Add configurable default chroma subsampling mode for JPEG encoder (#819)

This introduces a new `chroma_t` enum and the function `jpgSetChroma()`
to select the default chroma subsampling (4:4:4, 4:2:2, 4:2:0) used by
the JPEG encoder. This avoids changing existing function signatures and
allows globally configurable encoder behavior. The previous default
(H2V2 / 4:2:0) remains unchanged unless overridden.

Co-authored-by: Me No Dev <me-no-dev@users.noreply.github.com>

Author: DT-art1

conversions/include/img_converters.h

@@ -129,6 +129,26 @@ 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 Chroma subsampling modes for JPEG encoding.
+ *
+ * CHROMA_444: full chroma resolution (4:4:4)
+ * CHROMA_422: horizontal chroma subsampling (4:2:2)
+ * CHROMA_420: horizontal and vertical subsampling (4:2:0)
+ */
+typedef enum {
+    CHROMA_444 = 1,
+    CHROMA_422 = 2,
+    CHROMA_420 = 3
+} chroma_t;
+
+/**
+ * @brief Set default chroma subsampling mode for JPEG encoding.
+ *
+ * @param chroma Chroma subsampling mode (CHROMA_444, CHROMA_422, CHROMA_420)
+ */
+void jpgSetChroma(chroma_t chroma);
+
 /**
  * @brief Configure RGB565 input byte order for JPEG encoding.
  *

conversions/to_jpg.cpp

@@ -29,6 +29,8 @@
 static const char* TAG = "to_jpg";
 #endif
 
+
+static jpge::subsampling_t default_subsampling = jpge::H2V2;
 static bool rgb565_big_endian = true;
 
 static void *_malloc(size_t size)
@@ -99,7 +101,7 @@ static IRAM_ATTR void convert_line_format(uint8_t * src, pixformat_t format, uin
 bool convert_image(uint8_t *src, uint16_t width, uint16_t height, pixformat_t format, uint8_t quality, jpge::output_stream *dst_stream)
 {
     int num_channels = 3;
-    jpge::subsampling_t subsampling = jpge::H2V2;
+    jpge::subsampling_t subsampling = default_subsampling;
 
     if(format == PIXFORMAT_GRAYSCALE) {
         num_channels = 1;
@@ -242,6 +244,11 @@ 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 jpgSetChroma(chroma_t chroma)
+{
+    default_subsampling = static_cast<jpge::subsampling_t>(chroma);
+}
+
 void jpgSetRgb565BE(bool enable)
 {
     rgb565_big_endian = enable;