From 9787175b1366d643e6d748b8ac2370c3889f73de Mon Sep 17 00:00:00 2001
From: omigamedev <omarator@gmail.com>
Date: Wed, 9 Jan 2019 23:32:10 +0100
Subject: [PATCH] Implement raytraced lightmap

---
 data/layout.xml         |   3 +-
 src/app.cpp             |   5 +-
 src/app_shaders.cpp     |  60 +++++++++++++++++++
 src/node_panel_grid.cpp | 129 +++++++++++++++++++++++++++++++++++++---
 src/node_panel_grid.h   |   9 ++-
 src/rtt.cpp             |  33 ++++++++--
 src/rtt.h               |   4 +-
 src/shader.h            |   3 +
 src/shape.cpp           |  14 ++---
 src/shape.h             |   2 +
 src/texture.cpp         |   2 +-
 src/texture.h           |   4 +-
 12 files changed, 240 insertions(+), 28 deletions(-)

diff --git a/data/layout.xml b/data/layout.xml
index 1a5c89a..36a8908 100644
--- a/data/layout.xml
+++ b/data/layout.xml
@@ -292,7 +292,7 @@
               </node>
             </node>
             <node height="30" pad="1" width="100%" dir="row" margin="5 0 0 0">
-              <combobox id="grid-heightmap-shading" width="100%" height="30" combo-list="Transparent,Flat,Solid" default="0"/>
+              <combobox id="grid-heightmap-shading" width="100%" height="30" combo-list="Transparent,Flat,Solid,Textured" default="0"/>
             </node>
             <node height="20" pad="1" width="100%" margin="5 0 0 0"><slider-h id="grid-heightmap-wireframe" value="1.0"/></node>
             <node height="20" pad="1" width="100%"><slider-h id="grid-heightmap-height" value="0.25"/></node>
@@ -301,6 +301,7 @@
           </node>
         </node>
         <button id="grid-render" text="Render" height="30" margin="5 0 0 0"/>
+        <button id="grid-commit" text="Commit" height="30" margin="5 0 0 0"/>
       
     </border>
     </node>
diff --git a/src/app.cpp b/src/app.cpp
index 96537c6..a740bcb 100644
--- a/src/app.cpp
+++ b/src/app.cpp
@@ -328,7 +328,10 @@ void App::init()
             LOG("OPENGL: %.*s", length, message);
             FlushConsoleInputBuffer(GetStdHandle(STD_OUTPUT_HANDLE));
             SetConsoleTextAttribute(GetStdHandle(STD_OUTPUT_HANDLE), info.wAttributes);
-             //__debugbreak();
+#ifdef _WIN32
+            if (severity == GL_DEBUG_SEVERITY_HIGH)
+                __debugbreak();
+#endif
         }
     }, nullptr);
     glEnable(GL_DEBUG_OUTPUT);
diff --git a/src/app_shaders.cpp b/src/app_shaders.cpp
index 1531edf..3986a41 100644
--- a/src/app_shaders.cpp
+++ b/src/app_shaders.cpp
@@ -509,6 +509,7 @@ void App::initShaders()
         "uniform mat4 mvp;\n"
         "in vec4 pos;\n"
         "in vec3 nor;\n"
+        "in vec2 uvs;\n"
         "out vec3 n;\n"
         "void main() {\n"
         "   n = nor;\n"
@@ -522,6 +523,61 @@ void App::initShaders()
         "void main() {\n"
         "   mediump float d = max(0.0, dot(normalize(n), light_dir));\n"
         "   frag = vec4(vec3(d), 1.0);\n"
+        //"   frag = vec4(normalize(n) * 0.5 + 0.5, 1.0);\n"
+        "}\n";
+
+    // LAMBERT LIGHTMAP
+    static const char* shader_lambert_lightmap_v =
+        SHADER_VERSION
+        "uniform mat4 mvp;\n"
+        "in vec4 pos;\n"
+        "in vec3 nor;\n"
+        "in vec2 uvs;\n"
+        "out vec3 n;\n"
+        "out vec2 uv;\n"
+        "void main() {\n"
+        "   n = nor;\n"
+        "   uv = uvs;\n"
+        "   gl_Position = mvp * pos;\n"
+        "}\n";
+    static const char* shader_lambert_lightmap_f =
+        SHADER_VERSION
+        "uniform mediump sampler2D tex;\n"
+        "uniform mediump vec3 light_dir;\n"
+        "in mediump vec3 n;\n"
+        "in mediump vec2 uv;\n"
+        "out mediump vec4 frag;\n"
+        "void main() {\n"
+        "   mediump float d = max(0.0, dot(normalize(n), light_dir));\n"
+        "   mediump vec4 c = texture(tex, uv);\n"
+        "   frag = vec4(c.rgb * d, 1.0);\n"
+        "}\n";
+
+    // BAKE UVS
+    static const char* shader_bakeuv_v =
+        SHADER_VERSION
+        "uniform mat4 mvp;\n"
+        "in vec4 pos;\n"
+        "in vec3 nor;\n"
+        "in vec2 uvs;\n"
+        "out vec3 n;\n"
+        "out vec3 p;\n"
+        "void main() {\n"
+        "   n = nor;\n"
+        "   p = vec3(mvp * pos);\n"
+        "   gl_Position = vec4(uvs * 2.0 - 1.0, 0.0, 1.0);\n"
+        "}\n";
+    static const char* shader_bakeuv_f =
+        SHADER_VERSION
+        "uniform int mode;\n"
+        "in highp vec3 n;\n"
+        "in highp vec3 p;\n"
+        "out highp vec3 frag;\n"
+        "void main() {\n"
+        "   switch(mode) {\n"
+        "   case 0: frag = normalize(n); break;\n"
+        "   case 1: frag = p; break;\n"
+        "   }\n"
         "}\n";
 
 
@@ -566,6 +622,10 @@ void App::initShaders()
         LOG("Failed to create shader VertexColor");
     if (!ShaderManager::create(kShader::Lambert, shader_lambert_v, shader_lambert_f))
         LOG("Failed to create shader Lambert");
+    if (!ShaderManager::create(kShader::LambertLightmap, shader_lambert_lightmap_v, shader_lambert_lightmap_f))
+        LOG("Failed to create shader LambertLightmap");
+    if (!ShaderManager::create(kShader::BakeUV, shader_bakeuv_v, shader_bakeuv_f))
+        LOG("Failed to create shader BakeUV");
     LOG("shaders initialized");
 }
 
diff --git a/src/node_panel_grid.cpp b/src/node_panel_grid.cpp
index a4881bb..dca18c9 100644
--- a/src/node_panel_grid.cpp
+++ b/src/node_panel_grid.cpp
@@ -39,6 +39,7 @@ void NodePanelGrid::init_controls()
     m_hm_lpitch = find<NodeSliderH>("grid-heightmap-lpitch");
     m_hm_shading = find<NodeComboBox>("grid-heightmap-shading");
     m_render = find<NodeButton>("grid-render");
+    m_commit = find<NodeButton>("grid-commit");
 
     m_hm_preview->SetHeight(0);
     m_hm_plane.create(1, 1, 100);
@@ -49,12 +50,14 @@ void NodePanelGrid::init_controls()
             m_hm_plane.create(1, 1, m_hm_image, v * 5.f, get_height());
         else
             m_hm_plane.create(1, 1, 100 * v * 5.f);
+        m_rt_dirty = true;
         LOG("resolution value %f", v);
     };
 
     m_hm_height->on_value_final = [this](Node* target, float v) {
         if (m_hm_image.data())
             m_hm_plane.create(1, 1, m_hm_image, m_groud_resolution->get_value() * 5.f, get_height());
+        m_rt_dirty = true;
         LOG("height value %f", v);
     };
 
@@ -79,6 +82,7 @@ void NodePanelGrid::init_controls()
                 m_hm_preview->SetHeight(100);
                 if (m_groud_opacity->get_value() == 0.f)
                     m_groud_opacity->set_value(1.f);
+                m_rt_dirty = true;
             }
             async_update();
             async_end();
@@ -106,18 +110,31 @@ void NodePanelGrid::init_controls()
             m_hm_plane.create(1, 1, m_hm_image, 
                 m_groud_resolution->get_value() * 5.f, get_height());
             m_hm_preview->SetHeight(100);
+            m_rt_dirty = true;
         }
     };
 
     m_render->on_click = [this](Node*)
+    {
+        gl_state gl;
+        gl.save();
+        bake_uvs();
+        m_hm_shading->set_index(3);
+        m_shade_mode = ShadeMode::Textured;
+        gl.restore();
+    };
+    m_commit->on_click = [this](Node*)
     {
         gl_state gl;
         gl.save();
         Canvas::I->draw_objects([this](const glm::mat4& camera, const glm::mat4& proj, int i) {
             draw_heightmap(proj, camera);
         });
+        m_groud_opacity->set_value(0);
         gl.restore();
     };
+    m_texture.create(1024, 1024);
+    m_sampler_linear.create();
 }
 
 float NodePanelGrid::get_height() const
@@ -134,27 +151,26 @@ void NodePanelGrid::draw_heightmap(const glm::mat4& proj, const glm::mat4& camer
 {
     if (m_groud_opacity->get_value() > 0.f)
     {
-        glEnable(GL_BLEND);
         glEnable(GL_DEPTH_TEST);
         glClear(GL_DEPTH_BUFFER_BIT);
 
         auto mvp = proj * camera
-            * glm::translate(glm::vec3(0, get_offset(), 0))
-            * glm::scale(glm::vec3(1, get_height(), 1))
-            * glm::eulerAngleX(glm::radians(90.f));
+            * glm::translate(glm::vec3(0, get_offset(), 0));
 
         // DRAW SOLID
         if (m_hm_image.m_data)
         {
+            auto light_yaw = m_hm_lyaw->get_value() * glm::pi<float>() * 2.f;
+            auto light_pitch = m_hm_lpitch->get_value() * 5;
+            auto light_pos = glm::vec3(sinf(light_yaw), light_pitch, cosf(light_yaw));
+            auto light_dir = glm::normalize(light_pos);
+
             if (m_shade_mode == ShadeMode::Solid)
             {
                 glDisable(GL_BLEND);
                 ShaderManager::use(kShader::Lambert);
                 ShaderManager::u_mat4(kShaderUniform::MVP, mvp);
-                auto light_yaw = m_hm_lyaw->get_value() * glm::pi<float>() * 2.f;
-                auto light_pitch = m_hm_lpitch->get_value();
-                auto light_pos = glm::vec3(sinf(light_yaw), cosf(light_yaw), light_pitch);
-                ShaderManager::u_vec3(kShaderUniform::LightDir, glm::normalize(-light_pos));
+                ShaderManager::u_vec3(kShaderUniform::LightDir, light_dir);
                 m_hm_plane.draw_fill();
             }
             else if (m_shade_mode == ShadeMode::Flat)
@@ -167,6 +183,17 @@ void NodePanelGrid::draw_heightmap(const glm::mat4& proj, const glm::mat4& camer
                 ShaderManager::u_mat4(kShaderUniform::MVP, mvp);
                 m_hm_plane.draw_fill();
             }
+            else if(m_shade_mode == ShadeMode::Textured)
+            {
+                ShaderManager::use(kShader::LambertLightmap);
+                ShaderManager::u_mat4(kShaderUniform::MVP, mvp);
+                ShaderManager::u_vec3(kShaderUniform::LightDir, light_dir);
+                ShaderManager::u_int(kShaderUniform::Tex, 0);
+                m_sampler_linear.bind(0);
+                glActiveTexture(GL_TEXTURE0);
+                m_texture.bind();
+                m_hm_plane.draw_fill();
+            }
         }
 
         // DRAW GRIDS
@@ -191,3 +218,89 @@ void NodePanelGrid::draw_heightmap(const glm::mat4& proj, const glm::mat4& camer
         }
     }
 }
+
+void NodePanelGrid::bake_uvs()
+{
+    if (!m_hm_image.m_data)
+        return;
+
+    RTT fb;
+    fb.create(m_texture.size().x, m_texture.size().y, -1, GL_RGBA32F);
+    fb.bindFramebuffer();
+    fb.clear({ 1, 0, 0, 1 });
+    glDisable(GL_BLEND);
+    glDisable(GL_DEPTH_TEST);
+    glViewport(0, 0, fb.getWidth(), fb.getHeight());
+    ShaderManager::use(kShader::BakeUV);
+    ShaderManager::u_mat4(kShaderUniform::MVP, glm::mat4(1));
+    
+    // bake normal
+    ShaderManager::u_int(kShaderUniform::Mode, 0);
+    m_hm_plane.draw_fill();
+    std::unique_ptr<float[]> data_nor(fb.readTextureDataFloat());
+    //stbi_write_jpg("bake-nor.jpg", fb.getWidth(), fb.getHeight(), 4, fb.readTextureData(), 75);
+    
+    // bake position
+    ShaderManager::u_int(kShaderUniform::Mode, 1);
+    m_hm_plane.draw_fill();
+    std::unique_ptr<float[]> data_pos(fb.readTextureDataFloat());
+    //stbi_write_jpg("bake-pos.jpg", fb.getWidth(), fb.getHeight(), 4, fb.readTextureData(), 75);
+
+    fb.unbindFramebuffer();
+    fb.destroy();
+
+    if (m_rt_dirty)
+    {
+        nanort::BVHBuildOptions<float> build_options;  // Use default option
+        build_options.cache_bbox = false;
+        nanort::TriangleMesh<float> triangle_mesh(reinterpret_cast<float*>(m_hm_plane.vertices.data()), m_hm_plane.idx.data(), sizeof(vertex_t));
+        nanort::TriangleSAHPred<float> triangle_pred(reinterpret_cast<float*>(m_hm_plane.vertices.data()), m_hm_plane.idx.data(), sizeof(vertex_t));
+        bool ret = m_rt_accel.Build(static_cast<unsigned int>(m_hm_plane.idx.size() / 3), triangle_mesh, triangle_pred, build_options);
+        if (!ret)
+            return;
+        m_rt_dirty = false;
+    }
+    
+    auto light_yaw = m_hm_lyaw->get_value() * glm::pi<float>() * 2.f;
+    auto light_pitch = m_hm_lpitch->get_value() * 5;
+    auto light_pos = glm::vec3(sinf(light_yaw), light_pitch, cosf(light_yaw));
+    auto light_dir = glm::normalize(light_pos);
+
+    auto data_out = std::make_unique<uint8_t[]>(fb.getWidth() * fb.getHeight() * 4);
+    for (int y = 0; y < fb.getHeight(); y++)
+    {
+        for (int x = 0; x < fb.getWidth(); x++)
+        {
+            int i = y * fb.getHeight() + x;
+            auto nor = glm::make_vec4(&data_nor[i * 4]);
+            auto pos = glm::make_vec4(&data_pos[i * 4]);
+            auto& out = *reinterpret_cast<glm::i8vec4*>(&data_out[i * 4]);
+
+            nanort::Ray<float> ray;
+            ray.org[0] = pos.x;// + nor.x * 0.005;
+            ray.org[1] = pos.y;// + nor.y * 0.005;
+            ray.org[2] = pos.z;// + nor.z * 0.005;
+            ray.dir[0] = light_dir.x;
+            ray.dir[1] = light_dir.y;
+            ray.dir[2] = light_dir.z;
+
+            float kFar = 1.0e+30f;
+            ray.min_t = 0.001f;
+            ray.max_t = kFar;
+
+            nanort::TriangleIntersector<> triangle_intersector(reinterpret_cast<float*>(m_hm_plane.vertices.data()), m_hm_plane.idx.data(), sizeof(vertex_t));
+            nanort::TriangleIntersection<> isect;
+            bool hit = m_rt_accel.Traverse(ray, triangle_intersector, &isect);
+            if (hit) 
+            {
+                out = { 50, 50, 50, 255 };
+            }
+            else
+            {
+                out = { 255, 255, 255, 255 };
+            }
+        }
+    }
+    //stbi_write_jpg("bake-out.jpg", fb.getWidth(), fb.getHeight(), 4, data_out.get(), 75);
+    m_texture.update(data_out.get());
+}
diff --git a/src/node_panel_grid.h b/src/node_panel_grid.h
index aae4993..50c966b 100644
--- a/src/node_panel_grid.h
+++ b/src/node_panel_grid.h
@@ -9,11 +9,12 @@
 #include "node_button.h"
 #include "shape.h"
 #include "image.h"
+#include "nanort.h"
 
 class NodePanelGrid : public Node
 {
 public:
-    enum class ShadeMode : uint8_t { Transparent, Flat, Solid };
+    enum class ShadeMode : uint8_t { Transparent, Flat, Solid, Textured };
 
     NodeSliderH* m_groud_opacity;
     NodeSliderH* m_groud_value;
@@ -29,9 +30,14 @@ public:
     NodeSliderH* m_hm_lyaw;
     NodeSliderH* m_hm_lpitch;
     NodeButton* m_render;
+    NodeButton* m_commit;
     HeightmapPlane m_hm_plane;
     Image m_hm_image;
+    Texture2D m_texture;
+    Sampler m_sampler_linear;
     std::string m_file_path;
+    nanort::BVHAccel<float> m_rt_accel;
+    bool m_rt_dirty = true;;
     ShadeMode m_shade_mode{ ShadeMode::Transparent };
 
     virtual Node* clone_instantiate() const override;
@@ -41,4 +47,5 @@ public:
     float get_height() const;
     float get_offset() const;
     void draw_heightmap(const glm::mat4& proj, const glm::mat4& camera) const;
+    void bake_uvs();
 };
diff --git a/src/rtt.cpp b/src/rtt.cpp
index e32b619..2f50a72 100644
--- a/src/rtt.cpp
+++ b/src/rtt.cpp
@@ -167,6 +167,8 @@ void RTT::bindFramebuffer()
 
 void RTT::unbindFramebuffer()
 {
+    if (!bound)
+        return;
     glBindFramebuffer(GL_DRAW_FRAMEBUFFER, oldDFboID);
     glBindFramebuffer(GL_READ_FRAMEBUFFER, oldRFboID);
     oldRFboID = 0;
@@ -180,14 +182,28 @@ void RTT::clear(glm::vec4 color)
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 }
 
-void RTT::readTextureData(uint8_t* buffer)
+uint8_t* RTT::readTextureData(uint8_t* buffer)
 {
-    glReadBuffer(GL_BACK);
-    bindFramebuffer();
-    //glGetTexImage(GL_TEXTURE_2D, 0, GL_RGB, GL_UNSIGNED_BYTE, buffer);
+    if (!buffer)
+        buffer = createBuffer();
+    GLint old;
+    glGetIntegerv(GL_READ_FRAMEBUFFER_BINDING, &old);
+    glBindFramebuffer(GL_READ_FRAMEBUFFER, fboID);
     glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
-    unbindFramebuffer();
-    //glReadBuffer(GL_NONE);
+    glBindFramebuffer(GL_READ_FRAMEBUFFER, old);
+    return buffer;
+}
+
+float* RTT::readTextureDataFloat(float* buffer)
+{
+    if (!buffer)
+        buffer = createBufferFloat();
+    GLint old;
+    glGetIntegerv(GL_READ_FRAMEBUFFER_BINDING, &old);
+    glBindFramebuffer(GL_READ_FRAMEBUFFER, fboID);
+    glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, buffer);
+    glBindFramebuffer(GL_READ_FRAMEBUFFER, old);
+    return buffer;
 }
 
 uint8_t* RTT::createBuffer()
@@ -195,6 +211,11 @@ uint8_t* RTT::createBuffer()
     return new uint8_t[w * h * 4];
 }
 
+float * RTT::createBufferFloat()
+{
+    return new float[w * h * 4];
+}
+
 void RTT::bindTexture()
 {
     glBindTexture(GL_TEXTURE_2D, texID);
diff --git a/src/rtt.h b/src/rtt.h
index 925481e..77f923c 100644
--- a/src/rtt.h
+++ b/src/rtt.h
@@ -21,8 +21,10 @@ public:
     bool create(int width, int height, int tex = -1, GLint internal_format = GL_RGBA8);
     bool recreate() { return create(w, h); }
     void clear(glm::vec4 color = glm::vec4(0));
-    void readTextureData(uint8_t* buffer);
+    uint8_t* readTextureData(uint8_t* buffer = nullptr);
+    float* readTextureDataFloat(float* buffer = nullptr);
     uint8_t* createBuffer();
+    float* createBufferFloat();
     void bindFramebuffer();
     void unbindFramebuffer();
     void bindTexture();
diff --git a/src/shader.h b/src/shader.h
index bf9bc88..5a664fd 100644
--- a/src/shader.h
+++ b/src/shader.h
@@ -31,6 +31,7 @@ enum class kShaderUniform : uint16_t
     Direction = const_hash("dir"),
     UseFragCoordUV2 = const_hash("fragUV2"),
     LightDir = const_hash("light_dir"),
+    Mode = const_hash("mode"),
 };
 
 enum class kShader : uint16_t
@@ -56,6 +57,8 @@ enum class kShader : uint16_t
     BrushStroke = const_hash("brush-stroke"),
     VertexColor = const_hash("vertex-color"),
     Lambert     = const_hash("lambert"),
+    LambertLightmap = const_hash("lambert-lightmap"),
+    BakeUV      = const_hash("bakeuv"),
 };
 
 class Shader
diff --git a/src/shape.cpp b/src/shape.cpp
index aadbc8c..8c6584f 100644
--- a/src/shape.cpp
+++ b/src/shape.cpp
@@ -255,8 +255,8 @@ bool HeightmapPlane::create(float w, float h, const Image& img, float scale, flo
     int div = new_size.x - 1; // TODO: handle height also
     int idx_size = (div * div * 6) + (div * (div + 1) * 4);
     int vertices_size = (div + 1)*(div + 1);
-    std::vector<GLuint> idx(idx_size);
-    std::vector<vertex_t> vertices(vertices_size);
+    idx.resize(idx_size);
+    vertices.resize(vertices_size);
     std::vector<int> nor_count((size_t)vertices_size, 0);
 
     count[0] = div * div * 6;
@@ -277,8 +277,8 @@ bool HeightmapPlane::create(float w, float h, const Image& img, float scale, flo
         {
             vertex_t v;
             v.pos.x = ox + dx * (float)x;
-            v.pos.y = oy + dy * (float)y;
-            v.pos.z = (*px++).r / 255.f;
+            v.pos.y = (*px++).r / 255.f * height;
+            v.pos.z = oy + dy * (float)y;
             v.pos.w = 1;
             v.uvs2 = v.uvs = glm::vec2(x, y) / (float)div;
             *pv++ = v;
@@ -286,7 +286,7 @@ bool HeightmapPlane::create(float w, float h, const Image& img, float scale, flo
     }
 
     // generate indices
-    const glm::vec3 yscale(1, 1, height);
+    const glm::vec3 yscale(1, height, 1);
     for (int y = 0; y < div; y++)
     {
         int i = y * (div + 1);
@@ -363,8 +363,8 @@ bool HeightmapPlane::create(float w, float h, int div)
         {
             vertex_t v;
             v.pos.x = ox + dx * (float)x;
-            v.pos.y = oy + dy * (float)y;
-            v.pos.z = 0.f;
+            v.pos.y = 0.f;
+            v.pos.z = oy + dy * (float)y;
             v.pos.w = 1.f;
             v.uvs2 = v.uvs = glm::vec2(x, y) / (float)div;
             v.nor = { 0, 1, 0 };
diff --git a/src/shape.h b/src/shape.h
index fe9a65f..8ff8c0f 100644
--- a/src/shape.h
+++ b/src/shape.h
@@ -125,6 +125,8 @@ public:
 class HeightmapPlane : public Shape
 {
 public:
+    std::vector<GLuint> idx;
+    std::vector<vertex_t> vertices;
     bool create(float w, float h, const Image& img, float scale, float height);
     bool create(float w, float h, int div);
 };
diff --git a/src/texture.cpp b/src/texture.cpp
index a3b588b..a93de34 100644
--- a/src/texture.cpp
+++ b/src/texture.cpp
@@ -134,7 +134,7 @@ void Sampler::set_filter(GLint filter_min, GLint filter_mag)
     glSamplerParameteri(id, GL_TEXTURE_MAG_FILTER, filter_mag);
 #endif // USE_SAMPLER
 }
-void Sampler::bind(int unit)
+void Sampler::bind(int unit) const
 {
     current_unit = unit;
 #if USE_SAMPLER
diff --git a/src/texture.h b/src/texture.h
index 95004e0..bae25b1 100644
--- a/src/texture.h
+++ b/src/texture.h
@@ -27,12 +27,12 @@ public:
 class Sampler
 {
     GLuint id = 0;
-    GLint current_unit = 0;
+    mutable GLint current_unit = 0;
 public:
     bool create(GLint filter = GL_LINEAR, GLint wrap = GL_CLAMP_TO_EDGE);
     void set(GLint filter = GL_LINEAR, GLint wrap = GL_CLAMP_TO_EDGE);
     void set_filter(GLint filter_min, GLint filter_mag);
-    void bind(int unit);
+    void bind(int unit) const;
     void unbind();
     bool ready() const { return id != 0; }
 };