# OpenGL OpenGL png transparency

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Hi all,

This is my first post here (so go easy on me . I have spent the better part of a week attempting to properly load a png image,bind it to a texture, set it to an object and have the transparent layer stay transparent while rendering the rest of the sprite(like a decal would appear, or a billboarded sprite). I've searched this forum rather thoroughly, but every solution I've found has seemed not to work for me. Any help would be genuinely and greatly appreciated As I'm uncertain where I'm going wrong, I'm uncertain what information you may need, but I'll try to include the essentials, an image of the semi-transparent sprite is included.

To the best of my knowledge, the png file(created in gimp) does contain the alpha layer and was saved correctly.

Am I failing to set the OpenGL Alpha layer correctly, or perhaps calling the BlendFunc in the wrong location? Perhaps I'm misunderstanding the BlendFunc. (I'm getting my information here:https://www.opengl.org/archives/resources/faq/technical/transparency.htm).

Things:

I'm using c++, codeblocks, Ubuntu 14.04, glfw3,  freeImage (and a little loader found on this forum for png files, included in the code below), nvidia 560 ti,

When i set :

glEnable(GL_BLEND);

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

nothing is rendered at all. Though, if I set :

glEnable(GL_BLEND);

glBlendFunc(GL_ONE, GL_ONE);

things actually appear to display correctly, however, while the transparent layer is completely transparent, the rest of the sprite appears semi-transparent as well.

I've attempted to display the sprite using GL_DECAL, as well as trying to set the alpha_state and have ended up with similar results. Either nothing is displayed at all, the transparent layer is displayed as completely black, or this state, where it is semi-transparent. I've tried calling the glEnable and glBlendFunc in various places. The one thing I can't seem to successfully code is having the sprite display without the background and not be semi-transparent itself.

I should note as well that if I change the Fragment shader color value to vec4 (it's currently set to a vec3) and include an alpha color (set to anything between 0.1 and 1) the glBLendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA) will display the sprite, but (perhaps predictably) the entire sprite is semi-transparent, though the transparent layer of the sprite appears to be black(just slightly transparent) and, thus, not set correctly.

Here is (what i think would be) the relevant code:

Main:

#include "Main.h"

int main()
{
glEnable(GL_TEXTURE_2D);

if(!glfwInit())
{
fprintf( stderr, "Failed to initialize glfw\n");
return -1;
}
glfwWindowHint(GLFW_SAMPLES, 4);
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 4);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

window = glfwCreateWindow(1920, 1080, "OpenGL is an ******", NULL, NULL);

if(window == NULL)
{
fprintf(stderr, "Failed to open GLFW window.");
glfwTerminate();
return -1;
}
glfwMakeContextCurrent(window);

glewExperimental = true;
if(glewInit() != GLEW_OK)
{
fprintf(stderr, "Failled to initialize GLEW\n");
return -1;
}

glfwSetInputMode(window, GLFW_STICKY_KEYS, GL_TRUE);
glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_HIDDEN);

glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
glEnable(GL_CULL_FACE);

Object Obj2("background.png","plane5.obj");
Object Obj1("tree.png","tree3.obj");

do
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram(Obj1.programID);

Obj2.Render();
Obj1.Render();

glfwSwapBuffers(window);
glfwPollEvents();
}
while(glfwGetKey(window, GLFW_KEY_ESCAPE) != GLFW_PRESS &&
glfwWindowShouldClose(window) == 0);

Obj1.CleanUp();
Obj2.CleanUp();

glfwTerminate();

return 0;
}



Object:

#include "Object.h"

Object::Object(const char* textureHere, const char* objHere) {

VertexArrayID;
glGenVertexArrays(1, &VertexArrayID);
glBindVertexArray(VertexArrayID);

MatrixID = glGetUniformLocation(programID, "MVP");
ViewMatrixID = glGetUniformLocation(programID, "V");
ModelMatrixID = glGetUniformLocation(programID, "M");
PlaneMatrixID = glGetUniformLocation(programID, "P");

res = loadOBJ(objHere, vertices, uvs, normals);

glGenBuffers(1, &vertexbuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(glm::vec3), &vertices[0], GL_STATIC_DRAW);

glGenBuffers(1, &uvbuffer);
glBindBuffer(GL_ARRAY_BUFFER, uvbuffer);
glBufferData(GL_ARRAY_BUFFER, uvs.size() * sizeof(glm::vec2), &uvs[0], GL_STATIC_DRAW);

glGenBuffers(1, &normalbuffer);
glBindBuffer(GL_ARRAY_BUFFER, normalbuffer);
glBufferData(GL_ARRAY_BUFFER, normals.size() * sizeof(glm::vec3), &normals[0], GL_STATIC_DRAW);

glUseProgram(programID);
LightID = glGetUniformLocation(programID, "LightPosition_worldspace");
}

Object::Object(const Object& orig) {
}

Object::~Object() {
}

void Object::Render()
{
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE);

computeMatricesFromInputs();
glm::mat4 ProjectionMatrix = getProjectionMatrix();
glm::mat4 ViewMatrix = getViewMatrix();
glm::mat4 ModelMatrix = glm::mat4(1.0);
glm::mat4 MVP = ProjectionMatrix * ViewMatrix * ModelMatrix;

glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);
glUniformMatrix4fv(ModelMatrixID, 1, GL_FALSE, &ModelMatrix[0][0]);
glUniformMatrix4fv(ViewMatrixID, 1, GL_FALSE, &ViewMatrix[0][0]);

glm::vec3 lightPos = glm::vec3(4,4,4);
glUniform3f(LightID, lightPos.x, lightPos.y, lightPos.z);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, Texture);

glUniform1i(TextureID, 0);

glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glVertexAttribPointer(
0,              //attribute
3,              //size
GL_FLOAT,       //type
GL_FALSE,       //normalized?
0,              //stride...whateverthehell that is
(void*)0        //array buffer offset
);

// 2nd attribute buffer : uvs
glEnableVertexAttribArray(1);
glBindBuffer(GL_ARRAY_BUFFER, uvbuffer);
glVertexAttribPointer(
1,                                // attribute.
2,                                // size : u + v = 2
GL_FLOAT,                         // type
GL_FALSE,                         // normalized?
0,                                // stride
(void*)0                          // array buffer offset
);

//3rd attribute normals
glEnableVertexAttribArray(2);
glBindBuffer(GL_ARRAY_BUFFER, normalbuffer);
glVertexAttribPointer(
2,              //attribute
3,              //size
GL_FLOAT,        //type)
GL_FALSE,       //normalized?
0,              //stride
(void*)0        //array buffer offset

);
glDrawArrays(GL_TRIANGLES, 0, vertices.size() );

glDisableVertexAttribArray(0);
glDisableVertexAttribArray(1);
glDisableVertexAttribArray(2);
}

void Object::CleanUp()
{
glDeleteBuffers(1, &vertexbuffer);
glDeleteBuffers(1, &uvbuffer);
glDeleteBuffers(2, &normalbuffer);
glDeleteProgram(programID);
glDeleteTextures(1, &TextureID);
glDeleteVertexArrays(1, &VertexArrayID);
}

{
FREE_IMAGE_FORMAT formato =
FreeImage_GetFileType(imageName,0);

FIBITMAP* temp = imagen;

imagen = FreeImage_ConvertTo32Bits(imagen);

int w = FreeImage_GetWidth(imagen);
int h = FreeImage_GetHeight(imagen);

GLubyte* textura = new GLubyte[4*w*h];
char* pixels = (char*)FreeImage_GetBits(imagen);

for(int j= 0; j<w*h; j++){
textura[j*4+0]= pixels[j*4+2];
textura[j*4+1]= pixels[j*4+1];
textura[j*4+2]= pixels[j*4+0];
textura[j*4+3]= pixels[j*4+3];
}
GLuint TextureID;
glGenTextures(1, &TextureID);
glBindTexture(GL_TEXTURE_2D, TextureID);
if(FreeImage_GetBPP(imagen) == 24)
glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA, w, h, 0,
GL_RGB,GL_UNSIGNED_BYTE,(GLvoid*)textura );
else if(FreeImage_GetBPP(imagen) == 32)
glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA, w, h, 0,
GL_RGBA,GL_UNSIGNED_BYTE,(GLvoid*)textura );
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

return TextureID;
}



Let me know if you need any more relevant information. And, my apologies for posting yet another png transparency thread. I'm out of ideas, so create yet another thread on this out of simple desperation. The solution on almost every thread appears to be to simply set glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);, which results in nothing being displayed at all.

Thank you in advance for any assistance,

Edited by Misantes

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FREE_IMAGE_FORMAT formato =
FreeImage_GetFileType(imageName,0);

FIBITMAP* temp = imagen;

imagen = FreeImage_ConvertTo32Bits(imagen);

If your .png is authored and is loaded correctly, then it should already be a 32 bit image. I wonder if it's only 24-bit and FreeImage_ConvertTo32Bits is adding an empty alpha channel for you.

Also, could you post your fragment shader, please? The line "I should note as well that if I change the Fragment shader color value to vec4 (it's currently set to a vec3) and include an alpha color (set to anything between 0.1 and 1)" sounds a bit wrong, you probably should be dealing with vec4s when you're manipulating rgba colours.

I suppose that to narrow down whether it's a image loading or a rendering problem, you could experiment by modifying the "textura[j*4+3]= pixels[j*4+3];" line to "textura[j*4+3]= 128;" and see if that gives you a semi-transparent image. If so, then you know it's your image loading, otherwise it's your rendeiring. (Actually maybe it's textura[j*4+0] which represents alpha, I can never remember which order these things are supposed to be)

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Hey, thank you so much for the response.

#version 330 core

// Interpolated values from the vertex shaders
in vec2 UV;
in vec3 Position_worldspace;
in vec3 Normal_cameraspace;
in vec3 EyeDirection_cameraspace;
in vec3 LightDirection_cameraspace;

// Ouput data
out vec3 color;

// Values that stay constant for the whole mesh.
uniform sampler2D myTextureSampler;
uniform mat4 MV;
uniform vec3 LightPosition_worldspace;

void main(){

// Light emission properties
// You probably want to put them as uniforms
vec3 LightColor = vec3(1,1,1);
float LightPower = 50.0f;

// Material properties
vec3 MaterialDiffuseColor = texture2D( myTextureSampler, UV ).rgb;
vec3 MaterialAmbientColor = vec3(0.5,0.5,0.5) * MaterialDiffuseColor;
vec3 MaterialSpecularColor = vec3(0.3,0.3,0.3);

// Distance to the light
float distance = length( LightPosition_worldspace - Position_worldspace );

// Normal of the computed fragment, in camera space
vec3 n = normalize( Normal_cameraspace );
// Direction of the light (from the fragment to the light)
vec3 l = normalize( LightDirection_cameraspace );
// Cosine of the angle between the normal and the light direction,
// clamped above 0
//  - light is at the vertical of the triangle -> 1
//  - light is perpendicular to the triangle -> 0
//  - light is behind the triangle -> 0
float cosTheta = clamp( dot( n,l ), 0,1 );

// Eye vector (towards the camera)
vec3 E = normalize(EyeDirection_cameraspace);
// Direction in which the triangle reflects the light
vec3 R = reflect(-l,n);
// Cosine of the angle between the Eye vector and the Reflect vector,
// clamped to 0
//  - Looking into the reflection -> 1
//  - Looking elsewhere -> < 1
float cosAlpha = clamp( dot( E,R ), 0,1 );

color.rgb =
// Ambient : simulates indirect lighting
MaterialAmbientColor +
// Diffuse : "color" of the object
MaterialDiffuseColor * LightColor * LightPower * cosTheta / (distance*distance) +
// Specular : reflective highlight, like a mirror
MaterialSpecularColor * LightColor * LightPower * pow(cosAlpha,5) / (distance*distance);
//color.a = 0.3;
}


As I'm new to OpenGL and haven't gotten around to writing my own shaders yet, I used the shader from the tutorials on http://www.opengl-tutorial.org/, so it's entirely possible it's not suitable for what I'm trying to do as its just a simple example shader. I've left in the commented out line //color.a = 0.3; My previous comment was about trying to change  "out vec3 color;" to "out vec4 color" and adding the "color.a = 0.3" at the bottom. Again, when I do that, it sill appears semi-transparent, but the transparent layer appears as a semi-transparent black instead of being completely transparent (which kind of makes sense, i think, as I'm then having the shader make everything it renders semi-transparent..If I'm following how shaders work at all).

If I change textura[j*4+3]= pixels[j*4+3];" line to "textura[j*4+3]= 128; I do still end up with a semi-transparent image(it is the pixels[j*4+3] and not pixels[j*4+3] as that line appears to be the red value).

As I mentioned, the LoadImage function I got on this forum, but it seemed to work so I ran with it. It looks as though the convert function is just there to make sure the image ends up as 32 bits. However, If I comment out that line (as well as the one above and below), the image still appears semi-transparent.

FREE_IMAGE_FORMAT formato =
FreeImage_GetFileType(imageName,0);

//FIBITMAP* temp = imagen;

//imagen = FreeImage_ConvertTo32Bits(imagen);
//FreeImage_Unload(temp); 

I also tried commenting out

/*if(FreeImage_GetBPP(imagen) == 24)
glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA, w, h, 0,
GL_RGB,GL_UNSIGNED_BYTE,(GLvoid*)textura );
else if(FreeImage_GetBPP(imagen) == 32)*/


and it appears to have no effect either. I also tried changing the above code(still with the "FreeImage_ConvertTo32Bits" function commented out)  to:

if(FreeImage_GetBPP(imagen) == 24)
{
std::cout<<"This is 24 bits, dummy."<<std::endl;
}

else if(FreeImage_GetBPP(imagen) == 32)
glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA, w, h, 0,
GL_RGBA,GL_UNSIGNED_BYTE,(GLvoid*)textura );


and it appears to be loading the image as 32 bits when it runs (as the image still appears semi-transparent, and the console doesn't return the error).

I tried all of those changes with glBlendFunc set to both(GL_ONE, GL_ONE) and glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA).

Do you still think it may be the LoadImage Function, or does the shader maybe look problematic?

Thanks again for your help, I truly appreciate it! This has been one of those bang-my-head-against-the-desk problems for a week now . I'm trying to take a game I've made in 2D to a 3D environment with billboarding to give it a bit more depth. So, this is pretty much my first foray into OpenGL, other than some tinkering with it here and there. So, I'm not completely new, but new enough that i'm going to look stupid when we figure this out

Also, for reference, here is what it looks like with the shader changed to vec4 and color.a set to 0.3 and glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA).

and with color set to "color.a = 1.0;

When it is set to 1.0 it seems to be more of the typical problem where  the PNG is just displaying the transparency as black.

Edited by Misantes

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Hello Misantes, It seems to me that the shader you have there doesn't account for any change in alpha from the texture.
when you converted your vec3 color to a vec4 and inserted 0.3 into the alpha channel you essentially made every pixel be the same amount of transparency regardless of the texture.

To get this working and using your textures transparency you will need to change it to a vec4 but instead use the alpha channel from your texture sampler.

The easiest fix I can see is to just replace the line:

vec3 MaterialDiffuseColor = texture2D( myTextureSampler, UV ).rgb;

with

vec4 MaterialDiffuseColor = texture2D( myTextureSampler, UV );

you will then need to put .rgb on the end of anywhere you have you MaterialDiffuseColor.

Then replace your color.a = 0.3; with color.a = MaterialDiffuseColor.a;

I hope that makes sense, and if you would like for me to explain further why this needs to be done just ask :)

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#version 330 core

// Interpolated values from the vertex shaders
in vec2 UV;
in vec3 Position_worldspace;
in vec3 Normal_cameraspace;
in vec3 EyeDirection_cameraspace;
in vec3 LightDirection_cameraspace;

// Ouput data
out vec4 color;

// Values that stay constant for the whole mesh.
uniform sampler2D myTextureSampler;
uniform mat4 MV;
uniform vec3 LightPosition_worldspace;

void main(){

// Light emission properties
// You probably want to put them as uniforms
vec3 LightColor = vec3(1,1,1);
float LightPower = 50.0f;

// Material properties
vec4 MaterialDiffuseColor = texture2D( myTextureSampler, UV );
vec3 MaterialAmbientColor = vec3(0.5,0.5,0.5) * MaterialDiffuseColor;
vec3 MaterialSpecularColor = vec3(0.3,0.3,0.3);

// Distance to the light
float distance = length( LightPosition_worldspace - Position_worldspace );

// Normal of the computed fragment, in camera space
vec3 n = normalize( Normal_cameraspace );
// Direction of the light (from the fragment to the light)
vec3 l = normalize( LightDirection_cameraspace );
// Cosine of the angle between the normal and the light direction,
// clamped above 0
//  - light is at the vertical of the triangle -> 1
//  - light is perpendicular to the triangle -> 0
//  - light is behind the triangle -> 0
float cosTheta = clamp( dot( n,l ), 0,1 );

// Eye vector (towards the camera)
vec3 E = normalize(EyeDirection_cameraspace);
// Direction in which the triangle reflects the light
vec3 R = reflect(-l,n);
// Cosine of the angle between the Eye vector and the Reflect vector,
// clamped to 0
//  - Looking into the reflection -> 1
//  - Looking elsewhere -> < 1
float cosAlpha = clamp( dot( E,R ), 0,1 );

color.rgb =
// Ambient : simulates indirect lighting
MaterialAmbientColor +
// Diffuse : "color" of the object
MaterialDiffuseColor.rgb * LightColor * LightPower * cosTheta / (distance*distance) +
// Specular : reflective highlight, like a mirror
MaterialSpecularColor * LightColor * LightPower * pow(cosAlpha,5) / (distance*distance);
color.a = MaterialDiffuseColor.a;
} 

Hm, That results in an error:

0(28) : error C7011: implicit cast from "vec4" to "vec3"

And, I would welcome any explanation, but don't feel obligated as my knowledge of shaders is pretty limited still.
Also, thanks for the help

Shader now looks like the above.

Edit* nevermind, i fixed the error (it's 330am for me >,<), it now runs, but it is still showing up as semi-transparent.

Edit#2*
I changed the glBlendFunc to glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) and it now shows up perfectly!
Thanks so much to both you and C0lumbo! You've both been a giant help

See, I knew I'd look stupid Time to go read up on shaders  I'd still love a quick explanation if you have a moment though. I feel like I almost understand, but the concept is eluding me a little (a lot). I'm happy to do my own homework, usually, but sometimes I quick explanation can set me in the right direction to understanding it.

Cheers, and thanks again to both of you.

(and to anyone reading this later, no laughing at the art, It's just placeholder an artist-friend comes through with the finished sprites. I can't draw :))
Edited by Misantes

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No problem, it would have taken you a lot of work to figure that out considering you don't know much about shaders.

When using the texture2D function it samples a value from the texture that you have assigned to the shader. This combined with other values coming from light sources and the other material properties give you your final output colour. Without having an alpha channel it must force the alpha channel to be 1.0f making them opaque;

Shaders are great to play with and you can make some pretty cool effects. I would suggest checking out www.open.gl for some pretty decent tutorials into the basics of openGL and shaders.

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Just an FYI, your are using texture2D which has been deprecated since GLSL 1.30. You should just use texture instead, which overloads based on the sampler type.

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No problem, it would have taken you a lot of work to figure that out considering you don't know much about shaders.

When using the texture2D function it samples a value from the texture that you have assigned to the shader. This combined with other values coming from light sources and the other material properties give you your final output colour. Without having an alpha channel it must force the alpha channel to be 1.0f making them opaque;

Shaders are great to play with and you can make some pretty cool effects. I would suggest checking out www.open.gl for some pretty decent tutorials into the basics of openGL and shaders.

Yes, I have a feeling I would have been stuck on that for a very long time(well, longer than it already has).

And, thank you for the tutorial link.

Just an FYI, your are using texture2D which has been deprecated since GLSL 1.30. You should just use texture instead, which overloads based on the sampler type.

And thank you too. I seem to have had the damndest time finding any books/tutorials that aren't out of date. I'll check out the tutorial links above, and thanks again to everyone.

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