CODEWITHSUNDEEP
openglmatrix
user3325226
user3325226

Reputation: 339

Translate world with model view matrix

I try to understand the OpenGL model view matrix. I thought I understand it, but after I tested it I am confused. Why, if I set the OpenGL MODELVIEW matrix with

glMatrixMode(GL_MODELVIEW);

GLfloat model[] = { 1, 0, 0, -1,
                    0, 1, 0, -1,
                    0, 0, 1, 0,
                    0, 0, 0, 1 };

glLoadMatrixf(model);

glBegin(GL_TRIANGLES);

glVertex4f(0, 0, 0, 1);
glVertex4f(1, 0, 0, 1);
glVertex4f(0, 1, 0, 1);

glEnd();

, all things I draw are not translated -1 on the x and -1 on the y axis, I only get a crazy result ? I thought the passed vertices all multiplied with the model matrix.

Upvotes: 0

Views: 705

Answers (1)

bofjas
bofjas

Reputation: 1206

From the site: http://www.opengl.org/archives/resources/faq/technical/transformations.htm

"The translation components occupy the 13th, 14th, and 15th elements of the 16-element matrix, where indices are numbered from 1 to 16 as described in section 2.11.2 of the OpenGL 2.1 Specification."

In your float array the translation components are the 4th and 8th component.

You should change it to this:

GLfloat model[] = { 1, 0, 0, 0,
                    0, 1, 0, 0,
                    0, 0, 1, 0,
                   -1,-1, 0, 1 };

The matrix you created would cause the w component of the clip coordinates to be expressed as:

gl_Position.w = (position.x * -1.0) + (position.y * -1.0) + 1

Since ndc = gl_Position/gl_Position.w, This will cause the normalized device coordinates (ndc) to be:

ndc.x = position.x / ( (position.x * -1.0) + (position.y * -1.0) + 1 )
ndc.y = position.y / ( (position.x * -1.0) + (position.y * -1.0) + 1 )
ndc.z = position.z / ( (position.x * -1.0) + (position.y * -1.0) + 1 )

Which I can imagine would cause some weird results.

Upvotes: 1

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