Merge pull request #303 from tdaffin/fix_crop

Fix for scissor problem

Cargo.toml

@@ -31,6 +31,7 @@ image = "0.21.0"
 gl = "0.11.0"
 piston-shaders_graphics2d = "0.3.1"
 piston-texture = "0.6.0"
+piston-viewport = "0.5.0"
 shader_version = "0.3.0"
 fnv = "1.0.2"
 

src/back_end.rs

@@ -303,6 +303,8 @@ pub struct GlGraphics {
     current_program: Option<GLuint>,
     // Keeps track of the current draw state.
     current_draw_state: Option<DrawState>,
+    // Keeps track of the current viewport
+    current_viewport: Option<Viewport>,
 }
 
 impl<'a> GlGraphics {
@@ -321,6 +323,7 @@ impl<'a> GlGraphics {
             textured: Textured::new(glsl),
             current_program: None,
             current_draw_state: None,
+            current_viewport: None,
         }
     }
 
@@ -339,11 +342,12 @@ impl<'a> GlGraphics {
             textured: textured,
             current_program: None,
             current_draw_state: None,
+            current_viewport: None,
         }
     }
 
     /// Sets viewport with normalized coordinates and center as origin.
-    pub fn viewport(&mut self, x: i32, y: i32, w: i32, h: i32) {
+    fn viewport(&mut self, x: i32, y: i32, w: i32, h: i32) {
         unsafe {
             gl::Viewport(x as GLint, y as GLint, w as GLsizei, h as GLsizei);
         }
@@ -382,12 +386,12 @@ impl<'a> GlGraphics {
     pub fn use_draw_state(&mut self, draw_state: &DrawState) {
         match self.current_draw_state {
             None => {
-                draw_state::bind_scissor(draw_state.scissor);
+                draw_state::bind_scissor(draw_state.scissor, &self.current_viewport);
                 draw_state::bind_stencil(draw_state.stencil);
                 draw_state::bind_blend(draw_state.blend);
             }
             Some(ref old_state) => {
-                draw_state::bind_state(old_state, draw_state);
+                draw_state::bind_state(old_state, draw_state, &self.current_viewport);
             }
         }
         self.current_draw_state = Some(*draw_state);
@@ -405,6 +409,7 @@ impl<'a> GlGraphics {
         let rect = viewport.rect;
         let (x, y, w, h) = (rect[0], rect[1], rect[2], rect[3]);
         self.viewport(x, y, w, h);
+        self.current_viewport = Some(viewport);
         self.clear_program();
         unsafe {
             gl::Enable(gl::FRAMEBUFFER_SRGB);

src/draw_state.rs

@@ -1,9 +1,10 @@
 use gl;
 use graphics::draw_state::*;
+use viewport::Viewport;
 
-pub fn bind_state(old_state: &DrawState, new_state: &DrawState) {
+pub fn bind_state(old_state: &DrawState, new_state: &DrawState, viewport: &Option<Viewport>) {
     if old_state.scissor != new_state.scissor {
-        bind_scissor(new_state.scissor);
+        bind_scissor(new_state.scissor, viewport);
     }
     if old_state.stencil != new_state.stencil {
         bind_stencil(new_state.stencil);
@@ -13,14 +14,28 @@ pub fn bind_state(old_state: &DrawState, new_state: &DrawState) {
     }
 }
 
-pub fn bind_scissor(rect: Option<[u32; 4]>) {
+pub fn bind_scissor(rect: Option<[u32; 4]>, viewport: &Option<Viewport>) {
     match rect {
-        Some(r) => unsafe {
-            gl::Enable(gl::SCISSOR_TEST);
-            gl::Scissor(r[0] as gl::types::GLint,
-                        r[1] as gl::types::GLint,
-                        r[2] as gl::types::GLint,
-                        r[3] as gl::types::GLint);
+        Some(r) => {
+            // https://www.khronos.org/opengl/wiki/Scissor_Test indicates that 
+            // gl::Scissor takes x,y defined as lower left,
+            // but piston passes rect with x,y defined as upper left.
+            // To fix this we need to know height of the viewport
+            // so that we can transform y as top measured from top (yt)
+            // into y as bottom measured from bottom (yb)
+            // using yb = viewport_height - (yt + rect_height)
+            let yb = if let Some(vp) = viewport {
+                vp.rect[3] - (r[1] + r[3]) as i32
+            } else {
+                r[1] as i32
+            };
+            unsafe {
+                gl::Enable(gl::SCISSOR_TEST);
+                gl::Scissor(r[0] as gl::types::GLint,
+                            yb as gl::types::GLint,
+                            r[2] as gl::types::GLint,
+                            r[3] as gl::types::GLint);
+            }
         },
         None => unsafe { gl::Disable(gl::SCISSOR_TEST) },
     }

src/lib.rs

@@ -9,6 +9,7 @@ extern crate image;
 extern crate gl;
 extern crate graphics;
 extern crate texture as texture_lib;
+extern crate viewport;
 
 pub use shader_version::{OpenGL, Shaders};
 pub use shader_version::glsl::{GLSL};