[glfw] add "C" button to show stencil clip masks

include/mbgl/platform/default/glfw_view.hpp

@@ -52,6 +52,9 @@ private:
     makeSpriteImage(int width, int height, float pixelRatio);
 
     void nextOrientation();
+    void toggleClipMasks();
+
+    void renderClipMasks();
 
     void addRandomPointAnnotations(int count);
     void addRandomShapeAnnotations(int count);
@@ -81,6 +84,8 @@ private:
     int fbHeight;
     float pixelRatio;
 
+    bool showClipMasks = false;
+
     double lastX = 0, lastY = 0;
 
     double lastClick = -1;

platform/default/glfw_view.cpp

@@ -6,6 +6,7 @@
 #include <mbgl/gl/gl_values.hpp>
 #include <mbgl/gl/gl_helper.hpp>
 #include <mbgl/platform/log.hpp>
+#include <mbgl/platform/platform.hpp>
 #include <mbgl/util/string.hpp>
 #include <mbgl/util/chrono.hpp>
 
@@ -136,6 +137,9 @@ void GLFWView::onKey(GLFWwindow *window, int key, int /*scancode*/, int action,
             if (!mods)
                 view->map->resetPosition();
             break;
+        case GLFW_KEY_C:
+            view->toggleClipMasks();
+            break;
         case GLFW_KEY_S:
             if (view->changeStyleCallback)
                 view->changeStyleCallback();
@@ -232,6 +236,11 @@ void GLFWView::nextOrientation() {
     }
 }
 
+void GLFWView::toggleClipMasks() {
+    showClipMasks = !showClipMasks;
+    map->update(mbgl::Update::Repaint);
+}
+
 void GLFWView::addRandomCustomPointAnnotations(int count) {
     std::vector<mbgl::PointAnnotation> points;
 
@@ -439,9 +448,52 @@ void GLFWView::beforeRender() {
 }
 
 void GLFWView::afterRender() {
+    if (showClipMasks) {
+        renderClipMasks();
+    }
+
     glfwSwapBuffers(window);
 }
 
+void GLFWView::renderClipMasks() {
+    // Read the stencil buffer
+    auto pixels = std::make_unique<uint8_t[]>(fbWidth * fbHeight);
+    glReadPixels(0,                // GLint x
+                 0,                // GLint y
+                 fbWidth,          // GLsizei width
+                 fbHeight,         // GLsizei height
+                 GL_STENCIL_INDEX, // GLenum format
+                 GL_UNSIGNED_BYTE, // GLenum type
+                 pixels.get()      // GLvoid * data
+                 );
+
+    // Scale the Stencil buffer to cover the entire color space.
+    auto it = pixels.get();
+    auto end = it + fbWidth * fbHeight;
+    const auto factor = 255.0f / *std::max_element(it, end);
+    for (; it != end; ++it) {
+        *it *= factor;
+    }
+
+    using namespace mbgl::gl;
+    Preserve<PixelZoom> pixelZoom;
+    Preserve<RasterPos> rasterPos;
+    Preserve<StencilTest> stencilTest;
+    Preserve<DepthTest> depthTest;
+    Preserve<Program> program;
+    Preserve<ColorMask> colorMask;
+
+    MBGL_CHECK_ERROR(glPixelZoom(1.0f, 1.0f));
+    MBGL_CHECK_ERROR(glRasterPos2f(-1.0f, -1.0f));
+    MBGL_CHECK_ERROR(glDisable(GL_STENCIL_TEST));
+    MBGL_CHECK_ERROR(glDisable(GL_DEPTH_TEST));
+    MBGL_CHECK_ERROR(glUseProgram(0));
+    MBGL_CHECK_ERROR(glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE));
+    MBGL_CHECK_ERROR(glWindowPos2i(0, 0));
+
+    MBGL_CHECK_ERROR(glDrawPixels(fbWidth, fbHeight, GL_LUMINANCE, GL_UNSIGNED_BYTE, pixels.get()));
+}
+
 void GLFWView::report(float duration) {
     frames++;
     frameTime += duration;