OpenGL Mapping Textures to a Grid Stored Inside Vertex Array - opengl

Question

I have code that uses indices and vertices to draw a set of triangles in the shape of a grid. All the vertices are drawn using glDrawElements(). Now for each vertex I will set its corresponding Texture Coordinates to 0 or 1 for each set of triangles that form a square in the grid. Basically I want to draw a collage of random textures in each one of the "squares" (consisting of two triangles). I can do this using the glBegin() and glEnd() method calls inside a for loop using the fixed functional pipeline, but I would like to know how to do this using Vertex Arrays. A code view of what I am trying to do can be seen below.
#include "glwidget.h"
GLWidget::GLWidget(QWidget *parent, QGLWidget *glparent) :
QGLWidget(parent, glparent),
texture_ids_(NULL),
col_(30),
row_(30),
step_(16.0)
{
texture_ids_ = new GLuint[row_ * col_];
}
GLWidget::~GLWidget()
{
if (texture_ids_) {
glDeleteTextures(row_ * col_, texture_ids_);
}
}
void GLWidget::resizeEvent(QResizeEvent * /*event*/) {
initGL();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glViewport(0, 0, width(), height());
glOrtho(0, width(), 0, height(), -1, 1);
}
void GLWidget::initGL()
{
makeCurrent();
// Variables for vertices
vertices_.clear();
int32_t start_y = step_;
int32_t start_x = step_;
// Varaibles for indices
indices_.clear();
int32_t vertices_per_row = col_ + 1;
int32_t vertex_num = 0;
for (int32_t j = 0; j <= row_; ++j) {
// Generate Vertices on row j
for (int32_t i = 0; i <= col_; ++i) {
vertices_.push_back(Vertex<GLfloat>((start_x + (i * step_)),
(start_y + (j * step_)), 0.0f));
}
if (j == row_) {
break;
}
// Generate Indices to get right vertices for traingle
for (int32_t i = 0; i < col_; ++i) {
indices_.push_back(Indices<GLuint>(vertex_num, (vertex_num + 1),
(vertex_num + vertices_per_row)));
indices_.push_back(Indices<GLuint>((vertex_num + 1),
(vertex_num + vertices_per_row),
(vertex_num + vertices_per_row + 1)));
vertex_num++;
}
vertex_num++;
}
}
void GLWidget::textureInit()
{
makeCurrent();
for (int32_t i = 0; i < row_ * col_; ++i) {
QImage tmpQImage(step_, step_, QImage::Format_ARGB32);
tmpQImage = QGLWidget::convertToGLFormat(tmpQImage);
QPainter tmpQPainter;
tmpQPainter.begin(&tmpQImage);
tmpQPainter.fillRect(QRect(0, 0, width(), height()),
QColor(255, 0, 0));
tmpQPainter.setRenderHint(QPainter::Antialiasing, true);
tmpQPainter.end();
glGenTextures(1, &texture_ids_[i]);
glBindTexture(GL_TEXTURE_2D, texture_ids_[i]);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, tmpQImage.width(),
tmpQImage.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE,
tmpQImage.bits());
}
}
void GLWidget::updateGL() {
if (first_render_) {
textureInit();
first_render_ = false;
}
glMatrixMode(GL_MODELVIEW);
glScissor(0, 0, width(), height());
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glClearColor(0.5f, 0.5f, 0.5f, 0.5f);
glLoadIdentity();
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, vertices_.data());
glDrawElements(GL_TRIANGLES, indices_.size() * 3, GL_UNSIGNED_INT,
indices_.data());
glDisableClientState(GL_VERTEX_ARRAY);
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
}

Answer 1

So, you want to draw using a lot of textures, but you obviously can't re-bind new textures as it is all drawn from one array. One solution to this is to use a texture atlas. It is one single bitmap with all your textures inside it. For example, if you have 16 different textures, you make a bitmap with 4x4 sections. Instead of using texture coordinates from 0 to 1, you will use 0 to 0.25, or 0.25 to 0.50, etc.
There are some disadvantages you need to be aware of:
If you want high resolution, the texture atlas will obviously be quite big.
Minifying and magnifying can play tricks with you. GL_NEAREST won't be any problem, but using GL_LINEAR or variants of mipmapping will average values around a pixel. This can lead to artifacts for pixels at the border of one sub image.
As the UV coordinates will vary more, fewer vertices will have common vertex data, leading to a increased number of indices.
I assume you have done profiling that shows that using multiple iterations of drawing, rebinding the texture for each, is not good enough. This obvious solution can be surprisingly effective.