Why does RGB888 format file with only red channel data display in blue in Microsoft Photos app?

Question

I am working with a Espressif ESP32-CAM module. I can capture and display 128x128 JPEG images which display properly. With the associated Arduino library, I can also convert the JPEG image into RGB888 format, which also displays properly in Windows Photos. For my intended application, I want to capture and analyze just the RED channel of the RGB888 formatted file. To test this idea, I copied the first, fourth, seventh, etc bytes of the RGB888 file to another file, but padded with zeros in the G & B positions so that I wind up with a properly formatted RGB888 file with only RED channel data. However, when I view the file, everything is shades of blue instead of in shades of red. I thought maybe I had misunderstood the byte order for RGB888 but all the formatting information I found insists the byte ordering is R - G - B. What am I missing here?

Arduino code that produced the above images:

    Name:       ESPCAM_SDCARD.ino
    Created:    1/30/2025 8:59:52 PM
    Author:     FRANK_XPS_9530\Frank

    This program captures a JPEG image from the camera and writes it out to the SD card
    In addition it converts the JPEG image to BMP888 format (3x the size + header) and 
    writes it out to the SD card as well.
*/

#include "esp_camera.h"
#include "Arduino.h"
#include "FS.h"                // SD Card ESP32
#include "SD_MMC.h"            // SD Card ESP32
#include "soc/soc.h"           // Disable brownour problems
#include "soc/rtc_cntl_reg.h"  // Disable brownour problems
#include "driver/rtc_io.h"
#include <EEPROM.h>            // read and write from flash memory

// define the number of bytes you want to access
#define EEPROM_SIZE 1

// Pin definition for CAMERA_MODEL_AI_THINKER
#define PWDN_GPIO_NUM     32
#define RESET_GPIO_NUM    -1
#define XCLK_GPIO_NUM      0
#define SIOD_GPIO_NUM     26
#define SIOC_GPIO_NUM     27

#define Y9_GPIO_NUM       35
#define Y8_GPIO_NUM       34
#define Y7_GPIO_NUM       39
#define Y6_GPIO_NUM       36
#define Y5_GPIO_NUM       21
#define Y4_GPIO_NUM       19
#define Y3_GPIO_NUM       18
#define Y2_GPIO_NUM        5
#define VSYNC_GPIO_NUM    25
#define HREF_GPIO_NUM     23
#define PCLK_GPIO_NUM     22

int pictureNumber = 0;

void setup() {
  WRITE_PERI_REG(RTC_CNTL_BROWN_OUT_REG, 0); //disable brownout detector

  Serial.begin(115200);

  Serial.println("Starting Setup()");

  camera_config_t config;
  config.ledc_channel = LEDC_CHANNEL_0;
  config.ledc_timer = LEDC_TIMER_0;
  config.pin_d0 = Y2_GPIO_NUM;
  config.pin_d1 = Y3_GPIO_NUM;
  config.pin_d2 = Y4_GPIO_NUM;
  config.pin_d3 = Y5_GPIO_NUM;
  config.pin_d4 = Y6_GPIO_NUM;
  config.pin_d5 = Y7_GPIO_NUM;
  config.pin_d6 = Y8_GPIO_NUM;
  config.pin_d7 = Y9_GPIO_NUM;
  config.pin_xclk = XCLK_GPIO_NUM;
  config.pin_pclk = PCLK_GPIO_NUM;
  config.pin_vsync = VSYNC_GPIO_NUM;
  config.pin_href = HREF_GPIO_NUM;
  config.pin_sscb_sda = SIOD_GPIO_NUM;
  config.pin_sscb_scl = SIOC_GPIO_NUM;
  config.pin_pwdn = PWDN_GPIO_NUM;
  config.pin_reset = RESET_GPIO_NUM;
  config.xclk_freq_hz = 20000000;
  config.pixel_format = PIXFORMAT_JPEG;

  if (psramFound()) {
    config.frame_size = FRAMESIZE_128X128; // FRAMESIZE_ + QVGA|CIF|VGA|SVGA|XGA|SXGA|UXGA
    config.jpeg_quality = 10;
    config.fb_count = 2;
  }
  else {
    config.frame_size = FRAMESIZE_SVGA;
    config.jpeg_quality = 12;
    config.fb_count = 1;
  }

  // Init Camera
  esp_err_t err = esp_camera_init(&config);
  if (err != ESP_OK) {
    Serial.printf("Camera init failed with error 0x%x", err);
    return;
  }

  //Serial.println("Starting SD Card");
  if (!SD_MMC.begin()) {
    Serial.println("SD Card Mount Failed");
    return;
  }

  uint8_t cardType = SD_MMC.cardType();
  if (cardType == CARD_NONE) {
    Serial.println("No SD Card attached");
    return;
  }

  camera_fb_t* fb = NULL;

  // Take Picture with Camera
  fb = esp_camera_fb_get();
  delay(500);//This is key to avoid an issue with the image being very dark and green. If needed adjust total delay time.
  fb = esp_camera_fb_get();
  Serial.printf("Image acquired: fb = %x fb_len = %d\n", fb, fb->len);

  //02/01/25 per https://www.esp32.com/viewtopic.php?t=17479 can convert JPG to BMP
  uint8_t* bmp_buf = NULL; //pointer to buffer of uint8_t objects
  size_t bmp_buf_len = 0;
  bool converted = frame2bmp(fb, &bmp_buf, &bmp_buf_len);
  if (!converted) 
  {
    log_e("BMP Conversion failed");
  }
  else
  {
    Serial.printf("BMP Conversion succeeded with buf = %p, bmp_buf_len = %d\n", &bmp_buf, bmp_buf_len);
  }

  for (size_t i = 0; i < 100; i++)
  {
      Serial.printf("bmp_buf[%d] = %x\n",i, bmp_buf[i]);
  }

  if (!fb) {
    Serial.println("Camera capture failed");
    return;
  }
  // initialize EEPROM with predefined size
  EEPROM.begin(EEPROM_SIZE);
  pictureNumber = EEPROM.read(0) + 1;

  // Path where new picture will be saved in SD Card
  String path = "/picture" + String(pictureNumber) + ".jpg";

  //write JPEG file to SD card
  fs::FS& fs = SD_MMC;
  Serial.printf("Picture file name: %s\n", path.c_str());

  File file = fs.open(path.c_str(), FILE_WRITE);
  if (!file) {
    Serial.println("Failed to open file in writing mode");
  }
  else 
  {
    file.write(fb->buf, fb->len); // payload (image), payload length
    Serial.printf("Saved file to path: %s\n", path.c_str());
    EEPROM.write(0, pictureNumber);
    EEPROM.commit();
  }
  file.close();

  esp_camera_fb_return(fb);

//
//----------------------------------- write BMP888 file to SD card ----------------------
//
  path = "/picture" + String(pictureNumber) + ".bmp";

  Serial.printf("BMP888 file name: %s\n", path.c_str());

  file = fs.open(path.c_str(), FILE_WRITE);
  if (!file) {
    Serial.println("Failed to open BMP888 file in writing mode");
  }
  else 
  {
    file.write(bmp_buf, bmp_buf_len); // payload (image), payload length
    Serial.printf("Saved file to path: %s\n", path.c_str());
  }
  file.close();

//
//---------- Write out Red channel (with other bytes = 0) ----------------------
//

  uint8_t hdr_len = 54; 
  uint16_t image_len = hdr_len + 128 * 128 * 3; //49206
  path = "/picture" + String(pictureNumber) + "_red" + ".bmp";

  Serial.printf("BMP888 Red channel file name: %s\n", path.c_str());

  file = fs.open(path.c_str(), FILE_WRITE);
  if (!file) 
  {
    Serial.println("Failed to open BMP888 Red channel file in writing mode");
  }
  else 
  {
    file.write(bmp_buf, hdr_len); // write out header

    for (size_t i = hdr_len; i < bmp_buf_len; i+= 3)
    {
      file.write(bmp_buf[i]); // red channel
      file.write(0x0); // blue channel
      file.write(0x0); // green channel
    }
    Serial.printf("Saved BMP888 Red channel file to path: %s\n", path.c_str());
  }
  file.close();

//DEBUG!! Read Red channel data back into RAM and print out first 100 bytes
  uint8_t* red_buf = NULL;
  char temp;

  file = fs.open(path.c_str(), FILE_READ);
  for (size_t i = 0; i < 100; i++)
  {
    temp = file.read();
    printf("red[%d] = %x\n",i, temp);
  }

  // Turns off the ESP32-CAM white on-board LED (flash) connected to GPIO 4
  pinMode(4, OUTPUT);
  digitalWrite(4, LOW);
  rtc_gpio_hold_en(GPIO_NUM_4);
  esp_deep_sleep_start();
}

void loop() {

}