Transfer the inverted scene to the framebuffer

Question

I need help rendering an inverted scene in a framebuffer. I want to make a mirror effect. What I got as a result: Frame buffer initialization:

GLuint FramebufferName = 0; glGenFramebuffers(1, &FramebufferName); glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName); GLuint renderedTexture; glGenTextures(1, &renderedTexture); glBindTexture(GL_TEXTURE_2D, renderedTexture); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, SCREEN_WIDTH, SCREEN_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_COMPARE_R_TO_TEXTURE); glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, renderedTexture, 0); glDrawBuffer(GL_COLOR_ATTACHMENT0); glReadBuffer(GL_NONE); 

Draw the scene into the frame buffer:

  mView = camera.getViewMatrix(); mProjection = perspective(45.0f, 4.0f / 3.0f, 0.1f, 300.0f) * scale(mat4(1.0f), vec3(1.0f, -1.0f, 1.0f)); glBindFramebuffer(GL_DRAW_FRAMEBUFFER, FramebufferName); glViewport(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glCullFace(GL_FRONT); glEnable(GL_CULL_FACE); mWorld = rotate(mWorld, -15.0f, vec3(1, 0, 0)); mWorld = rotate(mWorld, -30.0f, vec3(0, 1, 0)); cubemap.Draw(mView, mProjection, mWorld); glDisable(GL_CULL_FACE); glEnable(GL_DEPTH_TEST); glUseProgram(program); mWorld = translate(mat4(1.0f), vec3(0, -10, 0)); glUniformMatrix4fv(mvp_object, 1, GL_FALSE, value_ptr(mProjection * mView * mWorld * scale(mat4(1.0f), vec3(10.0f)))); cannon.Draw(program); mWorld = mat4(1.0f); mWorld = translate(mat4(1.0f), vec3(50, -10, 0)); mWorld = rotate(mWorld, 90.0f, vec3(0, 1, 0)); glUniformMatrix4fv(mvp_object, 1, GL_FALSE, value_ptr(mProjection * mView * mWorld * scale(mat4(1.0f), vec3(10.0f)))); cannon.Draw(program); mWorld = mat4(1.0f); mWorld = translate(mat4(1.0f), vec3(0, -10, 50)); mWorld = rotate(mWorld, 140.0f, vec3(0, 1, 0)); glUniformMatrix4fv(mvp_object, 1, GL_FALSE, value_ptr(mProjection * mView * mWorld * scale(mat4(1.0f), vec3(10.0f)))); cannon.Draw(program); glDisable(GL_DEPTH_TEST); glBindFramebuffer(GL_FRAMEBUFFER, 0); 

I looked through several tutorials that use glScalef (1.0, -1.0, 1.0) to invert a scene. glScalef was used in older versions of opengl. How can I invert the scene in new versions of opengl? I multiplied the projection matrix by scale (mat4 (1.0f), vec3 (1.0, -1.0, 1.0)) for this effect. Right?

Answer 1

Here is the vertex shader written in GLSL.

 void main(void) { // Clean up inaccuracies vec2 Pos = sign(gl_Vertex.xy); gl_Position = vec4(Pos.xy, 0.0, 1.0); // Image-space gl_TexCoord[0].xy = vec2(-Pos.x, Pos.y) * 0.5 + 0.5; // x texture coord is inverted } 

Pos refers to the screen-aligned quadrant for framebuffer output.

Fragment shader

 uniform sampler2D framebufferSampler; void main(void) { gl_FragColor = texture2D(framebufferSampler, gl_TexCoord[0].xy); } 

Parity aligned square can be drawn

 glBegin(GL_QUADS) { glVertex2f(-1.0f, -1.0f); glVertex2f(1.0f, -1.0f); glVertex2f(1.0f, 1.0f); glVertex2f(-1.0f, 1.0f); } glEnd(); 

or using vertex arrays.

 GLfloat vertices[] = { -1.0f, -1.0f, 1.0f, -1.0f, 1.0f, 1.0f, -1.0f, 1.0f }; glEnableClientState(GL_VERTEX_ARRAY); glVertexPointer(2, GL_FLOAT, 0, (void*)vertices); glDrawArrays(GL_QUADS, 0, 4); glDisableClientState(GL_VERTEX_ARRAY); 

Texture

which is used for use in an ATV, is information about the framebuffer with the previous rendered content (multipass shader method).

You can also do this with other 2D texture coords or images instead of frame flags and screen alignment.

The original image.

Inverted image.

Further information can be found in Learn OpenGL .