into the library code reveal?

25

Code:

void printBitmap(unsigned char arr[]) {
  for (int i = 0; i < 8; ++i) {
    Serial.println(arr[i], BIN);
  }
}

Result:

image

The bitmap is displayed correctly in serial

26

But, did you put that debug printing directly inside the library code?

27

I will try

28

I finally found a solution. All that had to be done was to remove the const from the field where the bitmap is stored

#ifndef SOMECLASS_H
#define SOMECLASS_H
#include <Adafruit_SSD1306.h>

class SomeClass {
  public:
    void drawBitmap(Adafruit_SSD1306* display) {
      display->drawBitmap(10, 10, bitmap, 8, 8, SSD1306_WHITE);
    }
    unsigned char bitmap[8] = {
      B11111111, 
      B10000001, 
      B10000001, 
      B10000001, 
      B10000001, 
      B10000001,  
      B10000001,  
      B11111111
};
};

#endif

Many thanks to everyone who responded

29

If it were my project, I'd dig in deeper to figure out why that is the case. As it is, you have a "solution" that you don't understand.

30

Here is the code for an overload of the drawBitmap() function that accepts a bitmap from RAM:

void Adafruit_GFX::drawBitmap(int16_t x, int16_t y, uint8_t *bitmap, int16_t w,
                              int16_t h, uint16_t color) {

  int16_t byteWidth = (w + 7) / 8; // Bitmap scanline pad = whole byte
  uint8_t b = 0;

  startWrite();
  for (int16_t j = 0; j < h; j++, y++) {
    for (int16_t i = 0; i < w; i++) {
      if (i & 7)
        b <<= 1;
      else
        b = bitmap[j * byteWidth + i / 8];
      if (b & 0x80)
        writePixel(x + i, y, color);
    }
  }
  endWrite();
}

Given that it has a non-const bitmap parameter, it expects a non-const bitmap to be passed. But why the bitmap is displayed incorrectly, if, at first glance, there is no editing of the bitmap, I cannot understand. I lack knowledge

31

If that were the case, you would have gotten a compiler warning at the very least. When compiling for some platforms it would have caused a compiler error. Do you have the IDE's compiler warnings set to "All".

Also, there's an overload that take a constant array that the compiler would have picket (Adafruit_GFX.h):

  void drawBitmap(int16_t x, int16_t y, const uint8_t bitmap[], int16_t w,
                  int16_t h, uint16_t color);
32

Maybe because of overloading with a const, the compiler was choosing the one you sent. But because this overload was expecting a bitmap from progmem, it outputted badly.
As for the warnings, if I'm not mistaken, I didn't get any.

33

Good point. That could make a difference when working with an AVR processor. This is a long thread, I'm not sure if you ever told us which Arduino board you're using.

34

Nano

35

So, an ATmega328. Your analysis was correct, nice. The pgm_read_byte() function really means something specific on an AVR processor like that. It's more or less nothing on an ARM or ESP.