jpeg-encoder: add configurable RGB565 byte order support (#820)

Adds `jpgSetRgb565BE(bool)` to allow selecting byte order when
converting RGB565 input to RGB888 during JPEG encoding. The previous
behavior (big-endian MSB-first) is preserved as the default. This
improves compatibility with systems and pipelines producing
little-endian RGB565 buffers.

Author: DT-art1

conversions/include/img_converters.h

@@ -129,6 +129,17 @@ bool fmt2rgb888(const uint8_t *src_buf, size_t src_len, pixformat_t format, uint
 #define JPG_SCALE_MAX  JPEG_IMAGE_SCALE_1_8
 bool jpg2rgb565(const uint8_t *src, size_t src_len, uint8_t * out, esp_jpeg_image_scale_t scale);
 
+/**
+ * @brief Configure RGB565 input byte order for JPEG encoding.
+ *
+ * Controls how RGB565 source pixel data is interpreted before JPEG conversion.
+ * By default, the encoder assumes big-endian byte order (MSB first), which
+ * matches most ESP32 camera and frame buffer outputs.
+ *
+ * @param enable    True to use big-endian RGB565 (default), false for little-endian.
+ */
+void jpgSetRgb565BE(bool enable);
+
 #ifdef __cplusplus
 }
 #endif

conversions/to_jpg.cpp

@@ -29,6 +29,8 @@
 static const char* TAG = "to_jpg";
 #endif
 
+static bool rgb565_big_endian = true;
+
 static void *_malloc(size_t size)
 {
     void * res = malloc(size);
@@ -60,9 +62,15 @@ static IRAM_ATTR void convert_line_format(uint8_t * src, pixformat_t format, uin
         l = width * 2;
         src += l * line;
         for(i=0; i<l; i+=2) {
-            dst[o++] = src[i] & 0xF8;
-            dst[o++] = (src[i] & 0x07) << 5 | (src[i+1] & 0xE0) >> 3;
-            dst[o++] = (src[i+1] & 0x1F) << 3;
+            if (rgb565_big_endian) {
+                dst[o++] = src[i] & 0xF8;
+                dst[o++] = (src[i] & 0x07) << 5 | (src[i+1] & 0xE0) >> 3;
+                dst[o++] = (src[i+1] & 0x1F) << 3;
+            } else  {
+                dst[o++] = src[i+1] & 0xF8;
+                dst[o++] = (src[i+1] & 0x07) << 5 | (src[i] & 0xE0) >> 3;
+                dst[o++] = (src[i] & 0x1F) << 3;
+            }
         }
     } else if(format == PIXFORMAT_YUV422) {
         uint8_t y0, y1, u, v;
@@ -233,3 +241,8 @@ bool frame2jpg(camera_fb_t * fb, uint8_t quality, uint8_t ** out, size_t * out_l
 {
     return fmt2jpg(fb->buf, fb->len, fb->width, fb->height, fb->format, quality, out, out_len);
 }
+
+void jpgSetRgb565BE(bool enable)
+{
+    rgb565_big_endian = enable;
+}