Glow after image effect - how to save glow render buffer after render

1
votes

I implement glow like in
http://http.developer.nvidia.com/GPUGems/gpugems_ch21.html
And now i want to implement after image effect for glowing parts, so they make trails, like described in 21.5.4 The After-Image Effect part of article.

My approach is to pass glow texture after horizontal and vertical blooming to another frame buffer (trail holder) and render glow textured quad with modified alpha colour for fadeout effect, based on current trail count (how much times we already render to trail holder), and then mix trail holder texture with current frame final image. So we got series of glowing parts at previous sprite location.
If trail count == max trail size, then clear trail holder.

Problem is how to save trail holder buffer from cleaning, even if i comments all trail count checks and

//glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//glClearColor(...);

it will not hold information about previous render.

P.S. even without any buffers and blooming, if just move and draw to default buffer a textured quad and do not clear anything it will move on black screen, i think it must make trail of itself images, because we do not clear anything. Why and how to fix it? Question seems generic for OpenGL ES 2.0 and desktop OpenGL.

1
openglopengl-es-2.0

1 Answers

2
votes

I suggest you render to a stack of textures bound as color attachment in a off-screen framebuffer object. The glow effect is then created by combining the texturing layers in a postprocessing step rendering to the on-screen framebuffer.

By using framebuffer objects you get that segregation on clearing operations you seek.