如何使用矩阵在OpenGL中移动和旋转对象

Question

So im pretty new to opengl and i got an object to appear on the screen and i also have game loop but i cant get the triangle i made to move and if i do get it to move it doesn't redraw itself.

PaintGl draws my triangle to the screen

void MeGlWindow::paintGL()
{
GLint TriPositionUniformLocation = glGetUniformLocation(programID, "TriPosition");
TriPosition = rot * tra;
glUniformMatrix3fv(TriPositionUniformLocation, 1, GL_FALSE, &TriPosition[0][0]);

//glClearColor(1, 0, 0, 1);
//glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glViewport(0, 0, width(), height());
//glDrawArrays(GL_TRIANGLES, 0, 6);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_SHORT, 0);

}

my update gets called every frame and im trying to use this to get keyboard input to know when to move an object. right now im trying to translate if the u or down key is pressed and rotate if the left or right key is pressed.

void MeGlWindow::myUpdate()
{


short Left = GetAsyncKeyState(37);
short Up = GetAsyncKeyState(38);
short Right = GetAsyncKeyState(39);
short Down = GetAsyncKeyState(40);
float speed = .001f;
float angle = 0;


if (Left != 0)
{
    angle -= .5;
    rot = mat3(cos(angle), -sin(angle), rot[0][2], sin(angle), cos(angle), rot[1][2], rot[2][0], rot[2][1], rot[2][2]);
    cout << "moved" << endl;
}

if (Right != 0)
{
    angle += .5;
    rot = mat3(cos(angle), -sin(angle), rot[0][2], sin(angle), cos(angle), rot[1][2], rot[2][0], rot[2][1], rot[2][2]);
    cout << "moved" << endl;
}
if (Up != 0)
{
    tra = mat3(tra[0][0], tra[0][1], tra[0][2], tra[1][0], tra[1][1], tra[1][2] + speed, tra[2][0], tra[2][1], tra[2][2]);
    cout << "moved" << endl;
}
if (Down != 0)
{
    tra = mat3(tra[0][0], tra[0][1], tra[0][2], tra[1][0], tra[1][1], tra[1][2] + -speed, tra[2][0], tra[2][1], tra[2][2]);
    cout << "moved" << endl;
}
if (angle < 0) angle += 360;
if (angle > 360) angle -= 360;



paintGL();
}

my last piece of code is vertexshadercode that im using to draw the triangle aswell.

#version 430

in layout(location=0) vec2 position;
in layout(location=1) vec3 vertexColor;

uniform mat3 TriPosition;

out vec3 theColor;

void main() 
{
vec4 v = vec4(position, 0.0, 1.0);
gl_Position.x = gl_Position.x + TriPosition[0][2];
gl_Position.y = gl_Position.y + TriPosition[1][2];
theColor = vertexColor;
};

Why is my triangle not moving or rotating. I was able to do this when i wasnt using opengl but now im lost.

Answer 1

1.

I'm not sure without access to your constructor of TriPosition, but it is possible that your matrix have been transposed in the process of converting from 'mat3' class to your 'TriPosition' class.

glUniformMatrix3fv(TriPositionUniformLocation, 1, GL_FALSE, &TriPosition[0][0]);

Try to change the third argument to GL_TRUE, which will make your input transposed when passed to GPU. Test and see what happens.

glUniformMatrix3fv(TriPositionUniformLocation, 1, GL_TRUE, &TriPosition[0][0]);

The reason why nothing happens is explained in the comment by Reto.

2.

Although, I can see that your triangle is meant to live under 2D space, try to use mat4 instead of mat3 in vertex shader, so that you will only need to multiply matrix and vertex, instead of getting value out like what you are doing there.

 #version 430 in layout(location=0) vec2 position; in layout(location=1) vec3 vertexColor; uniform mat4 TriPosition; out vec3 theColor; void main() { vec4 v = vec4(position, 0.0, 1.0); gl_Posiiton = TriPosition * v; theColor = vertexColor; }; 

As you won't have any z translation or rotation, your matrix will look like the following where r0, r1, r2, r3 are rotation factors, tx, ty are translation factors:

 r0 r1 0 tx r0 r1 0 0 1 0 0 tx r2 r3 0 ty = r2 r3 0 0 * 0 1 0 ty 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 0 1 TRIPOSITION = ROTATION * TRANSLATION 

Notice everything is same as 3x3 matrix if you take out thrid row and thrid column.

3.

I'd say it is "Transform" rather than "TriPosition", because TriPosition sounds like only for translation.

4.

the class mat3 you have in c++. Is that from glm? Then you might want to check this for easy transform matrix generation.