Develop Reference

Render FBO to same FBO

I am trying to create a ghost-like camera filter. This requires mixing the previous frame to current one. I use one FBO to make the mixing and a second one to simply put the context to the screen.
My implementation works on 4 out of 5 devices I have tried. On the fifth (Samsung galaxy S7) I get some random pixels.
The simpler shader to reproduce the error is the following (the frame counter and cropping is just for debugging). The result is that I get on the center of the screen on line gradually going up.
uniform samplerExternalOES camTexture;
uniform sampler2D fbo;
uniform int frame_no;
varying vec2 v_CamTexCoordinate;
void main ()
{
vec2 uv = v_CamTexCoordinate;
if(frame_no<10){
gl_FragColor = texture2D(camTexture, uv);
}else{
if(uv.y>0.2 && uv.y<0.8 && uv.x>0.2 && uv.x<0.8)
gl_FragColor = texture2D(fbo, uv + vec2(0.0, +0.005));
else
gl_FragColor = texture2D(camTexture, uv);
}
}
But on the Samsung I get some correct pixels and some random ones as the following sample. Some black and other random pixels going up together with the camera's pixels. Any idea of what might be wrong?
Fault sample
Correct sample

Point sprite alpha blending issue (Android / OpenGL ES 2.0)

I've recently started looking into OpenGL ES for Android and am working on a drawing app. I've implemented some basics such as point sprites, path smoothing and FBO for double buffering. At the moment I am playing around with the glBlendFunc, more specifically when I put two textures close to each other with the same color/alpha values, the alpha gets added so it appears darker at the intersection of the sprites. This is a problem because the stroke opacity is not preserved if a lot of points are close together, as the color tends to more opaque rather than staying with the same opacity. Is there a way to make the textures have the same color on the intersection, i.e. have the same alpha value for the intersecting pixels, but keep the alpha values for the rest of the pixels?
Here's how I've done the relevant parts of the app:
for drawing the list of point sprites I use blending like this:
GLES20.glEnable(GLES20.GL_BLEND);
GLES20.glBlendFunc(GLES20.GL_ONE, GLES20.GL_ONE_MINUS_SRC_ALPHA);
the app uses an FBO with a texture, where it renders each brush stroke first and then this texture is rendered to the main screen. The blending func there is:
GLES20.glEnable(GLES20.GL_BLEND);
GLES20.glBlendFunc(GLES20.GL_SRC_ALPHA, GLES20.GL_ONE_MINUS_SRC_ALPHA);
OpenGL ES 2.0 doesn't support alpha masking;
there is no DEPTH_TEST function used anywhere in the app;
the textures for the point sprites are PNGs with transparent backgrounds;
the app supports texture masking which means one texture is used for the shape and one texture is used for the content;
my fragment shader looks like this:
precision mediump float;
uniform sampler2D uShapeTexture;
uniform sampler2D uFillTexture;
uniform float vFillScale;
varying vec4 vColor;
varying float vShapeRotation;
varying float vFillRotation;
varying vec4 vFillPosition;
vec2 calculateRotation(float rotationValue) {
float mid = 0.5;
return vec2(cos(rotationValue) * (gl_PointCoord.x - mid) + sin(rotationValue) * (gl_PointCoord.y - mid) + mid,
cos(rotationValue) * (gl_PointCoord.y - mid) - sin(rotationValue) * (gl_PointCoord.x - mid) + mid);
}
void main() {
// Calculations.
vec2 rotatedShape = calculateRotation(vShapeRotation);
vec2 rotatedFill = calculateRotation(vFillRotation);
vec2 scaleVector = vec2(vFillScale, vFillScale);
vec2 positionVector = vec2(vFillPosition[0], vFillPosition[1]);
// Obtain colors.
vec4 colorShape = texture2D(uShapeTexture, rotatedShape);
vec4 colorFill = texture2D(uFillTexture, (rotatedFill * scaleVector) + positionVector);
gl_FragColor = colorShape * colorFill * vColor;
}
my vertex shader is this:
attribute vec4 aPosition;
attribute vec4 aColor;
attribute vec4 aJitter;
attribute float aShapeRotation;
attribute float aFillRotation;
attribute vec4 aFillPosition;
attribute float aPointSize;
varying vec4 vColor;
varying float vShapeRotation;
varying float vFillRotation;
varying vec4 vFillPosition;
uniform mat4 uMVPMatrix;
void main() {
// Sey position and size.
gl_Position = uMVPMatrix * (aPosition + aJitter);
gl_PointSize = aPointSize;
// Pass values to fragment shader.
vColor = aColor;
vShapeRotation = aShapeRotation;
vFillRotation = aFillRotation;
vFillPosition = aFillPosition;
}
I've tried playing around with the glBlendFunc parameters but I can't find the right combination to draw what I want. I've attached some images showing what I would like to achieve and what I have at the moment. Any suggestions?
The Solution
Finally managed to get this working properly with a few lines thanks to # Rabbid76. First of all I had to configure my depth test function before I draw to the FBO:
GLES20.glEnable(GLES20.GL_DEPTH_TEST);
GLES20.glDepthFunc(GLES20.GL_LESS);
// Drawing code for FBO.
GLES20.glDisable(GLES20.GL_DEPTH_TEST);
Then in my fragment shader I had to make sure that any pixels with alpha < 1 in the mask are discarded like this:
...
vec4 colorMask = texture2D(uMaskTexture, gl_PointCoord);
if (colorMask.a < 1.0)
discard;
else
gl_FragColor = calculatedColor;
And the result is (flickering is due to Android emulator and gif capture tool):

If you set the glBlendFunc
with the functions (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) and you use
glBlendEquation with the equation GL_FUNC_ADD then the destination color is
calculated as follows:
C_dest = C_src * A_src + C_dest * (1-A_src)
If you blend for example C_dest = 1 with C_src = 0.5 and A_src = 0.5 then:
C_dest = 0.75 = 1 * 0.5 + 0.5 * 0.5
If you repeat blending the same color C_src = 0.5 and A_src = 0.5 then the destination color becomes darker:
C_dest = 0.625 = 0.75 * 0.5 + 0.5 * 0.5
Since the new target color is always a function of the original target color and the source color, the color can not remain equel when blending 2 times, because the target color has already changed after the 1st time blending (except GL_ZERO).
You have to avoid that any fragment is blended twice. If all fragments are drawn to the same depth (2D) then you can use the depth test for this:
glEnable( GL_DEPTH_TEST );
glDepthFunc( GL_LESS );
// do the drawing with the color
glDisable( GL_DEPTH_TEST );
Or the stencil test can be used. For example, the stencil test can be set to pass only when the stencil buffer is equal to 0.
Every time a fragment is to be written the stencil buffer is incremented:
glClear( GL_STENCIL_BUFFER_BIT );
glEnable( GL_STENCIL_TEST );
glStencilOp( GL_KEEP, GL_KEEP, GL_INCR );
glStencilFunc( GL_EQUAL, 0, 255 );
// do the drawing with the color
glDisable( GL_STENCIL_TEST );
Extension to the answer
Note that you can discard fragments which should not be drawn.
If the fragment in your sprite texture has an alpha channel of 0 you should discard it.
Note, if you discard a fragment neither the color nor the depth and stencil buffer will be written.
Fragment shaders also have access to the discard command. When executed, this command causes the fragment's output values to be discarded. Thus, the fragment does not proceed on to the next pipeline stages, and any fragment shader outputs are lost.
Fragment shader
if ( color.a < 1.0/255.0 )
discard;

It's not possible to do this using the fixed-function blending in OpenGL ES 2.0, because what you want isn't actually alpha blending. What you want is a logical operation (e.g. max(src, dst)) which is rather different to how OpenGL ES blending works.
If you want to do path / stroke / fill rendering with pixel-exact edges you might get somewhere with using stencil masks and stencil tests, but you can't do transparency in this case - just boolean operators.

GLSL - fragment shaders for image manipulation/blending

I'm trying to build a shader that allows you to combine two images by applying a gradient opacity mask to the one on top, like you do in photoshop. I've gotten to the point where I can overlay the masked image over the other but as a newbie am confused about a few things.
It seems that the images inside the shader are sometimes skewed to fit the canvas size, and they always start at position 0,0. I have played around with a few snippets I have found to try and scale the textures, but always end up with unsatisfactory results.
I am curious if there is a standard way to size, skew, and translate textures within a view, or if images in GLSL are necessarily limited in some way that will stop me from accomplishing my goal.
I'm also unsure of how I am applying the gradient/mask and if it is the right way to do it, because I do not have a lot of control over the shape of the gradient at the moment.
Here's what I have so far:
precision highp float;
varying vec2 uv;
uniform sampler2D originalImage;
uniform sampler2D image;
uniform vec2 resolution;
void main(){
float mask;
vec4 result;
vec2 position = gl_FragCoord.xy / ((resolution.x + resolution.y) * 2.0 );
mask = vec4(1.0,1.0,1.0,1.0 - position.x).a;
vec4 B = texture2D(image,uv);
vec4 A = texture2D(originalImage,uv) * (mask);
result = mix(B, A, A.a);
gl_FragColor = result;
}
Which produces an image like this:
What I would like to be able to do is change the positions of the images independently and also make sure that they conform to their proper dimensions.
I have tried naively shifting positions like this:
vec2 pos = uv;
pos.y = pos.y + 0.25;
texture2D(image, pos)
Which does shift the texture, but leads to a bunch of strange lines dragging:
I tried to get rid of them like this:
gl_FragColor = uv.y < 0.25 ? vec4(0.0,0.0,0.0,0.0) : result;
but it does nothing

You really need to decide what you want to happen when images are not the same size. What you probably want is for it to appear there's no image so check your UV coordinates and use 0,0,0,0 when outside of the image
//vec4 B = texture2D(image,uv);
vec4 getImage(sampler2D img, vec2 uv) {
if (uv.x < 0.0 || uv.x > 1.0 || uv.y < 0.0 || uv.y > 1.0) {
return vec4(0);
}
return texture2D(img, uv);
}
vec4 B = getImage(image, uv);
As for a standard way to size/skew/translate images use a matrix
uniform mat4 u_imageMatrix;
...
vec2 newUv = u_imageMatrix * vec4(uv, 0, 1).xy;
An example of implementing canvas 2d's drawImage using a texture matrix.
In general though I don't think most image manipulations programs/library would try to do everything in the shader. Rather they'd build up the image with very very simple primitives. My best guess would be they use a shader that's just A * MASK then draw B followed by A * MASK with blending on.
To put it another way, if you have 30 layers in photoshop they wouldn't generate a single shader that computes the final image in one giant shader taking in all 30 layers at once. Instead each layer would be applied on its own with simpler shaders.
I also would expect them to create an texture for the mask instead of using math in the shader. That way the mask can be arbitrarily complex, not just a 2 stop ramp.
Note I'm not saying you're doing it wrong. You're free to do whatever you want. I'm only saying I suspect that if you want to build a generic image manipulation library you'll have more success with smaller building blocks you combine rather than trying to do more complex things in shaders.
ps: I think getImage can be simplified to
vec4 getImage(sampler2D img, vec2 uv) {
return texture2D(img, uv) * step(0.0, uv) * step(-1.0, -uv);
}

Vertex texture Fetch (Texture read in vertex shader)

New to Opengl and GLSL.
I am using OpenGL es 3.0 and my GLSL version #version 300 es.
i want to get pixel(ARGB data) at every position in my vertex shader(vertex texture fetch). i have verified that my Android tablet supports vertex texture fetch.
Now i pass in the texture(image) and Texture coordinates to the vertex shader
and execute
GLES30.glDrawArrays(GLES30.GL_TRIANGLE_STRIP, 0, 4);
Is this the right way or should i use GL_POINTS .
if i am using GL_POINTS how to pass the texture cooordinate?
could you provide any samples/example code that does a full pixel read(ARGB) in the vertex shader.
attaching my vertex shader
uniform sampler2D sTexture;
in vec4 aTextureCoord;
out vec3 colorFactor;
vec2 vTextureCoord;
vec4 tex;
void main()
{
vTextureCoord = aTextureCoord.xy;
tex = texture(sTexture,vTextureCoord);
float luminance = 0.299 * tex.r + 0.587 * tex.g + 0.114 * tex.b;
colorFactor = vec3(1.0, 1.0, 1.0);
gl_Position = vec4(-1.0 + (luminance * 0.00784313725), 0.0, 0.0, 1.0);
gl_PointSize = 1.0;
};
My texture coordinates passed are
{0.f,1.f}
{1.f,1.f} {0.f,0.f}
{1.f,0.f}
and the shader is triggered by
GLES30.glDrawArrays(GLES30.GL_TRIANGLE_STRIP, 0, 4);

Just declare a sampler and sample it exactly as per usual. E.g.
"#version 150\n"
in vec4 position;
in vec2 texCoordinate;
uniform sampler2D texID;
uniform mat4 modelViewProjection;
void main()
{
gl_Position = modelViewProjection * (position + texture(texID, texCoordinate));
}
That will sample a 4d vector from the texture unit texID at location texCoordinate, using that to perturb position prior to applying the model-view-projection matrix. The type of geometry you're drawing makes no difference.

gl_Position = vec4(-1.0 + (luminance * 0.00784313725), 0.0, 0.0, 1.0);
That's clever. It's not going to work, but that's clever.
The thing that is confusing everyone is what you haven't told us. That you're computing the histogram by using blending. That you compute the location for each fragment based on the luminance, so you get lots of overlap. And blending just adds everything together, thus producing your histogram.
FYI: It's always best to explain what it is you're actually trying to accomplish in your question, rather than hoping that someone can deduce what you're attempting to do.
That's not going to work because you only have four vertices in this case. You have lots of fragments, but they will be generated based on interpolation from your 4 vertices. And you can't change the position of a fragment from within a fragment shader.
If you want to do what you're trying to do, you still have to render one vertex for every texel you fetch. You still need to use GL_POINTS.

GLSL for OpenGL ES Android

I want to use shading on my OpenGL objects but can't seem to access GLSL functions in my opengl package. Is there a GLSL package available for OpenGL ES in Eclipse?
EDIT: As Tim pointed out. Shaders are written as text files and then loaded using glShaderSoure, I have a C++ shader file which I had written once for a ray tracing application. But, I am really confused as to how I would go about implementing in java. Suppose I have a 2D square drawn in my Renderer class using a gl object MySquare , how will I go about implementing the java equivalent of the shader file below.
Shader.vert
varying vec3 N;
varying vec3 v;
void main()
{
// Need to transform the normal into eye space.
N = normalize(gl_NormalMatrix * gl_Normal);
// Always have to transform vertex positions so they end
// up in the right place on the screen.
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}
// Fragment shader for per-pixel Phong interpolation and shading.
Shader.Frag
// The "varying" keyword means that the parameter's value is interpolated
// between the nearby vertices.
varying vec3 N;
varying vec3 v;
//Used for Environmental mapping shader calculations
const vec3 xUnitVec=vec3(1.0, 0.0, 0.0), yUnitVec=vec3(1.0, 1.0, 0.0);
uniform vec3 BaseColor, MixRatio;
uniform sampler2D EnvMap;
void main()
{
// The scene's ambient light.
vec4 ambient = gl_LightModel.ambient * gl_FrontMaterial.ambient;
// The normal vectors is generally not normalized after being
// interpolated across a triangle. Here we normalize it.
vec3 Normal = normalize(N);
// Since the vertex is in eye space, the direction to the
// viewer is simply the normalized vector from v to the
// origin.
vec3 Viewer = -normalize(v);
// Get the lighting direction and normalize it.
vec3 Light = normalize(gl_LightSource[0].position.xyz);
// Compute halfway vector
vec3 Half = normalize(Viewer+Light);
// Compute factor to prevent light leakage from below the
// surface
float B = 1.0;
if(dot(Normal, Light)<0.0) B = 0.0;
// Compute geometric terms of diffuse and specular
float diffuseShade = max(dot(Normal, Light), 0.0);
float specularShade =
B * pow(max(dot(Half, Normal), 0.0), gl_FrontMaterial.shininess);
// Compute product of geometric terms with material and
// lighting values
vec4 diffuse = diffuseShade * gl_FrontLightProduct[0].diffuse;
vec4 specular = specularShade * gl_FrontLightProduct[0].specular;
ambient += gl_FrontLightProduct[0].ambient;
// Assign final color
gl_FragColor= ambient + diffuse + specular + gl_FrontMaterial.emission;
}

Check out the tutorials over at learnopengles.com. They'll answer all the questions you have.

There's no 'java equivalent' of a shader file. The shader is written in GLSL. The shader will be the same whether your opengl is wrapped in java, or c++, or python, or whatever. Aside from small API differences between OpenGL and OpenGLES, you can upload the exact same shader in Java as you used in C++, character for character.

You can use this source:
https://github.com/markusfisch/ShaderEditor
ShaderView class is the key!
GLSL files are in raw folder.