How would you produce a scrolling effect for an object? - c++

Question

I am using OpenGL in Visual C++ and I am displaying a rectangle using VBO and VAO and I have a basic vertex shader and fragment shader to apply a color and position for the object.
I would like to produce a scrolling effect of the object till I press the
Esc key. What would be the best way to do that?
I know that you can provide your Model, View and Projection matrices to your shader to perform the changes to the object. Especially the model matrix for performing the transformations. I can change the position of the object and rotate it but I am not sure how to produce a continuous or a scrolling effect of moving the object or rotating till a key is pressed.
My code so far:
GLfloat vb_data[] = {
-0.25f, 0.25f, 0.0f, // Top-left
0.25f, 0.25f, 0.0f, // Top-right
0.25f, -0.25f, 0.0f, // Bottom-right
-0.25f, -0.25f, 0.0f // Bottom-left
};
glGenBuffers(1, &vbo_ID);
glBindBuffer(GL_ARRAY_BUFFER, vbo_ID);
glBufferData(GL_ARRAY_BUFFER, sizeof(vb_data), vb_data, GL_DYNAMIC_DRAW);
GLfloat incre = 0.01f;
glClear(GL_COLOR_BUFFER_BIT);
while (1){
glm::mat4 trans = glm::mat4(1.0f);
glm::mat4 transBar = glm::translate(trans, glm::vec3(0.2f, 0.2f, 1.0f));
glm::mat4 MVP = transBar;
glUseProgram(pgID);
glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);
glBindBuffer(GL_ARRAY_BUFFER, vbo_ID);
glEnableVertexAttribArray(0);
glVertexAttribPointer(
0,
3,
GL_FLOAT,
GL_FALSE,
0,
(void*)0
);
glDrawArrays(GL_QUADS,0, 4);
glDisableVertexAttribArray(0);
SwapBuffers(g_pOpenGLWindow->hDC);
}

Answer 1

but I am not sure how to produce a continuous or a scrolling effect of moving the object or rotating ...
To generate a smooth motion (translation or rotation), the translation or rotation part of the model matrix has to be changed by small amount respectively angle in every frame. This can be achieved by control variables which are incremented by a small step in every frame. The model matrix has to be recalculated, by the use of this control variables in every frame.
See the concept in the following pseudo code:
translate_x = 0;
angle = 0;
while (1)
model_matrix = translate(translate_x, 0, 0) * roatateX(angle)
translate_x = translate_x + step_x
angle = angle + step_angle
Try the following code and play around with the parameters, to achieve the effect for your needs:
float angle = 0.0f;
float step_ang = 1.0f;
float trans_x = -0.5f;
float step_x = 0.01f;
bool move_forward = true;
while (1)
{
glm::mat4 model = glm::mat4(1.0f);
model = glm::translate(model, glm::vec3(trans_x, 0.2f, 1.0f));
model = glm::rotate(model, glm::radians(angle), glm::vec3(0.0f, 0.0f, 1.0f));
angle += step_ang;
trans_x += move_forward ? step_x : -step_x;
move_forward = move_forward ? trans_x < 0.5f : trans_x <= -0.5f;
glm::mat4 MVP = model;
.....
... till a key is pressed.
Since the question is tagged winapi, I assume that you have initilized a WNDCLASSEX data structure and that you have set the WindowProc callback function to the member lpfnWndProc.
You have to implement the WM_KEYDOWN message event in the window callback function and to check if the Esc (VK_ESCAPE) was pressed:
global variable:
bool esc_pressed = false;
Call back function:
LRESULT CALLBACK WindowProcedure( HWND hWnd, unsigned int msg, WPARAM wparam, LPARAM lparam )
{
switch(msg)
{
case WM_KEYDOWN:
if ( wparam == VK_ESCAPE )
esc_pressed = true;
break;
// other messages
// ...
}
return DefWindowProc( hWnd, msg, wparam, lparam );
}
Further you have to handle the window messages in the main loop. See GetMessage function and DispatchMessage function.
instead of
while (1)
{
.....
}
you have to
MSG msg;
while( GetMessage( &msg, 0, 0, 0 ) )
{
DispatchMessage( &msg );
.....
}