#概要
以前OV2640を使って画像を取得したので、今回はOV7725を使って画像を取得してみました。OV7725はjpegではなく、bmp形式で画像が出力されるので、少し制御方法が異なります。

今回はブラウザ上でOV7725のレジスタを変更できるようにしました！結論から申し上げると、30万画素ではあまりきれいな写真は撮れませんでした。初期の携帯に搭載されていたカメラと同性能らしいです…
きれいな写真を撮りたい場合はラズベリーパイを使って、OV5640で撮影したりした方がいいかと思います。

OV2640ではESP32-WROOM-32を使用しましたが、OV7725の場合はbmpで、メモリ使用量が増えるため、PSRAM搭載のESP32-WROVER-Bを使用しました。

公式
https://github.com/espressif/esp32-camera

たぶん公式に採用される前
https://github.com/igrr/esp32-cam-demo

ピン配置
https://www.instructables.com/id/The-Incredible-ESP32-Wrover-From-Espressif/

OV7725 データシート
https://cdn.sparkfun.com/datasheets/Sensors/LightImaging/OV7725.pdf

#設定
esp-idfのフォルダ内でsubmoduleをクローンします。

git submodule add https://github.com/espressif/esp32-camera.git components/esp32-camera
mv esp-idf/components/esp32-camera/driver/private_include/sccb.h esp-idf/components/esp32-camera/driver/include/sccb.h
mv esp-idf/components/esp32-camera/driver/private_include/sensor.h esp-idf/components/esp32-camera/driver/include/sensor.h

#注意点

driver公式のページ通りにbuildすると、camera_init()が重複していてエラーになったので、中身をapp_main関数に移動しました。
ピン配置は公式のものではなく、https://github.com/igrr/esp32-cam-demo を参考にしました。
XCLK:20MHz→6MHzへ変更

#コード
esp-idfのexampleにあるsimple serverを改良し、ブラウザで「http://<ローカルアドレス>/」へアクセスすると撮影画像が表示されます。ajaxで画像データ（base64で変換したデータ）を取得し、ブラウザに表示しています。

全コードは下記に載せています。
https://github.com/koki-iwaizumi/esp32-ov7725

サイズはVGA、出力形式はYUV422にしています。wifi周りの設定は変更してください。

app_main.c


#include <string.h>
#include <esp_wifi.h>
#include <esp_event_loop.h>
#include <esp_log.h>
#include <esp_system.h>
#include <nvs_flash.h>
#include <sys/param.h>
#include <esp_http_server.h>
#include <esp_camera.h>

#include "mbedtls/base64.h"
#include "cJSON.h"

#include "sensor.h"
#include "sccb.h"

/********* io ***********/
#define CAM_PIN_PWDN	2
#define CAM_PIN_RESET	15
#define CAM_PIN_XCLK	27
#define CAM_PIN_SIOD	25
#define CAM_PIN_SIOC	23
#define CAM_PIN_VSYNC	22
#define CAM_PIN_HREF	26
#define CAM_PIN_PCLK	21
#define CAM_PIN_D9		19
#define CAM_PIN_D8		36
#define CAM_PIN_D7		18
#define CAM_PIN_D6		39
#define CAM_PIN_D5		5
#define CAM_PIN_D4		34
#define CAM_PIN_D3		32
#define CAM_PIN_D2		35

/********* wifi ***********/
#define EXAMPLE_WIFI_SSID "*"
#define EXAMPLE_WIFI_PASS "*"

static const char *TAG = "APP";
sensor_t* sensor = NULL;

static camera_config_t camera_config = {
	.pin_pwdn  = CAM_PIN_PWDN,
	.pin_reset = CAM_PIN_RESET,
	.pin_xclk = CAM_PIN_XCLK,
	.pin_sscb_sda = CAM_PIN_SIOD,
	.pin_sscb_scl = CAM_PIN_SIOC,

	.pin_d7 = CAM_PIN_D9,
	.pin_d6 = CAM_PIN_D8,
	.pin_d5 = CAM_PIN_D7,
	.pin_d4 = CAM_PIN_D6,
	.pin_d3 = CAM_PIN_D5,
	.pin_d2 = CAM_PIN_D4,
	.pin_d1 = CAM_PIN_D3,
	.pin_d0 = CAM_PIN_D2,
	.pin_vsync = CAM_PIN_VSYNC,
	.pin_href = CAM_PIN_HREF,
	.pin_pclk = CAM_PIN_PCLK,

	.xclk_freq_hz = 6000000,
	.ledc_timer = LEDC_TIMER_0,
	.ledc_channel = LEDC_CHANNEL_0,

	.pixel_format = PIXFORMAT_YUV422,
	.frame_size = FRAMESIZE_VGA,

	.jpeg_quality = 12,
	.fb_count = 1
};

esp_err_t index_get_handler(httpd_req_t *req)
{
	extern const unsigned char index_start[] asm("_binary_index_html_start");
	extern const unsigned char index_end[]   asm("_binary_index_html_end");
	const size_t index_size = (index_end - index_start);

	httpd_resp_send_chunk(req, (const char *)index_start, index_size);
	httpd_resp_sendstr_chunk(req, NULL);

	return ESP_OK;
}

esp_err_t image_post_handler(httpd_req_t *req)
{
	int content_buf_size = 10 * 1000;
	char *content_buf = calloc(content_buf_size, sizeof(char));
    if(content_buf == NULL){
		ESP_LOGE(TAG, "Failed to allocate frame buffer - content_buf");
		httpd_resp_send_500(req);
		return ESP_FAIL;
    }

	if(httpd_req_recv(req, content_buf, req->content_len) <= 0){
		ESP_LOGE(TAG, "httpd_req_recv failed");
		httpd_resp_send_500(req);
		return ESP_FAIL;
	}

	ESP_LOGE(TAG, "httpd_req_recv content=%s", content_buf);

	cJSON *content_json = cJSON_Parse(content_buf);

	for(int i = 0; i <= 172; i++){
		char key[4];
		itoa(i,key,10);
		int value = strtol(cJSON_GetObjectItem(content_json, key)->valuestring, NULL, 16);

		SCCB_Write(sensor->slv_addr, i, value);
	}

	cJSON_Delete(content_json);
	free(content_buf);

	camera_fb_t *fb = NULL;
	esp_err_t res = ESP_OK;
	int64_t fr_start = esp_timer_get_time();

	fb = esp_camera_fb_get();
	if( ! fb){
		ESP_LOGE(TAG, "Camera capture failed");
		httpd_resp_send_500(req);
		return ESP_FAIL;
	}

	uint8_t *buf = NULL;
	size_t buf_len = 0;
	bool converted = frame2bmp(fb, &buf, &buf_len);
	esp_camera_fb_return(fb);
	if( ! converted){
		ESP_LOGE(TAG, "BMP conversion failed");
		httpd_resp_send_500(req);
		return ESP_FAIL;
	}

	ESP_LOGI(TAG, "esp_get_free_heap_size (%d bytes)", esp_get_free_heap_size());
	ESP_LOGI(TAG, "esp_get_minimum_free_heap_size (%d bytes)", esp_get_minimum_free_heap_size());
	ESP_LOGI(TAG, "heap_caps_get_free_size(MALLOC_CAP_8BIT) (%d bytes)", heap_caps_get_free_size(MALLOC_CAP_8BIT));
	ESP_LOGI(TAG, "heap_caps_get_minimum_free_size(MALLOC_CAP_8BIT) (%d bytes)", heap_caps_get_minimum_free_size(MALLOC_CAP_8BIT));
	ESP_LOGI(TAG, "heap_caps_get_largest_free_block(MALLOC_CAP_8BIT) (%d bytes)", heap_caps_get_largest_free_block(MALLOC_CAP_8BIT));

	int image_buf_size = 1300 * 1000;
	uint8_t *image_buf = calloc(image_buf_size, sizeof(char));
    if(image_buf == NULL){
		free(buf);
		ESP_LOGE(TAG, "Failed to allocate frame buffer - image_buf");
		httpd_resp_send_500(req);
		return ESP_FAIL;
    }

	size_t olen = 0;
	int base64_err = mbedtls_base64_encode(image_buf, image_buf_size, &olen, buf, buf_len);
	free(buf);
	if (base64_err != 0) {
		ESP_LOGE(TAG, "error base64 encoding, error %d, buff size: %d", base64_err, olen);
		httpd_resp_send_500(req);
		return ESP_FAIL;
	}

	ESP_LOGI(TAG, "esp_get_free_heap_size (%d bytes)", esp_get_free_heap_size());
	ESP_LOGI(TAG, "esp_get_minimum_free_heap_size (%d bytes)", esp_get_minimum_free_heap_size());
	ESP_LOGI(TAG, "heap_caps_get_free_size(MALLOC_CAP_8BIT) (%d bytes)", heap_caps_get_free_size(MALLOC_CAP_8BIT));
	ESP_LOGI(TAG, "heap_caps_get_minimum_free_size(MALLOC_CAP_8BIT) (%d bytes)", heap_caps_get_minimum_free_size(MALLOC_CAP_8BIT));
	ESP_LOGI(TAG, "heap_caps_get_largest_free_block(MALLOC_CAP_8BIT) (%d bytes)", heap_caps_get_largest_free_block(MALLOC_CAP_8BIT));

	uint8_t *image_json = calloc(image_buf_size, sizeof(char));
    if(image_json == NULL){
		free(image_buf);
		ESP_LOGE(TAG, "Failed to allocate frame buffer - image_json");
		httpd_resp_send_500(req);
		return ESP_FAIL;
    }

	sprintf((char *)image_json, "{\"image\":\"%s\"}", (const char *)image_buf);
	free(image_buf);
	ESP_LOGI(TAG, "image_json length=%d", strlen((const char*)image_json));

	res = httpd_resp_set_type(req, "application/json") || httpd_resp_set_hdr(req, "Access-Control-Allow-Origin", "*") || httpd_resp_send(req, (const char *)image_json, strlen((const char*)image_json));
	free(image_json);

	int64_t fr_end = esp_timer_get_time();
	ESP_LOGI(TAG, "BMP: %uKB %ums", (uint32_t)(buf_len/1024), (uint32_t)((fr_end - fr_start)/1000));
	return res;
}

esp_err_t config_get_handler(httpd_req_t *req)
{
	int data_json_size = 10 * 1000;
	char *data_json = calloc(data_json_size, sizeof(char));
    if(data_json == NULL){
		ESP_LOGE(TAG, "Failed to allocate frame buffer - data_json");
		httpd_resp_send_500(req);
		return ESP_FAIL;
    }

	strcat(data_json, "{");
	for(int i = 0; i <= 172; i++){
		char data_json_buf[255] = {'\0'};
		sprintf(data_json_buf, "\"%d\":\"%d\"", i, SCCB_Read(sensor->slv_addr, i));
		strcat(data_json, data_json_buf);
		if(i != 172) strcat(data_json, ",");
	}
	strcat(data_json, "}");

	ESP_LOGI(TAG, "data_json=%s", data_json);
	ESP_LOGI(TAG, "data_json length=%d", strlen((const char*)data_json));

	esp_err_t res = httpd_resp_set_type(req, "application/json") || httpd_resp_set_hdr(req, "Access-Control-Allow-Origin", "*") || httpd_resp_send(req, (const char *)data_json, strlen((const char*)data_json));
	free(data_json);

	return res;
}

httpd_uri_t index_uri = {
	.uri = "/",
	.method = HTTP_GET,
	.handler = index_get_handler,
};

httpd_uri_t image_uri = {
	.uri = "/image",
	.method = HTTP_POST,
	.handler = image_post_handler,
};

httpd_uri_t config_uri = {
	.uri = "/config",
	.method = HTTP_GET,
	.handler = config_get_handler,
};

httpd_handle_t start_webserver(void)
{
	httpd_handle_t server = NULL;
	httpd_config_t config = HTTPD_DEFAULT_CONFIG();

	ESP_LOGI(TAG, "Starting server on port: '%d'", config.server_port);
	if(httpd_start(&server, &config) == ESP_OK){
		ESP_LOGI(TAG, "Registering URI handlers");
		httpd_register_uri_handler(server, &index_uri);
		httpd_register_uri_handler(server, &image_uri);
		httpd_register_uri_handler(server, &config_uri);
		return server;
	}

	ESP_LOGI(TAG, "Error starting server!");
	return NULL;
}

void stop_webserver(httpd_handle_t server)
{
	httpd_stop(server);
}

static esp_err_t event_handler(void *ctx, system_event_t *event)
{
	httpd_handle_t *server = (httpd_handle_t *) ctx;

	switch(event->event_id){
		case SYSTEM_EVENT_STA_START:
			ESP_LOGI(TAG, "SYSTEM_EVENT_STA_START");
			ESP_ERROR_CHECK(esp_wifi_connect());
			break;
		case SYSTEM_EVENT_STA_GOT_IP:
			ESP_LOGI(TAG, "SYSTEM_EVENT_STA_GOT_IP");
			ESP_LOGI(TAG, "Got IP: '%s'", ip4addr_ntoa(&event->event_info.got_ip.ip_info.ip));

			if(*server == NULL){
				*server = start_webserver();
			}
			break;
		case SYSTEM_EVENT_STA_DISCONNECTED:
			ESP_LOGI(TAG, "SYSTEM_EVENT_STA_DISCONNECTED");
			ESP_ERROR_CHECK(esp_wifi_connect());

			if(*server){
				stop_webserver(*server);
				*server = NULL;
			}
			break;
		default:
			break;
	}
	return ESP_OK;
}

static void initialise_wifi(void *arg)
{
	tcpip_adapter_init();
	ESP_ERROR_CHECK(esp_event_loop_init(event_handler, arg));
	wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
	ESP_ERROR_CHECK(esp_wifi_init(&cfg));
	ESP_ERROR_CHECK(esp_wifi_set_storage(WIFI_STORAGE_RAM));
	wifi_config_t wifi_config = {
		.sta = {
			.ssid = EXAMPLE_WIFI_SSID,
			.password = EXAMPLE_WIFI_PASS,
		},
	};
	ESP_LOGI(TAG, "Setting WiFi configuration SSID %s...", wifi_config.sta.ssid);
	ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA));
	ESP_ERROR_CHECK(esp_wifi_set_config(ESP_IF_WIFI_STA, &wifi_config));
	ESP_ERROR_CHECK(esp_wifi_start());
}

void app_main()
{
	static httpd_handle_t server = NULL;
	ESP_ERROR_CHECK(nvs_flash_init());

	esp_err_t err = esp_camera_init(&camera_config);
	if(err != ESP_OK){
		ESP_LOGE(TAG, "Camera Init Failed");
		return;
	}

	sensor = esp_camera_sensor_get();

	initialise_wifi(&server);
}

#HTML周り

index.html


<!doctype html>
<html lang="ja">
	<head>
		<meta charset="utf-8">
		<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
		<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.4/css/all.css">
		<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
		<title>OV7725</title>
		<style>
			.alert{
				display:none;
			}
			#overlay{   
				position: fixed;
				top: 0;
				z-index: 100;
				width: 100%;
				height:100%;
				display: none;
				background: rgba(0,0,0,0.6);
			}
			.cv-spinner{
				height: 100%;
				display: flex;
				justify-content: center;
				align-items: center;  
			}
			.spinner{
				width: 40px;
				height: 40px;
				border: 4px #ddd solid;
				border-top: 4px #2e93e6 solid;
				border-radius: 50%;
				animation: sp-anime 0.8s infinite linear;
			}
			@keyframes sp-anime{
				0% { 
					transform: rotate(0deg); 
				}
				100% { 
					transform: rotate(359deg); 
				}
			}
			.is-hide{
				display:none;
			}
			.ov_a{
				height:600px;
				overflow-y:auto;
			}
		</style>
	</head>
	<body>
 
		<div id="overlay">
			<div class="cv-spinner">
				<span class="spinner"></span>
			</div>
		</div>

		<div class="text-center mt-3 mb-3">
			<h1>OV7725</h1>
		</div>

		<div class="container-fluid">
			<div class="row">
				<div class="col-lg-6 text-center mb-4">
					<div class="row">
						<div class="col-md-12">
							<div class="alert alert-success" role="alert">success</div>
							<div class="alert alert-danger" role="alert">failed</div>
						</div>
						<div class="col-md-12">
							<img id="img" class="mw-100">
						</div>
						<div class="col-md-12 mt-4">
							<button id="image_button" type="button" class="btn btn-success w-100">Take a Picture</button>
						</div>
					</div>
				</div>

				<div class="col-lg-6">
					<div class="ov_a">
						<form id="form">
							<table id="table_regs" class="table table-striped">
								<tr>
									<th>Address(Hex)</th>
									<th>Value(Hex)</th>
								</tr>
							</table>
						</form>
					</div>
				</div>
			</div>
		</div>
 
		<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
		<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
		<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
		<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>

		<script type="text/javascript">
			$(function(){

				var parseJson = function(data) {
					var returnJson = {};
					for(idx = 0; idx < data.length; idx++){
					returnJson[data[idx].name] = data[idx].value
					}
					return returnJson;
				};

				$(document).ready(function(){
					read_config();
					get_image();
				});

				$("#image_button").on("click", get_image);

				function get_image(){
					$.ajax({
						url: "./image",
						type: "POST",
						data: JSON.stringify(parseJson($("#form").serializeArray())),
						dataType: "json",
						beforeSend: function() {
							$(".alert").css("display", "none");
							$("#overlay").fadeIn(300);
						}
					})
					.done((data) => {
						$(".alert").css("display", "none");
						$(".alert-success").css("display", "block");
						$("#img").attr("src", "data:image/jpeg;base64," + data["image"]);
					})
					.fail((data) => {
						$(".alert").css("display", "none");
						$(".alert-danger").css("display", "block");
						console.log(data);
					})
					.always((data) => {
						setTimeout(function(){
							$("#overlay").fadeOut(300);
						},500);
					});
				}

				function read_config(){
					$.ajax({
						url: "./config",
						type: "GET",
						dataType: "json",
						async: false,
					})
					.done((data) => {
						$.each(data, function(i, item){
							$("#table_regs").append('<tr><td>0x' + Number(i).toString(16) + '</td><td><div class="input-group"><div class="input-group-prepend"><span class="input-group-text" id="text1a">0x</span></div><input name="' + i + '" value="' + Number(item).toString(16) + '" type="text" class="form-control"></div></td></tr>');
						});
					})
				}
			});
		</script>
	</body>
</html>