All geek questions in one place

How to create a texture stencil buffer (image) in OpenGL-ES 2.0

Is it possible to create a stencil with a texture (image) in OpenGL 2.0 so that some part of the image is transparent, and as a result it will be transferred as is to the Stencil buffer, and then use this stencil buffer for further drawing.

EDIT datenwolf to account for updated OPs questions in the answer:

By @InfiniteLoop:

@datenwolf thanks ur for the answer but fails :( here is my code

- (void)render { [EAGLContext setCurrentContext:context]; glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebuffer); glUseProgram(program); glClearColor(0, 104.0/255.0, 55.0/255.0, 1.0); glViewport(0, 0, backingWidth, backingHeight); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT); // Set the stencil clear value glClearStencil ( 0x1 ); // Set the depth clear value glClearDepthf( 0.75f ); //////// //// GLfloat threshold = 0.5f;//half transparency /* the order in which orthogonal OpenGL state is set doesn't matter */ glEnable(GL_STENCIL_TEST); glEnable(GL_ALPHA_TEST); /* don't write to color or depth buffer */ glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE); glDepthMask(GL_FALSE); glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE); /* first set alpha test so that fragments greater than the threshold will pass thus will set nonzero bits masked by 1 in stencil */ glStencilFunc(GL_ALWAYS, 1, 1);//set one glAlphaFunc(GL_GREATER, threshold);//greater than half transparency glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 0, fullVertices); glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, colorVertices); glVertexAttribPointer(textureLoc, 2, GL_FLOAT, GL_FALSE, 0, textureVertices); [self drawTexture:texture2DMaskImage]; /* second pass of the fragments less or equal than the threshold will pass thus will set zero bits masked by 1 in stencil */ glStencilFunc(GL_ALWAYS, 0, 1);//set zero glAlphaFunc(GL_LEQUAL, threshold);//Less than Half transparency glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 0, fullVertices); glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, colorVertices); glVertexAttribPointer(textureLoc, 2, GL_FLOAT, GL_FALSE, 0, textureVertices); [self drawTexture:texture2DMaskImage]; // till here as suggested by u after this I have added to draw // the next Image which will use the created Stencil glDepthMask(GL_TRUE); glDisable(GL_ALPHA_TEST); glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE); // glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);//stop further write to stencil buffer glStencilFunc(GL_EQUAL, 0, 1);//pass if 0 glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 0, fullVertices); glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, colorVertices); glVertexAttribPointer(textureLoc, 2, GL_FLOAT, GL_FALSE, 0, textureVertices); [self drawTexture:texture2D]; //// /////// // Validate program before drawing. This is a good check, // but only really necessary in a debug build. // DEBUG macro must be defined in your debug configurations // if that not already the case. #if defined(DEBUG) if (![self validateProgram:program]) { NSLog(@"Failed to validate program: %d", program); return; } #endif // draw [context presentRenderbuffer:GL_RENDERBUFFER];//Present } - (void) drawTexture:(Texture2D*)texture { // handle viewing matrices GLfloat proj[16], modelview[16], modelviewProj[16]; // setup projection matrix (orthographic) mat4f_LoadOrtho(-1.0f, 1.0f, -1.0f, 1.0f, -1.0f, 1.0f, proj); // glUniformMatrix4fv(uniforms[UNIFORM_PROJECTION], 1, 0, proj); // setup modelview matrix (rotate around z) mat4f_LoadIdentity(modelview); mat4f_LoadZRotation(0.0, modelview); mat4f_LoadTranslation3D(0.0, 0.0, 0.0, modelview); // projection matrix * modelview matrix mat4f_MultiplyMat4f(proj, modelview, modelviewProj); glUniformMatrix4fv(_modelViewUniform, 1, 0, modelviewProj); // update uniform values glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture.name); glUniform1i(_textureUniform, 0); glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, cubeIndices); }

Please shed some light. I am in desperate need :( Thanks again.

+6
source share
2 answers

Finally, I managed to do this using conditional color rendering using the discard keyword in the shader. I also used 2 textures in the same shader and merged them accordingly.

Thanks to everyone.

+1
source

Yes, you can use alpha testing ( glEnable(GL_ALPHA_TEST); glAlphaFunc(COMPARISION, VALUE); ) to remove fragments; discarded fragments will not be checked by stencil, so you can use the skipped fragments to create a stencil mask.

Sample EDIT Code

 /* the order in which orthogonal OpenGL state is set doesn't matter */ glEnable(GL_STENCIL_TEST); glEnable(GL_ALPHA_TEST); /* don't write to color or depth buffer */ glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE); glDepshMask(GL_FALSE); glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE); /* first set alpha test so that fragments greater than the threshold will pass thus will set nonzero bits masked by 1 in stencil */ glStencilFunc(GL_ALWAYS, 1, 1); glAlphaFunc(GL_GREATER, threshold); draw_textured_primitive(); /* second pass of the fragments less or equal than the threshold will pass thus will set zero bits masked by 1 in stencil */ glStencilFunc(GL_ALWAYS, 0, 1); glAlphaFunc(GL_LEQUAL, threshold); draw_textured_primitive();

Remember to return OpenGL state for later drawing.

EDIT to account for updated question:

Since you are actually using iOS, this way, OpenGL-ES 2.0 things get a little different. OpenGL-ES2 does not have an alpha test. Instead, you should discard the fragment in the fragment shader if it is below / above the selected threshold using the keyword / discard operator.

+7
source

Source: https://habr.com/ru/post/898151/

More articles:


All Articles