Why my iPhone OpenGL-ES texture does not cover the viewport?

Question

I have a square image of size 320x320, from which I create an OpenGL texture. I use the most basic Vertext and Fragment shaders and I want to display the texture in the entire view. The view (EAGLView derived from UIView as found in many OpenGL iOS samples) is also of size 320x320.

The problem is, the image is drawn on the top left corner, covering only around 50% of the entire view. It does not cover 100% of the view. I don't know why?

Here is my code:

position = glGetAttribLocation(m_shaderProgram, "position");
inputTextureCoordinate = glGetAttribLocation(m_shaderProgram, "inputTextureCoordinate");    
inputImageTexture = glGetUniformLocation(m_shaderProgram, "inputImageTexture");

static const GLfloat textureCoordinates[] = {
    0.0f, 1.0f,
    1.0f, 1.0f,
    0.0f,  0.0f,
    1.0f,  0.0f,
};

static const GLfloat imageVertices[] = {
    -1.0f, -1.0f,
    1.0f, -1.0f,
    -1.0f,  1.0f,
    1.0f,  1.0f,
};

[EAGLContext setCurrentContext:context];

glViewport(0, 0, backingWidth, backingHeight); // These are 320, 320

glUseProgram(m_shaderProgram);

glClearColor(0, 0, 0, 1);
glClear(GL_COLOR_BUFFER_BIT);

glActiveTexture(GL_TEXTURE2); 
glBindTexture(GL_TEXTURE_2D, sourceTextureID); // The texture is also of size 320x320

glUniform1i(inputImageTexture, 2);  

glVertexAttribPointer(position, 2, GL_FLOAT, 0, 0, imageVertices);
glVertexAttribPointer(textureCoordinate, 2, GL_FLOAT, 0, 0, textureCoordinates);

glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);    
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];

Vertext Shader.

 attribute vec4 position;
 attribute vec4 inputTextureCoordinate;

 varying vec2 textureCoordinate;

 void main()
 {
    gl_Position = position;
    textureCoordinate = inputTextureCoordinate.xy;
 }

Fragment Shader.

 varying highp vec2 textureCoordinate;
 uniform sampler2D inputImageTexture;

 void main()
 {
    gl_FragColor = texture2D(inputImageTexture, textureCoordinate);
 }

Answer 1

So the problem is that the texture's dimensions were not 2's power. So we need to scale the textureCoordinates accordingly. Inserting following lines solved the problem...

GLfloat textureCoordinates[] = {
    0.0f, 1.0f,
    1.0f, 1.0f,
    0.0f,  0.0f,
    1.0f,  0.0f,
};

float nearest2sPower = 2;

if (nearest2sPower < backingWidth) {
    while (nearest2sPower < backingWidth) {
        nearest2sPower *= 2;
    }
}

verticalFlipTextureCoordinates[2] = backingWidth/nearest2sPower;
verticalFlipTextureCoordinates[6] = backingWidth/nearest2sPower;

nearest2sPower = 2;

if (nearest2sPower < backingWidth) {
    while (nearest2sPower < backingWidth) {
        nearest2sPower *= 2;
    }
}

verticalFlipTextureCoordinates[1] = backingHeight/nearest2sPower;
verticalFlipTextureCoordinates[3] = backingHeight/nearest2sPower;