use a geometry shader to render rectangles

res/sh.geom.glsl

@@ -0,0 +1,25 @@
+#version 330 core
+layout(points) in;
+layout(triangle_strip, max_vertices = 4) out; // output the vertices of this rectangle
+
+void push_vert(float x, float y) {
+	gl_Position = vec4(x, y, 0.0F, 1.0F);
+	EmitVertex();
+}
+
+void main() {
+	// position data is stored as (x,y,w,h)
+	vec4 rect_a = gl_in[0].gl_Position; // get the rectangle data which will be our first point of the rectangle
+	vec2 rect_d = vec2(                 // compute point D
+		rect_a.x + rect_a.z,
+		rect_a.y - rect_a.w);
+
+	// A---B
+	// C---D
+	// triangle_strip will use the last two vertices to create a triangle
+	push_vert(rect_a.x, rect_a.y); // A
+	push_vert(rect_d.x, rect_a.y); // B
+	push_vert(rect_a.x, rect_d.y); // C
+	push_vert(rect_d.x, rect_d.y); // D
+	EndPrimitive();
+}

res/sh.vert.glsl

@@ -1,7 +1,22 @@
 #version 330 core
+layout(location = 0) in ivec4 rect; // rectangle data
+uniform ivec2 screen;               // current screen dimensions
 
-layout(location = 0) in vec4 pos;
-
+/* this vertex shader takes in unsigned integers formatted as (x,y,w,h), which denote rectangles
+ * the shader is responsible to output the data for this rectangle in the same format,
+ * but transformed to match the OpenGL coördinate space.
+ * I found the formula `(x-0.5*l)/0.5*l` to work well for scaling an axis to fit within the -1—1 coördinate space.
+ * And for transforming width/height, you just divide by the length of the corresponding axis.
+ * I demonstrate this here: https://www.desmos.com/calculator/ptwuyiv6bh */
 void main() {
-	gl_Position = vec4(pos.x, pos.y, 0.0F, 1.0F);
+	// divide the screen x and y coördinates by 2
+	// using bit shift because I don't trust the compiler optimiser since I've got less control.
+	int w_2 = screen.x >> 1;
+	int h_2 = screen.y >> 1;
+
+	gl_Position = vec4(
+		(rect.x - w_2) / float(w_2),            // scale the rect X point
+		(screen.y - rect.y - h_2) / float(h_2), // scale the rect Y point and invert the axis for top left alignment
+		rect.z / float(w_2),                    // scale the rect width
+		rect.w / float(h_2));                   // scale the rect height
 }

src/io/render.c

@@ -8,23 +8,27 @@
 #include <stdint.h>
 #include <stdlib.h>
 
-#include "../util/vec/float2.h"
-#include "shader.h"
 #include "../error.h"
+#include "shader.h"
 
-#define VERTC 6
-GLuint pipe;
-float2 verts[VERTC] = {
-	{-1, -1   }, // pnt A
-	{1,  -1   }, // pnt B
-	{1,  -0.9F}, // pnt C
+// include GLFW
+#include <GLFW/glfw3.h>
 
-	{-1, -0.9F}, // pnt D
-	{1,  -0.9F}, // pnt C
-	{-1, -1   }, // pnt A
-};
-GLuint vbo; // vertex buffer object
-GLuint vao; // vertex array object
+#define VERTC 1
+static GLuint pipe;
+static GLuint vbo;        // vertex buffer object
+static GLuint vao;        // vertex array object
+static GLuint screen_loc; // location to where OpenGL sends to the shaders of the screen dimensions
+
+static void screen_resize(int w, int h) {
+	int32_t verts[VERTC][4] = {
+		{0, h, w, -20},
+	};
+
+	glUniform2i(screen_loc, w, h);                                        // send the screen dimensions to the shader pipeline
+	glViewport(0, 0, w, h);                                               // update the viewport
+	glBufferData(GL_ARRAY_BUFFER, sizeof(verts), verts, GL_DYNAMIC_DRAW); // bind the data to it
+}
 
 int render_init(void) {
 	pipe = glCreateProgram();
@@ -32,11 +36,6 @@ int render_init(void) {
 	glLinkProgram(pipe);     // link the application
 	glValidateProgram(pipe); // validate that what we've done is correct
 
-	// init the VBO
-	glGenBuffers(1, &vbo);
-	glBindBuffer(GL_ARRAY_BUFFER, vbo);
-	glBufferData(GL_ARRAY_BUFFER, sizeof(verts), verts, GL_STATIC_DRAW);
-
 	int len;
 	glGetProgramiv(pipe, GL_INFO_LOG_LENGTH, &len);
 	if (len > 0) {
@@ -46,14 +45,18 @@ int render_init(void) {
 		fatal("error whilst linking the pipe: '%s'", log);
 	}
 
-	// init the VAO
+	screen_loc = glGetUniformLocation(pipe, "screen");
+
+	glGenBuffers(1, &vbo);              // create the vertex buffer objects
+	glBindBuffer(GL_ARRAY_BUFFER, vbo); // bind to it
+
 	glGenVertexArrays(1, &vao);
 	glBindVertexArray(vao);
 
 	// set VBO info
 	glBindBuffer(GL_ARRAY_BUFFER, vbo);
 	glEnableVertexAttribArray(0);
-	glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float2), NULL);
+	glVertexAttribIPointer(0, 4, GL_INT, 4 * sizeof(int32_t), NULL);
 
 	glBindVertexArray(0);
 	return 0;
@@ -65,11 +68,16 @@ void render_update(GLFWwindow* win) {
 
 	int w, h;
 	glfwGetWindowSize(win, &w, &h);
-	glViewport(0, 0, w, h);
-	glClearColor(0.1F, 0.1F, 0.1F, 1.0F);
-	glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
 	glUseProgram(pipe);
 
+	static int d = -1;  // initialize d to an impossible value so the condition below is always true on first execution
+	if ((h ^ w) != d) { // false negative when h and w swap integers, but this is quite a rare occurrence and it's impact is minimal
+		screen_resize(w, h);
+		d = h ^ w;
+	}
+
+	glClearColor(0.1F, 0.1F, 0.1F, 1.0F);
+	glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
 	glBindVertexArray(vao);
-	glDrawArrays(GL_TRIANGLES, 0, VERTC);
+	glDrawArrays(GL_POINTS, 0, VERTC);
 }

src/io/shader.c

@@ -16,6 +16,8 @@ extern char const NAM_S(sh_vert_glsl)[];
 extern char const NAM_E(sh_vert_glsl)[];
 extern char const NAM_S(sh_frag_glsl)[];
 extern char const NAM_E(sh_frag_glsl)[];
+extern char const NAM_S(sh_geom_glsl)[];
+extern char const NAM_E(sh_geom_glsl)[];
 // NOLINTEND
 
 /* compile a shader */
@@ -41,10 +43,14 @@ static GLuint shader_compile(GLenum type, char const* src, size_t len) {
 int shader_init(GLuint pipe) {
 	GLuint vs = COMPILE_NAME(GL_VERTEX_SHADER, sh_vert_glsl);
 	GLuint fs = COMPILE_NAME(GL_FRAGMENT_SHADER, sh_frag_glsl);
+	GLuint gs = COMPILE_NAME(GL_GEOMETRY_SHADER, sh_geom_glsl);
 
 	glAttachShader(pipe, vs);
 	glAttachShader(pipe, fs);
+	glAttachShader(pipe, gs);
+
 	glDeleteShader(vs);
 	glDeleteShader(fs);
+	glDeleteShader(gs);
 	return 1;
 }