Using multiple textures on the same FrameBuffer

OpenGL OpenGL: Basic Coding
1

Hello, I’m making a drawing board-like application and i need help in clearing something up… My current standing is that i have a frameBuffer with a rendertexture attached to it… I draw vertices to the render texture and i show that texture on a quad, this is how my drawing application is working. Now i want to implement a selection tool where i cut out a portion and move it and drop it where i want. I have done the cutting and storing pixels in a data structure… but i cant display it without ruining my current texture on the rendertexture.

So i think the solution is to get a secondary texture and overlap that on the first quad. I can move the stored pixels onto the second quad and move them as i like and when i’m done with it, i transfer the final pixels (at new locations) to the primary render texture. So the problem i’m getting is that i cant seem to get my second texture binded to the framebuffer. Here is the code for my primary texture


    glGetIntegerv(GL_FRAMEBUFFER_BINDING, &screenFrameBuffer);
    glGenFramebuffers(1, &renderFrameBuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, renderFrameBuffer);
    checkGLError("glBindFramebuffer");

    // The texture we're going to render to
    glGenTextures(1, &renderedTexture);
    checkGLError("glGenTextures");

    // Primary Texture
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, renderedTexture);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); //GL_LINEAR
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); //GL_NEAREST
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, _Width, _Height, 0, GL_RGBA, GL_UNSIGNED_BYTE, primaryTextureDataStorage);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, renderedTexture, 0);
    checkGLError("glFramebufferTexture2D");

Here is what i’m doing right after it for the secondary texture


    // Secondary Texture
    glActiveTexture(GL_TEXTURE1); // activating the 2nd texture
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); //GL_LINEAR
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); //GL_NEAREST
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, _Width, _Height, 0, GL_RGBA, GL_UNSIGNED_BYTE, secondaryTextureDataStorage);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, renderedTexture, 0);
    checkGLError("glFramebufferTexture2D Secondary");

From my understanding, we can bind multiple textures on the same framebuffer but at different color attachment slots. I even tried changing the texture buffer by generating, binding another texture buffer and attaching that to the color attachment 1… i dont know if i did that wrong or not, i changed back to using a single texture buffer because my current understanding says color attachments can work around that.

Here is the code i use to draw:


    GLuint texPos = glGetAttribLocation(glProgram.textureProgram, "a_tex_texCoord");
    GLuint vertPos = glGetAttribLocation(glProgram.textureProgram, "a_tex_position");
    glVertexAttribPointer(vertPos, 2, GL_FLOAT, GL_FALSE, 2*sizeof(GLfloat), squareVertices);
    glEnableVertexAttribArray(vertPos);
    glVertexAttribPointer(texPos, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(GLfloat), textureVertices);
    glEnableVertexAttribArray(texPos);

    glDisable(GL_BLEND);
    glActiveTexture(GL_TEXTURE0);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);;
    checkGLError("glDrawArrays-PrimaryTexture");

    // drawing secondary on top the primary
    glEnable(GL_BLEND);
    glActiveTexture(GL_TEXTURE1);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    checkGLError("glDrawArrays-SecondaryTexture");

    glDisableVertexAttribArray(vertPos);
    glDisableVertexAttribArray(texPos);

EDIT:I changed uniform location as well between the two glDrawArrays method… 0 when i need the primary and 1 when i need secondary

2

You can, but I don’t think that’s of much use for your application. Having more than one colour attachment is only useful if you have a fragment shader with multiple outputs.

I suspect that you just need to change which texture is bound to the FBO (or have multiple FBOs and change which FBO is bound).

3

Thanks for correcting me… When you said not changing attachment and still changing texture made figure it out… i was confusing color attachment with active texture… i thought for texture 0 i have to use attachment 0 and 1 for 1. Now it seems to appear correctly i have attachment 0 (which i dont change) and i only change the active texture number.

Tho i seem to have encountered another problem… can we select which texture on a render texture do we write to? Like i want to write on texture 0 with the renderframebuffer technique. But it seems to be writing over both the textures.

4

Rendering operations write to the texture(s) bound to the current framebuffer, and read from textures bound to texture units.

5

Thanks for the help, i had found the issue. I was wrongly rebinding another render texture on the same frame buffer… I only had to do


    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, _Width, _Height, 0, GL_RGBA, GL_UNSIGNED_BYTE, secondaryTextureDataStorage);

when ever i had to change the texture… Tho it didnt fullfil my later need because i had to read from it later, so i ended up making another fbo as well.