OpenGL - blend two textures on the same object

Question

I want to apply two textures on the same object (actually just a 2D rectangle) in order to blend them. I thought I would achieve that by simply calling glDrawElements with the first texture, then binding the other texture and calling glDrawElements a second time. Like this:

//create vertex buffer, frame buffer, depth buffer, texture sampler, build and bind model
//...

glEnable(GL_BLEND);
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ZERO);
glBlendEquation(GL_FUNC_ADD);

// Clear the screen
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Bind our texture in Texture Unit 0
GLuint textureID;
//create or load texture
//...
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, textureID);
// Set our  sampler to use Texture Unit 0
glUniform1i(textureSampler, 0);

// Draw the triangles !
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (void*)0);

//second draw call
GLuint textureID2;
//create or load texture
//...
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, textureID2);
// Set our sampler to use Texture Unit 0
glUniform1i(textureSampler, 0);

// Draw the triangles !
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (void*)0);

Unfortunately, the 2nd texture is not drawn at all and I only see the first texture. If I call glClear between the two draw calls, it correctly draws the 2nd texture.

Any pointers? How can I force OpenGL to draw on the second call?

Answer 1

As an alternative to the approach you followed so far I would like to suggest using two texture samplers within your GLSL shader and perform the blending there. This way, you would be done with just one draw call, thus reducing CPU/GPU interaction. To do so, just define to texture samplers in your shader like

layout(binding = 0) uniform sampler2D texture_0;
layout(binding = 1) uniform sampler2D texture_1;

Alternatively, you can use a sampler array:

layout(binding = 0) uniform sampler2DArray textures;

In your application, setup the textures and samplers using

enum Sampler_Unit{BASE_COLOR_S = GL_TEXTURE0 + 0, NORMAL_S = GL_TEXTURE0 + 2};
glActiveTexture(Sampler_Unit::BASE_COLOR_S);
glBindTexture(GL_TEXTURE_2D, textureBuffer1);
glTexStorage2D( ....)
glActiveTexture(Sampler_Unit::NORMAL_S);
glBindTexture(GL_TEXTURE_2D, textureBuffer2);
glTexStorage2D( ....)

Answer 2

Thanks to @tkausl for the tip.

I had depth testing enabled during the initialization phase.

// Enable depth test
glEnable(GL_DEPTH_TEST);
// Accept fragment if it closer to the camera than the former one
glDepthFunc(GL_LESS);

The option needs to be disabled in my case, for the blend operation to work.

//make sure to disable depth test
glDisable(GL_DEPTH_TEST);