single header C api for webcam frame capture

cam.h

/*
	// gcc -s -O3 -std=c99 -Wall -o cam cam.c -lvfw32
	#include <stdio.h>
	#define STB_IMAGE_WRITE_IMPLEMENTATION
	#include "stb_image_write.h"
	#include "cam.h"

	int main(void)
	{
		long w = 1280;
		long h = 720;
		unsigned char *rgb = malloc(w * h * 3);
		Cam c = {0};
		cam_init(&c, w, h, rgb);

		cam_on(&c);
		{
			cam_cap(&c);
			stbi_write_jpg("tmp1.jpg", w, h, 3, rgb, 90);

			cam_cap(&c);
			stbi_write_jpg("tmp2.jpg", w, h, 3, rgb, 90);
		}
		cam_off(&c);

		cam_on(&c);
		{
			cam_cap(&c);
			stbi_write_jpg("tmp3.jpg", w, h, 3, rgb, 90);
		}
		cam_off(&c);

		cam_destroy(&c);

		free(rgb);

		return 0;
	}
*/
#ifdef _WIN32
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <vfw.h>

#define FOURCC(a, b, c, d) ((DWORD)(((d) << 24) | ((c) << 16) | ((b) << 8) | (a)))

static unsigned char clip(int i)
{
	if (i <= 0)
		return 0;
	if (i >= 255)
		return 255;
	return (unsigned char)i;
}

static void YUV444toRGB888(unsigned char y, unsigned char u, unsigned char v, unsigned char *dst)
{
	int C = y - 16;
	int D = u - 128;
	int E = v - 128;
	*dst++ = clip((298 * C + 409 * E + 128) >> 8);
	*dst++ = clip((298 * C - 100 * D - 208 * E + 128) >> 8);
	*dst++ = clip((298 * C + 516 * D + 128) >> 8);
}

static void YUYVToRGB24(int w, int h, unsigned char *src, unsigned char *dst)
{
	int i;
	unsigned char u, y1, v, y2;
	for (i = 0; i < w * h; i += 2)
	{
		y1 = *src++;
		u = *src++;
		y2 = *src++;
		v = *src++;
		YUV444toRGB888(y1, u, v, dst);
		dst += 3;
		YUV444toRGB888(y2, u, v, dst);
		dst += 3;
	}
}

typedef struct
{
	HWND hwnd;
	long w, h;
	BITMAPINFO bmi;
	void *rgb;
} Cam;

LRESULT CALLBACK capVideoStreamCallback(HWND hwnd, LPVIDEOHDR vhdr)
{
	Cam *c = (Cam *)capGetUserData(hwnd);
	if (c->bmi.bmiHeader.biCompression == FOURCC('Y', 'U', 'Y', '2'))
		YUYVToRGB24(c->w, c->h, vhdr->lpData, c->rgb);
	return 0;
}

static void cam_init(Cam *c, long w, long h, void *rgb)
{
	c->hwnd = capCreateCaptureWindow(0, 0, 0, 0, 0, 0, 0, 0);
	c->w = w;
	c->h = h;
	c->rgb = rgb;
}

static void cam_on(Cam *c)
{
	capSetUserData(c->hwnd, c);
	capDriverConnect(c->hwnd, 0);
	capGetVideoFormat(c->hwnd, &c->bmi, sizeof(c->bmi));
	c->bmi.bmiHeader.biWidth = c->w;
	c->bmi.bmiHeader.biHeight = c->h;
	c->bmi.bmiHeader.biSizeImage = 0;
	capSetVideoFormat(c->hwnd, &c->bmi, sizeof(c->bmi));
	capSetCallbackOnFrame(c->hwnd, capVideoStreamCallback);
}

static void cam_cap(Cam *c)
{
	capGrabFrameNoStop(c->hwnd);
}

static void cam_off(Cam *c)
{
	capDriverDisconnect(c->hwnd);
}

static void cam_destroy(Cam *c)
{
	DestroyWindow(c->hwnd);
}
#else
#error "not supported"
#endif