From a94861b09ae9e7e8edce1eb518cb5e5f20e328a3 Mon Sep 17 00:00:00 2001
From: Wolfgang Draxinger <code@datenwolf.net>
Date: Thu, 31 Dec 2020 16:52:07 +0100
Subject: commit it

---
 copy_pixels_swap_hack.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 copy_pixels_swap_hack.c

(limited to 'copy_pixels_swap_hack.c')

diff --git a/copy_pixels_swap_hack.c b/copy_pixels_swap_hack.c
new file mode 100644
index 0000000..653eeaf
--- /dev/null
+++ b/copy_pixels_swap_hack.c
@@ -0,0 +1,67 @@
+// copy_pixels_swap_hack.c
+//
+// Call glXSwapBuffers instead of glCopyPixels if OpenGL read and draw buffers
+// would perform a copy from back to front buffer.
+//
+// compile with
+// gcc -shared -o copy_pixels_swap_hack.so -fPIC -O2 copy_pixels_swap.c -ldl
+//
+// use with
+// LD_PRELOAD=copy_pixels_swap_hack.so ${YOUR_PROGRAM}
+//
+// Refer to the ld.so(8) manpage for the path at which the preloaded shared
+// objects is searched for.
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <GL/gl.h>
+#include <GL/glx.h>
+void glCopyPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum type)
+{
+	static int first = 1;
+	static Display*    (*glx_get_current_display)(void) = NULL;
+	static GLXDrawable (*glx_get_current_drawable)(void) = NULL;
+	static void (*glx_swap_buffers)(Display*,GLXDrawable) = NULL;
+	static void (*gl_copy_pixels)(GLint,GLint,GLsizei,GLsizei,GLenum) = NULL;
+	static void (*gl_get_integerv)(GLenum,GLint*) = NULL;
+	static void (*gl_draw_buffer)(GLenum) = NULL;
+	static void (*gl_read_buffer)(GLenum) = NULL;
+	if( first ){
+		*(void**)(&glx_get_current_display) = dlsym(RTLD_NEXT, "glXGetCurrentDisplay");
+		*(void**)(&glx_get_current_drawable) = dlsym(RTLD_NEXT, "glXGetCurrentDrawable");
+		*(void**)(&glx_swap_buffers) = dlsym(RTLD_NEXT, "glXSwapBuffers");
+		*(void**)(&gl_copy_pixels) = dlsym(RTLD_NEXT, "glCopyPixels");
+		*(void**)(&gl_get_integerv) = dlsym(RTLD_NEXT, "glGetIntegerv");
+		*(void**)(&gl_draw_buffer) = dlsym(RTLD_NEXT, "glDrawBuffer");
+		*(void**)(&gl_read_buffer) = dlsym(RTLD_NEXT, "glReadBuffer");
+		first = 0;
+	}
+
+	Display *dpy;
+	GLXDrawable drw;
+	if( !glx_get_current_display || !glx_get_current_drawable
+	 || !gl_get_integerv || !gl_copy_pixels || !glx_swap_buffers
+	 || !gl_draw_buffer || !gl_read_buffer
+	 || !(dpy = glx_get_current_display())
+	 || !(drw = glx_get_current_drawable())
+	){
+		return;
+	}
+
+	GLint buffer_draw =  0;
+	GLint buffer_read = ~0;
+	gl_get_integerv(GL_DRAW_BUFFER, &buffer_draw);
+	gl_get_integerv(GL_READ_BUFFER, &buffer_read);
+	if( GL_COLOR == type
+	 && buffer_draw == GL_FRONT
+	 && buffer_read == GL_BACK
+	){
+		glx_swap_buffers(dpy, drw);
+		gl_draw_buffer(GL_BACK);
+		gl_read_buffer(GL_FRONT);
+		gl_copy_pixels(x,y,width,height,type);
+		gl_draw_buffer(GL_FRONT);
+		gl_read_buffer(GL_BACK);
+	} else {
+		gl_copy_pixels(x,y,width,height,type);
+	}
+}
-- 
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