pointer to array problem

Projects Programming
1

I am trying to modify the software pendant but I do not understand the next step

a = pgm_read_byte(ptr++);     // New frame X1/Y1
  if(a >= 0x90) {               // EOD marker? (valid X1 never exceeds 8)
    ptr = anim;                 // Reset animation data pointer to start
    a   = pgm_read_byte(ptr++); // and take first value
  }
  x1 = a >> 4;                  // X1 = high 4 bits
  y1 = a & 0x0F;                // Y1 = low 4 bits
  a  = pgm_read_byte(ptr++);    // New frame X2/Y2
  x2 = a >> 4;                  // X2 = high 4 bits
  y2 = a & 0x0F;                // Y2 = low 4 bits

  // Read rectangle of data from anim[] into portion of img[] buffer
  for(x=x1; x<=x2; x++) { // Column-major
    for(y=y1; y<=y2; y++) img[(x << 4) + y] = pgm_read_byte(ptr++);
  }

anim[] is a array 23086 byte long and img[] is a array 144 byte long

who explains it to me?

2

who explains it to me?

What is the next step, and what don't you understand?

It would appear that that snippet of code is locating the start of a 144 byte block of data in a large array, and then copying the bytes to an array in SRAM.

3 
pgm_read_byte(ptr++)

This part reads the next byte from PROGMEM

It reads a byte and if the high half of the byte contains 9 that marks the end of the animation and it starts over.

If the top half isn't 9 the two halves are an X,Y coordinate pair. The next byte is also a coordinate pair. The loops scan the rectangle defined by the two coordinate pairs and fills that part of the image from PROGMEM.

What's your question?