Make camera sensor auto-exposure faster, usable

src/config.h

@@ -6,4 +6,13 @@
 
 // Use a primitive autoexposure by shifting each frames luminosity based on the
 // ratio of white to black pixels in the previous frames
-// #define CONFIG_USE_EXPOSURE_CORRECTION
+// #define CONFIG_USE_EXPOSURE_CORRECTION
+
+// Capture a frame when it is within this tolerance of 50% black pixels
+#define CONFIG_TOLERANCE        0.2
+
+// Wait this number of frames before a frame is eligible for beaing captured
+#define CONFIG_MIN_FRAMES       2
+
+// Maximum number of frames to wait. Capture even if it is not within tolerance.
+#define CONFIG_MAX_FRAMES       15

src/ov7670.c

@@ -1,7 +1,6 @@
 #include "ov7670.h"
 #include "pinning.h"
 #include "debug.h"
-#include "config.h"
 
 #define REG_GAIN                0x00
 #define REG_BLUE                0x01
@@ -127,12 +126,12 @@
 #define REG_HAECC2              0xa0
 #define REG_SCALING_PCLK_DELAY  0xa2
 #define REG_NT_CTRL             0xa4
-#define REG_BD50MAX             0xa5
+#define REG_AECGMAX             0xa5
-#define REG_HAECC3              0xa6
+#define REG_LPH                 0xa6
-#define REG_HAECC4              0xa7
+#define REG_UPL                 0xa7
-#define REG_HAECC5              0xa8
+#define REG_TPL                 0xa8
-#define REG_HAECC6              0xa9
+#define REG_TPH                 0xa9
-#define REG_HAECC7              0xaa
+#define REG_NALG                0xaa
 #define REG_BD60MAX             0xab
 #define REG_STR_OPT             0xac
 #define REG_STR_R               0xad
@@ -161,6 +160,9 @@ static unsigned int BlackPixels = 0;
 static volatile int ExposureCorrection = 0;
 #endif
 
+volatile unsigned int Camera_BlackPixelCounts[] = {0};
+volatile int Camera_FinalFrameCount = -1;
+
 static uint8_t ReadRegister(uint8_t reg)
 {
     while(I2C1->SR2 & I2C_SR2_BUSY);
@@ -302,6 +304,24 @@ void Camera_Init(void)
     WriteRegister(REG_COM14, 0x18 | 1);
     WriteRegister(REG_SCALING_PCLK_DIV, 1);
 
+    // Set maximum gain ceiling
+    WriteRegister(REG_COM9, 0xca);
+
+    // Disable auto-exposure, enable auto-gain
+    WriteRegister(REG_COM8, 0xec);
+
+    // // Disable auto-gain, enable auto-exposure
+    // WriteRegister(REG_COM8, 0xe9);
+
+    // Use histogram-based auto-exposure
+    WriteRegister(REG_NALG, 0x94);
+
+    // Increase AEC speed:
+    // ‘Decrease TPL but not less than UPL [default 0xc1] and increase TPH but
+    // not bigger than LPH [default 0xf0]’
+    WriteRegister(REG_TPL, 0xc1);
+    WriteRegister(REG_TPH, 0xf0);
+
     // Enable VSYNC interrupts. TIM3 is enabled at the first VSYNC.
     FrameCount = 0;
     Camera_Captured = 0;
@@ -334,14 +354,28 @@ void TIM1_CC_IRQHandler(void)
 #endif
 
     // Check if the last frame's exposure is reasonable
-    if(FrameCount >= 5)
+    if(!Camera_Captured && FrameCount >= CONFIG_MIN_FRAMES
+        && (
+            (BlackPixels > (int)(CAMERA_PIXELS / 2
+                - CAMERA_PIXELS * CONFIG_TOLERANCE)
+            && BlackPixels < (int)(CAMERA_PIXELS / 2
+                + CAMERA_PIXELS * CONFIG_TOLERANCE))
+            || FrameCount >= CONFIG_MAX_FRAMES
+        ))
     {
         Camera_Captured = 1;
+        Camera_FinalFrameCount = FrameCount;
         // Disable everything
         TIM3->CR1 = 0;
         TIM1->CR1 = 0;
     }
 
+    if(FrameCount < (int)(sizeof(Camera_BlackPixelCounts)
+        / sizeof(Camera_BlackPixelCounts[0])))
+    {
+        Camera_BlackPixelCounts[FrameCount] = BlackPixels;
+    }
+
     // Reset black pixel counter
     BlackPixels = 0;
 

src/ov7670.h

@@ -2,6 +2,7 @@
 
 #include <stdlib.h>
 #include "stm32f1xx.h"
+#include "config.h"
 
 #define CAMERA_IMAGE_WIDTH      160
 #define CAMERA_IMAGE_HEIGHT     144
@@ -10,4 +11,7 @@
 
 extern volatile int Camera_Captured;
 
+extern volatile unsigned int Camera_BlackPixelCounts[CONFIG_MAX_FRAMES];
+extern volatile int Camera_FinalFrameCount;
+
 void Camera_Init(void);