Question

I cannot get my cube to show after trying to do a manual perspective transformation My code is below. I have a suspicion it maybe my near and far plane numbers.

-(void)drawRect:(NSRect)dirtyRect
{
    // get the dimensions of the window
    NSSize dim = [self frame].size;

    // clear the background with color
    glClearColor(0.0, 0.0, 0.0, 0.4);
    glViewport(0, 0, dim.width, dim.height);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // variables
    GLfloat cubeHeight = 90.0;
    GLfloat cubeWidth = 90.0;
    GLfloat cubeLength = 200.0;
    //GLfloat alpha = 0.0;
    //GLfloat beta = 0.0;
    //GLfloat gamma = 0.0;

    // cube position data
    GLfloat cubePositionData[] = {0.0, 0.0, 0.0,
        0.0, 0.0, cubeLength,
        cubeWidth, 0.0, cubeLength,
        cubeWidth, 0.0, 0.0,
        0.0, cubeHeight, 0.0,
        0.0, cubeHeight, cubeLength,
        cubeWidth, cubeHeight, cubeLength,
        cubeWidth, cubeHeight, 0.0};

    // cube indices data
    GLubyte cubeIndices[] = {0, 1, 2, 0, 2, 3,
        4, 5, 6, 4, 6, 7,
        1, 2, 6, 1, 6, 5,
        2, 3, 7, 2, 7, 6,
        3, 0, 4, 3, 4, 7,
        0, 1, 5, 0, 5, 4};

    // cube color data
    GLfloat cubeColorData[] = {0.0, 0.3, 0.8, 1.0,
        0.0, 1.0, 0.0, 1.0,
        0.0, 1.0, 0.0, 1.0,
        0.0, 0.3, 0.8, 1.0,
        0.0, 0.3, 0.8, 1.0,
        0.0, 1.0, 0.0, 1.0,
        0.0, 1.0, 0.0, 1.0,
        0.0, 0.3, 0.8, 1.0};

    // array to hold buffer IDs
    GLuint vertexBuffers[2];

    // bind each array of data to separate buffers
    // bind cube position data to the first buffer
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(cubePositionData), cubePositionData, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    // bind the cube color data to the second buffer
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(cubeColorData), cubeColorData, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // enable the shader program
    GLuint programID = [self loadShaders];
    glUseProgram(programID);

    // enable vertex attributes
    // enable cube position attributes
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[0]);
    glEnableVertexAttribArray(VERTEX_POS_INDEX);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    // enable cube color attributes
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[1]);
    glEnableVertexAttribArray(VERTEX_COLOR_INDEX);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // point to the enabled attribute data
    glVertexAttribPointer(VERTEX_POS_INDEX, VERTEX_POS_SIZE, GL_FLOAT, GL_FALSE, 0, cubePositionData);
    glVertexAttribPointer(VERTEX_COLOR_INDEX, VERTEX_COLOR_SIZE, GL_FLOAT, GL_FALSE, 0, cubeColorData);

    GLfloat offset[] = {0.5, 0.5};
    GLint offsetUnif = glGetUniformLocation(programID, "offset");
    GLint zNearUnif = glGetUniformLocation(programID, "zNear");
    GLint zFarUnif = glGetUniformLocation(programID, "zFar");
    GLint frustumScaleUnif = glGetUniformLocation(programID, "frustumScale");

    glUniform2fv(offsetUnif, 1, offset);
    glUniform1f(frustumScaleUnif, 1.0f);
    glUniform1f(zNearUnif, 1.0);
    glUniform1f(zFarUnif, 3.0);

    // draw the elements
    glDrawElements(GL_TRIANGLES, sizeof(cubeIndices)/sizeof(GLubyte), GL_UNSIGNED_BYTE, cubeIndices);

    glDisableVertexAttribArray(VERTEX_POS_INDEX);
    glDisableVertexAttribArray(VERTEX_COLOR_INDEX);
    glUseProgram(0);

    // flush buffer
    glFlush();
    [[self openGLContext] flushBuffer];

}

What might the problem be? Oh, and the shaders too:

#version 120

attribute vec3 position;
attribute vec4 inColor;

uniform vec2 offset;
uniform float zNear;
uniform float zFar;
uniform float frustumScale;

varying vec4 outColor;

void main()
{
    vec4 cameraPos = vec4(position, 1.0) + vec4(offset.x, offset.y, 0.0, 0.0);
    vec4 clipPos;

    clipPos.xy = cameraPos.xy * frustumScale;

    clipPos.z = cameraPos.z * (zNear + zFar) / (zNear - zFar);
    clipPos.z += 2 * zNear * zFar / (zNear - zFar);

    clipPos.w = -cameraPos.z;

    gl_Position = clipPos;
    outColor = inColor;
}

#version 120

varying vec4 outColor;

void main()
{
    gl_FragColor = outColor;
}

---

OK, changed a section of my code to look like this:

GLfloat offset[] = {-2.0, -2.0};
GLint offsetUnif = glGetUniformLocation(programID, "offset");
GLint zNearUnif = glGetUniformLocation(programID, "zNear");
GLint zFarUnif = glGetUniformLocation(programID, "zFar");
GLint frustumScaleUnif = glGetUniformLocation(programID, "frustumScale");

glUniform2fv(offsetUnif, 1, offset);
glUniform1f(frustumScaleUnif, 1.0);
glUniform1f(zNearUnif, 1.0);
glUniform1f(zFarUnif, 25.0);

I get this:

Shouldn’t the back end (green) look smaller in the distance and not bigger, like a trapezoid base?

Answer 1

OK, I think I fixed it. Changed my vertex shader to look like this:

#version 120

attribute vec3 position;
attribute vec4 inColor;

uniform vec2 offset;
uniform float zNear;
uniform float zFar;
uniform float frustumScale;

varying vec4 outColor;

void main()
{
    vec4 cameraPos = vec4(position.x, position.y, -position.z, 1.0) + vec4(offset.x, offset.y, -10.0, 0.0);
    vec4 clipPos;

    clipPos.xy = cameraPos.xy * frustumScale;

    clipPos.z = cameraPos.z * (zNear + zFar) / (zNear - zFar);
    clipPos.z += 2 * zNear * zFar / (zNear - zFar);

    clipPos.w = -cameraPos.z;

    gl_Position = clipPos;
    outColor = inColor;
}

And I get this:

Maybe maybe?