Setting Up OpenGL ES 2.0 Textures

Question

Hi everyone,

I'm working on an iPhone project in Xcode 3.2.4 for which I'd like to display textures using OpenGL ES 2.0. I don't have much previous experience working with OpenGL, but so far it doesn't seem too awful. So far I've mainly been referring to The OpenGL ES 2.0 Programming Guide (gold book).

Right now I've got a project based on Xcode's default OpenGL template, with the drawFrame function replaced with my texture-displaying code, which in turn is based on the Simple_Texture2D project from the gold book. However, the square I should be drawing my texture onto does not display it; it's black instead, and I'm not sure why this is happening.

Here are the contents of drawFrame:

[(EAGLView *)self.view setFramebuffer];

GLuint textureID;

static const GLfloat squareVertices[] =
{
    -0.5f, -0.33f,
    0.5f, -0.33f,
    -0.5f,  0.33f,
    0.5f,  0.33f
};

// Generate a 2 x 2 rgb image at 3 bytes per pixel.
GLubyte image[4 * 3] =
{
    255,   0,   0,
    0,   255,   0,
    0,     0, 255,
    255, 255,   0
};

glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

glUseProgram(program);

glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
glEnableVertexAttribArray(ATTRIB_VERTEX);

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

glGenTextures(1, &textureID);

glBindTexture(GL_TEXTURE_2D, textureID);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 2, 2, 0, GL_RGB, GL_UNSIGNED_BYTE, image);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

GLushort indices[] = {0, 1, 2, 3, 2, 1};

glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices);

[(EAGLView *)self.view presentFramebuffer];

Perhaps to an experienced OpenGL user it'll be obvious what I'm doing wrong. Otherwise, any suggestions, guesses, or constructive comments of any kind are great too.

Thanks in advance!

Answer 1

You need not only vertex positions but also texture coordinates. As it stands you're only specifying vertex positions, so you're only going to vertices with whatever color was set last, no texture.

Try setting up a texCoord array and doing another glVertexAttribPointer(ATTRIB_TEXCOORD, ...) and a glEnableVertexArray(ATTRIB_TEXCOORD).

(If you're lighting you may want normals as well.)

There's sample code off songho's OpenGL Vertex Array page (glDrawElements section). See the draw3 function and prior setup.

Note that vertex buffer objects (and other buffer objects) are generally replacing vertex arrays (and other arrays) as the preferred solution for rendering batches of data at once. VBOs have more opportunity to be stored in a high-performance way by the driver.