Issue with drawXBM() on STM32F103C8 + SH1106 (Byte Swapping Issue)

[Peter-VM]

Hello!

I am using STM32F103C8 (Bluepill) + OLED SH1106_128X64 with the U8g2 library.

I convert an image into a byte array using image2cpp (XBM Format, Little Endian, Horizontal Scan).

🔹 Issue:

When I display the image using u8g2.drawBitmap(), it appears correctly.
However, when using u8g2.drawXBM(), the image is distorted: pairs of bytes (every 2 bytes) are swapped.
🔹 What I have checked:

I manually verified the byte array – it is correct.
drawBitmap() takes the same array and works without distortion.
Could you please help me understand what causes this issue and how to fix drawXBM() on STM32?

Thank you!

[Peter-VM]

I would also like to highlight that your library is truly impressive. I recently started using it and greatly appreciate its high-quality code and well-designed architecture.

P.S. I am currently using version 2.32.15.

[olikraus]

I did my tests with "GIMP" to produce the xbm file. This did work as expected.
Maybe you can post the related code.

[Peter-VM]

Here is my code.
Size of my picture is 32x10 picsels.

#include <Arduino.h>
#include <U8g2lib.h>
#include <Wire.h>

U8G2_SH1106_128X64_NONAME_F_HW_I2C u8g2(U8G2_R0, U8X8_PIN_NONE);

// X and O in bitmap
const unsigned char pict[] = {
0xff, 0xaa, 0xff, 0xff, 0xef, 0xef, 0xf8, 0x3f, 0xf3, 0xdf, 0xf7, 0xdf, 0xfd, 0x3f, 0xef, 0xef,
0xfe, 0xff, 0xdf, 0xf7, 0xfd, 0x3f, 0xdf, 0xf7, 0xf3, 0xdf, 0xdf, 0xf7, 0xef, 0xef, 0xef, 0xef,
0xff, 0xff, 0xf7, 0xdf, 0xff, 0xff, 0xf8, 0x3f
};
/*cat face
0x04, 0x0c, 0x00, 0x00,
0x0e, 0x1c, 0x01, 0x10,
0x1a, 0x16, 0x01, 0xf0,
0x13, 0xf2, 0x00, 0x40,
0x10, 0x02, 0x00, 0x40,
0x14, 0x22, 0x02, 0x50,
0x16, 0x32, 0x00, 0x40,
0x30, 0x03, 0x0c, 0x46,
0x24, 0x91, 0x0f, 0xfe,
0x23, 0x61, 0x00 0x00,
*/

// for test bytes order in memory
static uint16_t *b16;
static uint32_t *b32;

void setup()
{
b16 = (uint16_t *)pict;
b32 = (uint32_t *)pict;
Serial1.begin(9600);
Serial1.println("Initializing display...");

u8g2.begin();
u8g2.setContrast(255);
u8g2.setDrawColor(0);
}

void loop()
{
static int x = 0, y = 0;
u8g2.clearBuffer();
//u8g2.drawBitmap(x, y, 32/8, 10, pict);
u8g2.drawXBMP(x, y, 32, 10, pict);
u8g2.sendBuffer();

// move on screen
x++;
if (x > 128)
{
x = 0;
y += 16;
if (y > 64)
y = 0;
}
delay(50);
}

[olikraus]

Is it an AVR based uC? If so you need to use drawXBM (without P, however I am not sure whether I have added the version without P)

[Peter-VM]

No, I use STM32F103C8T6.
My steps:
I converted the image to a byte array using image2cpp.
Then I placed this array in the program code. I use platformio.
I draw this image on the OLED SH1106_128X64.
If I draw using the drawBitmap function, the image is displayed correctly.
But if I use the drawXBMP or drawXBM functions, the image is displayed with distortion. This distortion is due to the fact that the byte pairs have swapped.

[olikraus]

drawXBM really differs from drawBitmap. They do use a different format. XBM actually is a kind of an official file format: https://de.wikipedia.org/wiki/X_BitMap

Many drawing tools (like GIMP) are able to export as XBM.

[Peter-VM]

Thanks for the wrap! Okay, I'll use GIMP and see what the result will be.

[Peter-VM]

Thanks for the hint, I figured out the problem. It really turns out that the BMP and XBM formats are different formats.
The Gimp graphic editor saves the image in the format XBM. This format turned out to be text, which can be pasted directly into a C program.