graphics/gl: Add Features.resolve_attachments

Just as instancing is only available on GL3, resolve attachments are only available on WebGL2 and gl3

examples/msaa_render_texture.rs

@@ -37,7 +37,13 @@ impl Stage {
             sample_count: 4,
             ..Default::default()
         });
-
+        // Without this check, new_render_pass_mrt might panic on GL2/WebGl1.
+        // It is recommended to handle it and create a normal,
+        // non-msaa render pass instead.
+        assert!(
+            ctx.info().features.resolve_attachments,
+            "MSAA render targets are not supported on current rendering backend!"
+        );
         let offscreen_pass =
             ctx.new_render_pass_mrt(&[color_img], Some(&[color_resolve_img]), Some(depth_img));
 
@@ -235,7 +241,7 @@ impl EventHandler for Stage {
         self.ctx.apply_pipeline(&self.display_pipeline);
         self.ctx.apply_bindings(&self.display_bind);
         self.ctx.apply_uniforms(UniformsSource::table(&vs_params));
-        self.ctx.draw(0, 36, 1);
+        self.ctx.draw(0, 6, 1);
         self.ctx.end_render_pass();
 
         self.ctx.commit_frame();
@@ -245,6 +251,7 @@ impl EventHandler for Stage {
 fn main() {
     let mut conf = conf::Conf::default();
     let metal = std::env::args().nth(1).as_deref() == Some("metal");
+    conf.platform.webgl_version = conf::WebGLVersion::WebGL2;
     conf.platform.apple_gfx_api = if metal {
         conf::AppleGfxApi::Metal
     } else {

examples/quad.rs

@@ -123,7 +123,6 @@ fn main() {
     } else {
         conf::AppleGfxApi::OpenGl
     };
-    conf.platform.webgl_version = conf::WebGLVersion::WebGL2;
 
     miniquad::start(conf, move || Box::new(Stage::new()));
 }

src/graphics.rs

@@ -392,7 +392,7 @@ pub struct TextureParams {
     /// On OpenGL, for a `sample_count > 1` render texture, render buffer object will
     /// be created instead of a regulat texture.
     ///
-    /// The only way to use 
+    /// The only way to use
     pub sample_count: i32,
 }
 
@@ -569,11 +569,21 @@ pub const MAX_SHADERSTAGE_IMAGES: usize = 12;
 #[derive(Clone, Debug)]
 pub struct Features {
     pub instancing: bool,
+    /// Does current rendering backend support automatic resolve of
+    /// multisampled render passes on end_render_pass.
+    /// Would be false on WebGl1 and GL2.
+    ///
+    /// With resolve_attachments: false, not-none resolve_img in new_render_pass will
+    /// result in a runtime panic.
+    pub resolve_attachments: bool,
 }
 
 impl Default for Features {
     fn default() -> Features {
-        Features { instancing: true }
+        Features {
+            instancing: true,
+            resolve_attachments: true,
+        }
     }
 }
 
@@ -1203,7 +1213,10 @@ pub trait RenderingBackend {
     /// Same as "new_render_pass", but allows multiple color attachments.
     /// if `resolve_img` is set, MSAA-resolve operation will happen in `end_render_pass`
     /// this operation require `color_img` to have sample_count > 1,resolve_img have
-    /// sample_count == 1, and color_img.len() should be equal to resolve_img.len() 
+    /// sample_count == 1, and color_img.len() should be equal to resolve_img.len()
+    ///
+    /// Note that resolve attachments may be not supported by current backend!
+    /// They are only available when `ctx.info().features.resolve_attachments` is true.
     fn new_render_pass_mrt(
         &mut self,
         color_img: &[TextureId],

src/graphics/gl.rs

@@ -501,11 +501,7 @@ impl GlContext {
 
             glGenVertexArrays(1, &mut vao as *mut _);
             glBindVertexArray(vao);
-            let features = Features {
-                instancing: !crate::native::gl::is_gl2(),
-                ..Default::default()
-            };
-            let info = gl_info(features);
+            let info = gl_info();
             GlContext {
                 default_framebuffer,
                 shaders: ResourceManager::default(),
@@ -782,14 +778,25 @@ impl GlContext {
     }
 }
 
-fn gl_info(features: Features) -> ContextInfo {
+fn gl_info() -> ContextInfo {
     let version_string = unsafe { glGetString(super::gl::GL_VERSION) };
     let gl_version_string = unsafe { std::ffi::CStr::from_ptr(version_string as _) }
         .to_str()
         .unwrap()
         .to_string();
     //let gles2 = !gles3 && gl_version_string.contains("OpenGL ES");
 
+    let gl2 = gl_version_string.is_empty()
+        || gl_version_string.starts_with("2")
+        || gl_version_string.starts_with("OpenGL ES 2");
+    let webgl1 = gl_version_string == "WebGL 1.0";
+
+    let features = Features {
+        instancing: !gl2,
+        resolve_attachments: !webgl1 && !gl2,
+        ..Default::default()
+    };
+
     let mut glsl_support = GlslSupport::default();
 
     // this is not quite documented,