Displaying graphics with Data from a SD card

Other Hardware Displays
1

Hello

I have a I2C 128x32 OLED Display (wide.hk) that I currently talk to with the amazing u8g Library.

I came to the point where I want to show way more graphics then my ATMega32u4 can hold in progmem. Because of this I tried to integrate an microSD card reader into my setup.

I can read files from the microSD card with the (also) amazing SDfat Library. But I have problem with handling this data.

It starts with the problem that every function in the two graphic library I tried (Adafruit_GFX, u8G) that are able to Display graphics in bulk (drawBitmap, drawXBM, etc.) wants to use PROGMEM (or U8G_PROGMEM). Also those variables are mostly defined as static. I see no way how I could pass Data on runtime to the Display if those functions nly except static progmem data.

Because of this I'm currently thinking about using binary files (010101) to show on-pixel and off-pixel. But walking down this road the last days seems exhausting and cant be right. Am I missing something?

Has anyone done something like this and could give me a hint?

2

If you want to just shift bitmaps from the SD to the display, then you have to adopt a 'bit at a time approach' - ie read some data from the SD, send it to the display, read a bit more, send it to the display until you have transferred the whole image.

The amount you can transfer at a time is dependent on the amount of RAM you have available - this will be much less than the amount of flash (progmem) you're likely to have.

I'm currently doing something similar for a ST7735 based display - I'm using the Adafruit GFX and ST7735 libraries but have extended the latter by adding in a handy function that will write image data in the display's native 65k colour format (2 bytes/pixel) directly from a buffer in RAM to the display. I've got enough free RAM to read one line of the image off the SD and then send to the display at a time.

I then added another function to my main program that takes the name of the file to load, the x & y of where to show the bitmap, and the width of the bitmap, as parameters and then then reads the file one line at a time into ram & then calls the library function to transfer this to the display.

3

Mh, I see what you mean. Space really is extremly tight on those chips.

I just tried said technique of reading binary files and printing them as single pixel of 1 and 0.
It is disappointing that the whole idea of loading XBM is just worth to display a lame splashscreen-image. One can only image how nice it would be to use it for dynamic data as well.

Ok. Thank you, I guess I have to go down that gravel road with my inline skater.

4

You can write directly to that display. If you have your bitmap stored on the SD card organized like the 1306 screen memory you could do something like this:

#define BUFFER_SIZE 32
char buffer[BUFFER_SIZE];

ssd1306_command(SSD1306_SETLOWCOLUMN);
ssd1306_command(SSD1306_SETHIGHCOLUMN);
ssd1306_command(SSD1306_SETSTARTPAGE);

for (int i=0; i<((128*32/8)/BUFFER_SIZE); i++) {
  if (file.read(buffer, BUFFER_SIZE) == BUFFER_SIZE) {
    for (int j=0; j<BUFFER_SIZE; j++)
      ssd1306_data(buffer[j]);
  } else {
    break;	// error
  }
}

The ssd1306_command and _data functions are from the Adafruit library but you could cut and paste them into your sketch if you're using a different library.

Ideally the buffer would be 512 bytes but a smaller one would still work okay. I haven't tried this but I suspect it would be pretty fast if you're using hardware SPI.