Switch to translating the quad using a translation matrix

shaders/frag.glsl

@@ -3,8 +3,6 @@
 in vec3 vert_colours;
 out vec4 color;
 
-uniform float u_offset;
-
 void main() {
-  color = vec4(vert_colours.r, vert_colours.g + u_offset, vert_colours.b, 1.0f);
+  color = vec4(vert_colours.r, vert_colours.g, vert_colours.b, 1.0f);
 };

shaders/vert.glsl

@@ -3,11 +3,11 @@
 layout(location=0) in vec3 position;
 layout(location=1) in vec3 vert_colour;
 
-uniform float u_offset;
+uniform mat4 u_model;
 
 out vec3 vert_colours;
 
 void main() {
   vert_colours = vert_colour;
-  gl_Position = vec4(position.x, position.y + u_offset, position.z, 1.0f);
+  gl_Position = u_model * vec4(position, 1.0f);
 };

src/main.cc

@@ -1,9 +1,13 @@
 #include "glad/glad.h"
+#include "glm/gtc/type_ptr.hpp"
 #include <SDL2/SDL.h>
 #include <SDL2/SDL_error.h>
 #include <SDL2/SDL_video.h>
 #include <SDL2/SDL_events.h>
 #include <SDL2/SDL_keycode.h>
+#include <glm/vec3.hpp>
+#include <glm/mat4x4.hpp>
+#include <glm/ext.hpp>
 #include <cstdint>
 #include <cstdio>
 #include <string>
@@ -41,13 +45,13 @@ struct App {
   // OpenGL refers to as element, hence the name Element Buffer Object
   GLuint        ebo;
 
-  float         offset_modifier;
-  // offset value passed to shaders as uniform
-  float         offset;
-  // offset uniform index
-  GLint         u_offset_idx;
-
   GLuint        shader_program;
+
+  float         speed;
+  // Model matrix to be sent as uniform to the vertex shader
+  glm::mat4     model_mat;
+  GLint         u_model_idx;
+
   bool          running;
 };
 
@@ -119,7 +123,8 @@ int init(App &app) {
          glGetString(GL_SHADING_LANGUAGE_VERSION)
   );
 
-  app.offset_modifier = 0.02f;
+  app.speed = 0.02f;
+  app.model_mat = glm::translate(glm::mat4(1.0f), glm::vec3(0.0f));
 
   return EXIT_CODE_SUCCESS;
 }
@@ -200,10 +205,10 @@ void create_graphics_pipeline(App &app) {
 
   app.shader_program = create_shader_program(vs_source, fs_source);
 
-  const char *u_offset = "u_offset";
-  app.u_offset_idx = glGetUniformLocation(app.shader_program, u_offset);
-  if (app.u_offset_idx < 0) {
-    printf("Can't find uniform %s", u_offset);
+  const char *u_model = "u_model";
+  app.u_model_idx = glGetUniformLocation(app.shader_program, u_model);
+  if (app.u_model_idx < 0) {
+    printf("Failed to find uniform %s\n", u_model);
   }
 }
 
@@ -218,9 +223,9 @@ void run_main_loop(App &app) {
         break;
       case SDL_KEYDOWN:
         if (app.event.key.keysym.sym == SDLK_UP) {
-          app.offset += app.offset_modifier;
+          app.model_mat = glm::translate(app.model_mat, glm::vec3(0.0f, app.speed, 0.0f));
         } else if (app.event.key.keysym.sym == SDLK_DOWN) {
-          app.offset -= app.offset_modifier;
+          app.model_mat = glm::translate(app.model_mat, glm::vec3(0.0f, -app.speed, 0.0f));
         }
       }
     }
@@ -234,8 +239,8 @@ void run_main_loop(App &app) {
     glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
 
     glUseProgram(app.shader_program);
-    if (app.u_offset_idx >= 0) {
-      glUniform1f(app.u_offset_idx, app.offset);
+    if (app.u_model_idx >= 0) {
+      glUniformMatrix4fv(app.u_model_idx, 1, GL_FALSE, glm::value_ptr(app.model_mat));
     }
     // End pre draw setup