问题
I am trying to render a small region of the screen to an off-screen texture. This is part of a screenshot function in my app where the user selects a region on the screen and saves this to an image. While the region on the screen might be 250x250px, the saved image can be a lot larger like 1000x1000px.
I understand the process of rendering to a texture using an FBO. I'm mostly stuck when it comes to defining the projection matrix that clips the scene so that only the screenshot region is rendered.
回答1:
I believe you can do this without changing the projection matrix. After all, if you think about, you don't really want to change the projection. You want to change which part of the projected geometry gets mapped to your rendering surface. The coordinate system after projection is NDC (normalized device coordinates). The transform that controls how NDC is mapped to the rendering surface is the viewport transformation. You control the viewport transformation by the parameters to glViewport().
If you set the viewport dimensions to the size of your rendering surface, you map the NDC range of [-1.0, 1.0] to your rendering surface. To render a sub-range of that NDC range to your surface, you need to scale up the specified viewport size accordingly. Say to map 1/4 of your original image to the width of your surface, you set the viewport width to 4 times your surface width.
To map a sub-range of the standard NDC range to your surface, you will also need to adjust the origin of the viewport. The viewport origin values become negative in this case. Continuing the previous example, to map 1/4 or the original image starting in the middle of the image, the x-value of your viewport origin will be -2 times the surface width.
Here is what I came up with on how the viewport needs to be adjusted. Using the following definitions:
winWidth: width of original window
winHeight: height of original window
xMin: minimum x-value of zoomed region in original window coordinates
xMax: maximum x-value of zoomed region in original window coordinates
yMin: minimum y-value of zoomed region in original window coordinates
yMax: maximum y-value of zoomed region in original window coordinates
fboWidth: width of FBO you use for rendering zoomed region
fboHeight: height of FBO you use for rendering zoomed region
To avoid distortion, you will probably want to maintain the aspect ratio:
fboWidth / fboHeight = (xMax - xMin) / (yMax - yMin)
In all of the following, most of the operations (particularly the divisions) will have to be executed in floating point. Remember to use type casts if the original variables are integers, and round the results back to integer for the final results.
xZoom = winWidth / (xMax - xMin);
yZoom = winHeight / (yMax - yMin);
vpWidth = xZoom * fboWidth;
vpHeight = yZoom * fboHeight;
xVp = -(xMin / (xMax - xMin)) * fboWidth;
yVp = -(yMin / (yMax - yMin)) * fboHeight;
glViewport(xVp, yVp, vpWidth, vpHeight);
回答2:
You might want to look into how gluPickMatrix works and replicate its functionality using modern OpenGL methods. You can find the gluPickMatrix source code in the implementation of Mesa3D.
While the original intent of gluPickMatrix was for selection mode rendering, it can be used for what you want to do as well.
来源：https://stackoverflow.com/questions/24310536/opengl-render-portion-of-screen-to-texture