From 75baf054e5b77c3267bb9f12eddd9bc5dff45a2d Mon Sep 17 00:00:00 2001
From: David Oberhollenzer <goliath@infraroot.at>
Date: Sun, 17 Jul 2022 01:58:46 +0200
Subject: [PATCH] Implement "dump raw frame buffer to stdout"

Signed-off-by: David Oberhollenzer <goliath@infraroot.at>
---
 shadermeh.c | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 shadermeh.h |   2 +
 2 files changed, 119 insertions(+), 3 deletions(-)

diff --git a/shadermeh.c b/shadermeh.c
index 7d1fdc6..79bfd45 100644
--- a/shadermeh.c
+++ b/shadermeh.c
@@ -21,14 +21,55 @@ static double diff_timespec(const struct timespec *time1,
 		+ (time1->tv_nsec - time0->tv_nsec) / 1000000000.0;
 }
 
+static void convert_for_ffmpeg(const uint8_t *in, uint8_t *out,
+			       size_t width, size_t height)
+{
+	size_t x, y;
+
+	for (y = 0; y < height; ++y) {
+		const uint8_t *src = in + y * width * 4;
+		uint8_t *dst = out + (height - 1 - y) * width * 3;
+
+		for (x = 0; x < width; ++x) {
+			*(dst++) = *(src++);
+			*(dst++) = *(src++);
+			*(dst++) = *(src++);
+			++src;
+		}
+	}
+}
+
+static int write_retry(int fd, const void *buffer, size_t size)
+{
+	while (size > 0) {
+		int ret = write(fd, buffer, size);
+
+		if (ret < 0) {
+			if (errno == EINTR)
+				continue;
+			perror("write");
+			return -1;
+		}
+
+		if (ret == 0)
+			return -1;
+
+		size -= ret;
+		buffer = (const char *)buffer + ret;
+	}
+
+	return 0;
+}
+
 static const struct option long_opts[] = {
 	{ "width", required_argument, NULL, 'w' },
  	{ "height", required_argument, NULL, 'h' },
 	{ "shader", required_argument, NULL, 's' },
+	{ "to-stdout", no_argument, NULL, 'S' },
 	{ NULL, 0, NULL, 0 },
 };
 
-static const char *short_opts = "w:h:s:";
+static const char *short_opts = "w:h:s:S";
 
 static const char *usage_str =
 "shadermeh OPTIONS...\n"
@@ -38,6 +79,8 @@ static const char *usage_str =
 "  --width, -w <pixels>\n"
 "  --height, -h <pixels>\n"
 "\n"
+"  --to-stdout, -S\n"
+"\n"
 "  --shader, -s <shader file>\n"
 "\n";
 
@@ -46,10 +89,12 @@ int main(int argc, char **argv)
 	GLuint u_iResolution, u_iTime, u_iTimeDelta, u_iFrame;
 	struct timespec start, frame_start, frame_end;
 	unsigned int width, height, iFrame = 0;
+	void *fb32 = NULL, *fb24 = NULL;
 	const char *shader_file = NULL;
+	GLuint vao, vbo, fbo, fbo_tex;
 	GLint major, minor, prog;
 	float iTime, iTimeDelta;
-	GLuint vao, vbo;
+	bool to_stdout = false;
 	window *wnd;
 	int i;
 
@@ -72,6 +117,9 @@ int main(int argc, char **argv)
 		case 's':
 			shader_file = optarg;
 			break;
+		case 'S':
+			to_stdout = true;
+			break;
 		default:
 			fputs(usage_str, stderr);
 			return EXIT_FAILURE;
@@ -84,11 +132,28 @@ int main(int argc, char **argv)
 		return EXIT_FAILURE;
 	}
 
+	if (to_stdout) {
+		fb32 = calloc(1, width * height * 4);
+		if (!fb32) {
+			perror("allocating scratch framebuffer");
+			return EXIT_FAILURE;
+		}
+
+		fb24 = calloc(1, width * height * 3);
+		if (!fb24) {
+			perror("allocating scratch framebuffer");
+			free(fb32);
+			return EXIT_FAILURE;
+		}
+	}
+
 	/********** create window and make context current **********/
 	wnd = window_create(width, height, "shader meh...");
 
 	if (!wnd) {
 		fputs("failed to create window", stderr);
+		free(fb32);
+		free(fb24);
 		return EXIT_FAILURE;
 	}
 
@@ -101,6 +166,8 @@ int main(int argc, char **argv)
 	if (glewInit() != GLEW_OK) {
 		fputs("glewInit() error", stderr);
 		window_destroy(wnd);
+		free(fb32);
+		free(fb24);
 		return EXIT_FAILURE;
 	}
 
@@ -148,12 +215,34 @@ int main(int argc, char **argv)
 
 	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 
+	/******************** framebuffer object ********************/
+	if (to_stdout) {
+		glGenFramebuffers(1, &fbo);
+		glGenTextures(1, &fbo_tex);
+
+		glBindTexture(GL_TEXTURE_2D, fbo_tex);
+		glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0,
+			     GL_RGBA, GL_UNSIGNED_BYTE, NULL);
+		glBindTexture(GL_TEXTURE_2D, 0);
+
+		glBindFramebuffer(GL_FRAMEBUFFER, fbo);
+		glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+				     fbo_tex, 0);
+	}
+
 	/******************** drawing loop ********************/
 	while (window_handle_events()) {
+		/* render image to FBO */
+		if (to_stdout)
+			glBindFramebuffer(GL_FRAMEBUFFER, fbo);
+
 		clock_gettime(CLOCK_MONOTONIC_RAW, &frame_start);
 		glClear(GL_COLOR_BUFFER_BIT);
 		glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
-		window_swap_buffers(wnd);
+
+		if (!to_stdout)
+			window_swap_buffers(wnd);
+
 		clock_gettime(CLOCK_MONOTONIC_RAW, &frame_end);
 
 		iFrame += 1;
@@ -163,6 +252,23 @@ int main(int argc, char **argv)
 		glUniform1f(u_iTimeDelta, iTimeDelta);
 		glUniform1f(u_iTime, iTime);
 		glUniform1ui(u_iFrame, iFrame);
+
+		if (to_stdout) {
+			/* get image from FBO */
+			glBindFramebuffer(GL_FRAMEBUFFER, 0);
+			glBindTexture(GL_TEXTURE_2D, fbo_tex);
+			glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA,
+				      GL_UNSIGNED_BYTE, fb32);
+			glBindTexture(GL_TEXTURE_2D, 0);
+
+			/* convert to 24 bps and dump to stdout */
+			convert_for_ffmpeg(fb32, fb24, width, height);
+
+			if (write_retry(STDOUT_FILENO, fb24,
+					width * height * 3)) {
+				break;
+			}
+		}
 	}
 
 	/******************** cleanup ********************/
@@ -173,7 +279,15 @@ int main(int argc, char **argv)
 	glDeleteVertexArrays(1, &vao);
 	glDeleteProgram(prog);
 
+	if (to_stdout) {
+		glBindFramebuffer(GL_FRAMEBUFFER, 0);
+		glDeleteFramebuffers(1, &fbo);
+		glDeleteTextures(1, &fbo_tex);
+	}
+
 	window_make_current(NULL);
+	free(fb32);
+	free(fb24);
 	window_destroy(wnd);
 	return EXIT_SUCCESS;
 }
diff --git a/shadermeh.h b/shadermeh.h
index 419bc47..d568af4 100644
--- a/shadermeh.h
+++ b/shadermeh.h
@@ -10,11 +10,13 @@
 #include <GL/gl.h>
 #include <GL/glx.h>
 
+#include <stdbool.h>
 #include <string.h>
 #include <stdlib.h>
 #include <getopt.h>
 #include <unistd.h>
 #include <stdio.h>
+#include <errno.h>
 #include <time.h>
 
 typedef struct {