Created examples/filters.c to demonstrate how to draw in one window based on

the position of a different window.

This illustrates the idea of filters; windows that draw their contents based
on the content of a different window (similar to what you'd see if you viewed
something through a colored filter).  This works on OS X.  Untested on other
platforms.

examples/CMakeLists.txt

@@ -38,10 +38,12 @@ add_executable(simple WIN32 MACOSX_BUNDLE simple.c ${ICON} ${GLAD})
 add_executable(splitview WIN32 MACOSX_BUNDLE splitview.c ${ICON} ${GLAD})
 add_executable(transparent WIN32 MACOSX_BUNDLE transparent.c ${ICON} ${GLAD})
 add_executable(wave WIN32 MACOSX_BUNDLE wave.c ${ICON} ${GLAD})
+add_executable(filters WIN32 MACOSX_BUNDLE filters.c ${ICON} ${TINYCTHREAD} ${GETOPT} ${GLAD})
 
 target_link_libraries(particles "${CMAKE_THREAD_LIBS_INIT}" "${RT_LIBRARY}")
+target_link_libraries(filters "${CMAKE_THREAD_LIBS_INIT}" "${RT_LIBRARY}")
 
-set(WINDOWS_BINARIES boing gears heightmap particles simple splitview wave)
+set(WINDOWS_BINARIES boing gears heightmap particles simple splitview wave filters)
 
 set_target_properties(${WINDOWS_BINARIES} PROPERTIES FOLDER "GLFW3/Examples")
 
@@ -56,6 +58,7 @@ if (APPLE)
     set_target_properties(gears PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Gears")
     set_target_properties(heightmap PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Heightmap")
     set_target_properties(particles PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Particles")
+    set_target_properties(filters PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Filters")
     set_target_properties(simple PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Simple")
     set_target_properties(splitview PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "SplitView")
     set_target_properties(wave PROPERTIES MACOSX_BUNDLE_BUNDLE_NAME "Wave")

examples/filters.c

@@ -0,0 +1,281 @@
+/**
+ * This code was originally derived from the threads test code that came with
+ * the license below.  However, its purpose has changed; while it does show
+ * how to use threads, its real purpose is to show how to draw images on a
+ * window based on the location of a different window.
+ */
+
+//========================================================================
+// Multi-threading test
+// Copyright (c) Camilla Berglund <elmindreda@elmindreda.org>
+//
+// This software is provided 'as-is', without any express or implied
+// warranty. In no event will the authors be held liable for any damages
+// arising from the use of this software.
+//
+// Permission is granted to anyone to use this software for any purpose,
+// including commercial applications, and to alter it and redistribute it
+// freely, subject to the following restrictions:
+//
+// 1. The origin of this software must not be misrepresented; you must not
+//    claim that you wrote the original software. If you use this software
+//    in a product, an acknowledgment in the product documentation would
+//    be appreciated but is not required.
+//
+// 2. Altered source versions must be plainly marked as such, and must not
+//    be misrepresented as being the original software.
+//
+// 3. This notice may not be removed or altered from any source
+//    distribution.
+//
+//========================================================================
+//
+// This test is intended to verify whether the OpenGL context part of
+// the GLFW API is able to be used from multiple threads
+//
+//========================================================================
+
+#include "tinycthread.h"
+
+#include <glad/glad.h>
+#include <GLFW/glfw3.h>
+
+#include "linmath.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+static const struct
+{
+    float x, y;
+    float r, g, b;
+} vertices[3] =
+{
+    { -0.6f, -0.4f, 1.f, 0.f, 0.f },
+    {  0.6f, -0.4f, 0.f, 1.f, 0.f },
+    {   0.f,  0.6f, 0.f, 0.f, 1.f }
+};
+
+static const char* vertex_shader_text =
+"uniform mat4 MVP;\n"
+"attribute vec3 vCol;\n"
+"attribute vec2 vPos;\n"
+"varying vec3 color;\n"
+"void main()\n"
+"{\n"
+"    gl_Position = MVP * vec4(vPos, 0.0, 1.0);\n"
+"    color = vCol;\n"
+"}\n";
+
+static const char* fragment_shader_text =
+"varying vec3 color;\n"
+"void main()\n"
+"{\n"
+"    gl_FragColor = vec4(color, 1.0);\n"
+"}\n";
+
+typedef struct
+{
+    GLFWwindow* primary_window;
+    GLFWwindow* window;
+    const char* title;
+    float r, g, b, a;
+    thrd_t id;
+} Thread;
+
+static volatile int running = GLFW_TRUE;
+
+static void error_callback(int error, const char* description)
+{
+    fprintf(stderr, "Error: %s\n", description);
+}
+
+static int thread_main(void* data)
+{
+    const Thread* thread = data;
+
+    glfwMakeContextCurrent(thread->window);
+    glfwSwapInterval(1);
+
+    while (running)
+    {
+        const float v = (float) fabs(sin(glfwGetTime() * 2.f));
+        glClearColor(thread->r * v, thread->g * v, thread->b * v, thread->a * v);
+
+        glClear(GL_COLOR_BUFFER_BIT);
+        glfwSwapBuffers(thread->window);
+    }
+
+    glfwMakeContextCurrent(NULL);
+    return 0;
+}
+
+static int other_thread_main(void* data)
+{
+    const Thread* thread = data;
+
+    GLuint vertex_buffer, vertex_shader, fragment_shader, program;
+    GLint mvp_location, vpos_location, vcol_location;
+
+    glfwMakeContextCurrent(thread->window);
+    gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
+    glfwSwapInterval(1);
+
+    // NOTE: OpenGL error checks have been omitted for brevity
+
+    glGenBuffers(1, &vertex_buffer);
+    glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer);
+    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
+
+    vertex_shader = glCreateShader(GL_VERTEX_SHADER);
+    glShaderSource(vertex_shader, 1, &vertex_shader_text, NULL);
+    glCompileShader(vertex_shader);
+
+    fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
+    glShaderSource(fragment_shader, 1, &fragment_shader_text, NULL);
+    glCompileShader(fragment_shader);
+
+    program = glCreateProgram();
+    glAttachShader(program, vertex_shader);
+    glAttachShader(program, fragment_shader);
+    glLinkProgram(program);
+
+    mvp_location = glGetUniformLocation(program, "MVP");
+    vpos_location = glGetAttribLocation(program, "vPos");
+    vcol_location = glGetAttribLocation(program, "vCol");
+
+    glEnableVertexAttribArray(vpos_location);
+    glVertexAttribPointer(vpos_location, 2, GL_FLOAT, GL_FALSE,
+                          sizeof(float) * 5, (void*) 0);
+    glEnableVertexAttribArray(vcol_location);
+    glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
+                          sizeof(float) * 5, (void*) (sizeof(float) * 2));
+
+    while (running)
+    {
+        int primary_x_pos, primary_y_pos;
+        int my_x_pos, my_y_pos;
+        float delta_x, delta_y;
+
+        glfwGetWindowPos(thread->primary_window, &primary_x_pos, &primary_y_pos);
+        glfwGetWindowPos(thread->window, &my_x_pos, &my_y_pos);
+
+        float ratio;
+        int width, height;
+        mat4x4 m, p, mvp;
+        const float v = (float) fabs(sin(glfwGetTime() * 2.f));
+        glfwGetFramebufferSize(thread->window, &width, &height);
+        ratio = width / (float) height;
+        glViewport(0, 0, width, height);
+
+        delta_x = (((float)(primary_x_pos - my_x_pos)) / ((float) width)) * 2;
+        delta_y = (-((float)(primary_y_pos - my_y_pos)) / ((float) height)) * 2;
+
+        glClearColor(thread->r * v, thread->g * v, thread->b * v, thread->a * v);
+        glClear(GL_COLOR_BUFFER_BIT);
+
+        mat4x4_identity(m);
+        mat4x4_rotate_Z(m, m, (float) glfwGetTime());
+        mat4x4_ortho(p, -ratio, ratio, -1.f, 1.f, 1.f, -1.f);
+        mat4x4_translate_in_place(p, delta_x, delta_y, 0.0f);
+        mat4x4_mul(mvp, p, m);
+        //mat4x4_translate_in_place(mvp, 1.0f, 0.0f, 0.0f);
+
+        glUseProgram(program);
+        glUniformMatrix4fv(mvp_location, 1, GL_FALSE, (const GLfloat*) mvp);
+        glDrawArrays(GL_TRIANGLES, 0, 3);
+
+        glfwSwapBuffers(thread->window);
+    }
+
+    glfwMakeContextCurrent(NULL);
+    return 0;
+}
+
+int main(void)
+{
+    int i, result;
+    Thread threads[] =
+    {
+        { NULL, NULL, "Red",   1.0f, 0.0f, 0.0f, 0.5f, 0 },
+        { NULL, NULL, "Green", 0.0f, 1.0f, 0.0f, 0.5f, 0 },
+        { NULL, NULL, "Blue",  0.0f, 0.0f, 1.0f, 0.5f, 0 }
+    };
+    const int count = sizeof(threads) / sizeof(Thread);
+
+    glfwSetErrorCallback(error_callback);
+
+    if (!glfwInit())
+        exit(EXIT_FAILURE);
+
+    glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);
+
+    glfwWindowHint(GLFW_ALPHA_BITS, 8);
+    glfwWindowHint(GLFW_TRANSPARENT, GLFW_TRUE);
+    glfwWindowHint(GLFW_DECORATED, GLFW_TRUE);
+
+    for (i = 0;  i < count;  i++)
+    {
+        threads[i].window = glfwCreateWindow(200, 200,
+                                             threads[i].title,
+                                             NULL, NULL);
+        if (!threads[i].window)
+        {
+            glfwTerminate();
+            exit(EXIT_FAILURE);
+        }
+
+        glfwSetWindowPos(threads[i].window, 200 + 250 * i, 200);
+        glfwShowWindow(threads[i].window);
+    }
+
+    for (i = 0; i < count; i++)
+    {
+        threads[i].primary_window = threads[0].window;
+    }
+
+    glfwMakeContextCurrent(threads[0].window);
+    gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
+    glfwMakeContextCurrent(NULL);
+
+    for (i = 0;  i < count;  i++)
+    {
+        if (i == (count - 1))
+        {
+            result = thrd_create(&threads[i].id, other_thread_main, threads + i);
+        }
+        else
+        {
+            result = thrd_create(&threads[i].id, thread_main, threads + i);
+        }
+
+        if (result != thrd_success)
+        {
+            fprintf(stderr, "Failed to create secondary thread\n");
+
+            glfwTerminate();
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    while (running)
+    {
+        glfwWaitEvents();
+
+        for (i = 0;  i < count;  i++)
+        {
+            if (glfwWindowShouldClose(threads[i].window))
+                running = GLFW_FALSE;
+        }
+    }
+
+    for (i = 0;  i < count;  i++)
+        glfwHideWindow(threads[i].window);
+
+    for (i = 0;  i < count;  i++)
+        thrd_join(threads[i].id, &result);
+
+    exit(EXIT_SUCCESS);
+}
+