Finished it...

src/CRE/renderer/transparency.cpp

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+/*
+    by korenkonder
+    GitHub/GitLab: korenkonder
+*/
+
+#include "transparency.hpp"
+#include "../../KKdLib/str_utils.hpp"
+#include "../render_context.hpp"
+#include "../gl_state.hpp"
+#include "../shader_ft.hpp"
+
+extern render_context* rctx_ptr;
+
+namespace renderer {
+    Transparency::Transparency(GLuint color_texture, GLuint depth_texture,
+        int32_t width, int32_t height) : sampler(), vao() {
+        GLuint color_textures[1];
+        color_textures[0] = color_texture;
+        fbo.init(width, height, color_textures, 1, depth_texture);
+
+        glGenSamplers(1, &sampler);
+        glSamplerParameteri(sampler, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+        glSamplerParameteri(sampler, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_NEAREST);
+        glSamplerParameteri(sampler, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+        glSamplerParameteri(sampler, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+
+        glGenVertexArrays(1, &vao);
+    }
+
+    Transparency::~Transparency() {
+        if (vao) {
+            glDeleteVertexArrays(1, &vao);
+            vao = 0;
+        }
+
+        if (sampler) {
+            glDeleteSamplers(1, &sampler);
+            sampler = 0;
+        }
+    }
+
+    void Transparency::combine(RenderTexture* rt, float_t alpha) {
+        transparency_batch_shader_data shader_data = {};
+        shader_data.g_opacity = { alpha, 0.0f, 0.0f, 0.0f };
+        rctx_ptr->transparency_batch_ubo.WriteMemory(shader_data);
+
+        gl_state_disable_blend();
+        gl_state_disable_depth_test();
+        rctx_ptr->render_buffer.Bind();
+        shaders_ft.set(SHADER_FT_TRANSPARENCY);
+        rctx_ptr->transparency_batch_ubo.Bind(0);
+        gl_state_active_bind_texture_2d(0, fbo.textures[0]);
+        gl_state_active_bind_texture_2d(1, rt->GetColorTex());
+        gl_state_bind_vertex_array(vao);
+        shaders_ft.draw_arrays(GL_TRIANGLE_STRIP, 0, 4);
+        gl_state_enable_depth_test();
+
+        glCopyImageSubData(rctx_ptr->render_buffer.GetColorTex(),
+            GL_TEXTURE_2D, 0, 0, 0, 0, rt->GetColorTex(),
+            GL_TEXTURE_2D, 0, 0, 0, 0, fbo.width, fbo.height, 1);
+    }
+
+    void Transparency::copy(GLuint texture) {
+        glCopyImageSubData(texture,
+            GL_TEXTURE_2D, 0, 0, 0, 0, fbo.textures[0],
+            GL_TEXTURE_2D, 0, 0, 0, 0, fbo.width, fbo.height, 1);
+        gl_state_bind_framebuffer(fbo.buffer);
+        glViewport(0, 0, fbo.width, fbo.height);
+    }
+
+    void Transparency::resize(GLuint color_texture, GLuint depth_texture, int32_t width, int32_t height) {
+        GLuint color_textures[1];
+        color_textures[0] = color_texture;
+        fbo.init(width, height, color_textures, 1, depth_texture);
+    }
+}