Python OpenGL How to render off-screen correctly

Question

I have a task to render things on off-screen in OpenGL.I have to render things in frame buffer but no need to display it on OpenGL Window.

import sys
from PIL import Image
from OpenGL.GL import *
from OpenGL.GLU import *
from OpenGL.GLUT import *
import numpy as np

# define shader code
vertex_code='''
uniform float scale;
attribute vec2 position;
attribute vec4 color;
varying vec4 v_color;
attribute vec2 TexCoordIn; 
varying vec2 TexCoordOut;

void main()
{

    gl_Position = vec4(position*scale, 0.0, 1.0);
    v_color = color;
    TexCoordOut = TexCoordIn;

}'''

fragment_code='''
varying vec4 v_color;
varying vec2 TexCoordOut; 
uniform sampler2D Texture; 
uniform vec2 originPosition;
uniform vec2 targetPosition;
vec2 curveWarp(vec2 textureCoord, vec2 originPosition, vec2 targetPosition, float radius)
{
    vec2 offset = vec2(0.0);
    vec2 result = vec2(0.0);

    vec2 direction = targetPosition - originPosition;

    float infect = distance(textureCoord, originPosition)/radius;

    infect = 1.0 - infect;
    infect = clamp(infect, 0.0, 1.0);
    offset = direction * infect;

    result = textureCoord - offset;
    return result;
}

void main()
{
    vec2 coordinate = vec2(0.0);
    float radius = 0.5;
    coordinate = curveWarp(TexCoordOut,originPosition,targetPosition,radius);
    gl_FragColor = v_color*0.000000000001 + texture2D(Texture, coordinate);

}'''

#define vertex and color array
data = np.zeros(4, dtype = [ ("position", np.float32, 2), ("color", np.float32, 4),("textureCoord",np.float32,2)] )
data['color']    = [ (1,1,0,1), (1,1,0,1), (1,1,0,1), (1,1,0,1) ]
data['position'] = [ (-1,-1),   (-1,+1),   (+1,-1),   (+1,+1)  ]
data['textureCoord']= [ (0,0),   (0,1),   (1,0),   (1,1) ]

#define debug function
def showTextureInGLTexture2D():
    im = glGetTexImage( GL_TEXTURE_2D, 0, GL_RGBA,  GL_UNSIGNED_BYTE )
    image=Image.frombuffer("RGBA",(512,512),im, 'raw', 'RGBA', 0, 1)
    image.show()

#define useful function
def display():
    glClear(GL_COLOR_BUFFER_BIT)
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4)

    glutSwapBuffers()

def reshape(width,height):
    glViewport(0, 0, width, height)



#step1 init the context
def  init(img_path):
    image = Image.open(img_path)
    glutInit()
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB)
    glutCreateWindow('Hello world!')
    glutReshapeWindow(image.width,image.height)
    glutReshapeFunc(reshape)
    glutDisplayFunc(display)
    glutKeyboardFunc(keyboard)
    return image

#step 2 
def initShaderProgram():
    program  = glCreateProgram()
    vertex   = glCreateShader(GL_VERTEX_SHADER)
    fragment = glCreateShader(GL_FRAGMENT_SHADER)

    # Set shaders source
    glShaderSource(vertex, vertex_code)
    glShaderSource(fragment, fragment_code)

    # Compile shaders
    glCompileShader(vertex)
    glCompileShader(fragment)

    fragSuccess=glGetShaderiv(fragment, GL_COMPILE_STATUS)
    vertSuccess=glGetShaderiv(vertex, GL_COMPILE_STATUS)
    print("vertext shader compile  success  [%s]" %(vertSuccess,))
    print( "fragment shader compile  success  [%s]" %(fragSuccess,))

    if vertSuccess==0:
        print (glGetShaderInfoLog(vertex))
        sys.exit(0)

    if fragSuccess==0:
        print (glGetShaderInfoLog(fragment))
        sys.exit(0)


    glAttachShader(program, vertex)
    glAttachShader(program, fragment)
    glLinkProgram(program)
    linksucc=glGetProgramiv(program,GL_LINK_STATUS);
    print( "link  program success [%s]" %(linksucc,))
    glUseProgram(program)

    return program

#step 3.1 optional setup texture 
def getTextureFromFile(image_file):
    #convert file to bytes
    image = image_file.transpose(Image.FLIP_TOP_BOTTOM)
    image = image.convert("RGBA")
    byteImage =np.array(list(image.getdata()), np.uint8)

    #setup texture 
    texIndex=glGenTextures(1)
    glEnable( GL_TEXTURE_2D )
    glBindTexture(GL_TEXTURE_2D,texIndex)  

    glPixelStorei(GL_UNPACK_ALIGNMENT,1)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR)
     #make the texture the default
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texIndex, 0)
    glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA,image.width,image.height,0,GL_RGBA,GL_UNSIGNED_BYTE,byteImage)

    return texIndex




#step 4.1 optional get the image from BufferFrame 
def saveImageFromFBO(image):
    glReadBuffer(GL_COLOR_ATTACHMENT0)
    glPixelStorei(GL_PACK_ALIGNMENT, 1)
    data = glReadPixels (0, 0, image.width, image.height, GL_RGB,  GL_UNSIGNED_BYTE)
    image = Image.new ("RGB", (image.width, image.height), (0, 0, 0))
    image.frombytes (data)
    image = image.transpose(Image.FLIP_TOP_BOTTOM)
    image.save ('9_result.jpg')


#step 4 optional if need to OSR generate a BufferFrame
def setupSelfDefineFBO(program,image):

    # Setup framebuffer
    framebuffer = glGenFramebuffers (1)
    glBindFramebuffer(GL_FRAMEBUFFER, framebuffer)

    # Setup depthbuffer
    depthbuffer = glGenRenderbuffers (1)
    glBindRenderbuffer (GL_RENDERBUFFER,depthbuffer)
    glRenderbufferStorage (GL_RENDERBUFFER, GL_DEPTH_COMPONENT, image.width, image.height)
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthbuffer)

    #first init VBO, then other parameters
    buffer = glGenBuffers(1)     # Request a buffer slot from GPU
    glBindBuffer(GL_ARRAY_BUFFER, buffer)     # Make this buffer the default one
    glBufferData(GL_ARRAY_BUFFER, data.nbytes, data, GL_DYNAMIC_DRAW)    # Upload data    

    # Create texture to render to
    # glBufferData(GL_FRAMEBUFFER, data.nbytes, data, GL_DYNAMIC_DRAW)
    loc = glGetAttribLocation(program, "position")   #get the index of  the attribute in program
    glEnableVertexAttribArray(loc)  #allow this attribute decide by index can be use 
    stride = data.strides[0]   #define how to read buffer
    offset = ctypes.c_void_p(0) #define the offset where the data begin in buffer
    glVertexAttribPointer(loc, 2, GL_FLOAT, False, stride, offset)
    offset = ctypes.c_void_p(data.dtype["position"].itemsize)
    loc = glGetAttribLocation(program, "color")
    glEnableVertexAttribArray(loc)
    glVertexAttribPointer(loc, 4, GL_FLOAT, False, stride, offset)

    #setup other parameters
    loc = glGetUniformLocation(program, "scale")
    glUniform1f(loc, 1.0)


    originPosition = glGetUniformLocation(program, "originPosition")
    glUniform2f(originPosition, 0.5, 0.5 )

    targetPosition = glGetUniformLocation(program, "targetPosition")
    glUniform2f(targetPosition, 0.47, 0.47 )

    # following code to bind uniform texture if needed
    aTexture=getTextureFromFile(image)

    glActiveTexture(GL_TEXTURE0)
    glBindTexture(GL_TEXTURE_2D,aTexture)
    loc=glGetUniformLocation(program,"Texture")
    textureId=glGetIntegerv(GL_TEXTURE_BINDING_2D)  #find out the id of the current texture bind to GL_TEXTURE_2D
    glUniform1i(loc,textureId)    

    loc= glGetAttribLocation(program, "TexCoordIn")
    glEnableVertexAttribArray(loc)
    offset=ctypes.c_void_p(data.dtype["color"].itemsize+8)
    glVertexAttribPointer(loc,2,GL_FLOAT,False,stride,offset)

    status = glCheckFramebufferStatus (GL_FRAMEBUFFER)
    if status != GL_FRAMEBUFFER_COMPLETE:
        print( "Error in framebuffer activation")

    saveImageFromFBO(image)


    glBindFramebuffer(GL_FRAMEBUFFER, GL_NONE)
    glDeleteTextures ([aTexture])
    glDeleteFramebuffers (1,[framebuffer])

    print( 'save image from FBO success')

def keyboard( keycode, x, y ):
    # print(keycode)
    # if (keycode == 'b'+'q'):
    #     sys.exit()
    # elif (keycode == 'b'+' '):
    saveImageFromFBO(image)



if (__name__ == '__main__'):
   image = init('9.jpg')
   program = initShaderProgram()
   setupSelfDefineFBO(program,image)

The code does not render pictures correctly, the shader does not work, but it can save the original image. I didn't find useful information on the Internet. Any ideas why this would not work? And how I can fix it?

Looks like a duplicate for me: https://stackoverflow.com/questions/12157646/how-to-render-offscreen-on-opengl — user2988257, Mar 22 '20 at 15:15

score 3 · Accepted Answer · answered Mar 22 '20 at 15:12

First of all the framebuffer needs a color buffer:

def setupSelfDefineFBO(program,image):

    fbWidth, fbHeight = image.width, image.height

    # Setup framebuffer
    framebuffer = glGenFramebuffers (1)
    glBindFramebuffer(GL_FRAMEBUFFER, framebuffer)

    # Setup colorbuffer
    colorbuffer = glGenRenderbuffers (1)
    glBindRenderbuffer(GL_RENDERBUFFER, colorbuffer)
    glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA, fbWidth, fbHeight)
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, colorbuffer) 

    # Setup depthbuffer
    depthbuffer = glGenRenderbuffers (1)
    glBindRenderbuffer(GL_RENDERBUFFER,depthbuffer)
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, fbWidth, fbHeight)
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthbuffer)

    # check status
    status = glCheckFramebufferStatus (GL_FRAMEBUFFER)
    if status != GL_FRAMEBUFFER_COMPLETE:
        print( "Error in framebuffer activation")

    # [...]

The size of the view port has to be adapted to the size of the framebuffer (glViewport):

def setupSelfDefineFBO(program,image):
    # [...]

    glViewport(0, 0, fbWidth, fbHeight)

    # [...]

The binding boinpoint of the texture object an the texture sampler uniform is the texture unit. You have to assign the texture unit to the texture sampler uniform rather than the texture object (name) id (0 for GL_TEXTURE0, 1 for GL_TEXTURE1 ...):

def setupSelfDefineFBO(program,image):
    # [...]

    glActiveTexture(GL_TEXTURE0)
    glBindTexture(GL_TEXTURE_2D,aTexture)
    loc=glGetUniformLocation(program,"Texture")
    glUniform1i(loc, 0) # <----

    # [...]

To run the shader program, you have to draw some geometry. The application sets up the vertex coordinates and attributes, but the geometry id never drawn. Draw a screen space quad by a GL_TRIANGLE_STRIP Triangle primitives:

def setupSelfDefineFBO(program,image):
    # [...]

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4)
    saveImageFromFBO(fbWidth, fbHeight)

    glBindFramebuffer(GL_FRAMEBUFFER, GL_NONE)
    glViewport(0, 0, 512, 512)

    # [...]

def saveImageFromFBO(width, height):
    glReadBuffer(GL_COLOR_ATTACHMENT0)
    glPixelStorei(GL_PACK_ALIGNMENT, 1)
    data = glReadPixels (0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE)
    image = Image.new("RGB", (width, height), (0, 0, 0))
    image.frombytes(data)
    image = image.transpose(Image.FLIP_TOP_BOTTOM)
    image.save ('9_result.jpg')

Thank you very much. Your answer helped me a lot – MOON Sun Mar 22 '20 at 16:45 — MOON Sun, Mar 22 '20 at 16:45

Python OpenGL How to render off-screen correctly

1 Answers1