CODEWITHSUNDEEP
c++openglglfwglm-mathray-picking
yan
yan

Reputation: 1

OpenGL: mouse move objects, object can't follow mouse

After picking an object with the mouse, I want to be able to move the object using the mouse. First, I translate mouse position to world position, and use glReadPixels() to read the depth of the object as z's:

double xpos, ypos, zpos;
glfwGetCursorPos(window_ptr, &xpos, &ypos);
float xPercent = (xpos + 0.5f) / scr_width_ * 2.0f - 1;  // range is -1 to +1
float yPercent = (ypos + 0.5f) / scr_height_ * 2.0f - 1;  // range is -1 to +1
yPercent = -yPercent;

glReadPixels(xpos, scr_height_ - ypos - 1, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &zPercent);

then we move the mouse until we want to release the mouse.

last_position = (xPercent, yPercent, zPercent);

Finally we use the same value as z's value and calculate the x's world position and y's position:

current_position = (xPercent, yPercent, zPercent);

then we translate the object model:

model = glm::translate(model, current_position - last_postion);

the issue is:

waiting for your answer.

Upvotes: 0

Views: 652

Answers (1)

the_Demongod
the_Demongod

Reputation: 78

The problem is caused by the difference in coordinate systems between the screen and the world. Your cursor position is in "percent" (fraction of the screen width), but your objects are likely placed in some other coordinate system that is determined by your projection matrix (for example, in meters). Unless you are using an orthographic projection and the world-space coordinates of the object are in the same space as the screen-space coordinates, you will get different motion.

For example, even if you had an orthographic projection, it may be configured such that the screen maps to a region of game world that is 20 meters wide, so moving your mouse anywhere within the range [-1, 1] (across the full width of the screen) will only translate to the object moving over 1/10th of the screen.

Furthermore, you may be working with a perspective projection. In that case, not only will your world coordinates differ from the screen coordinates, but there is actually a nonlinear transformation between the screen coordinates and the world coordinates that will cause the object's motion to be distorted near the edges of the screen. You can correct for this by un-projecting your mouse cursor's screen-space coordinates back into world coordinates using glm::unproject. Here is a good explanation of this process.

Upvotes: 1

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