• Advertisement
Sign in to follow this  

OpenGL glTexImage2D with GL_ALPHA

This topic is 2861 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Hi, I've got a floating point buffer of pixel values, with only one float per pixel (so a single component instead of 4 components for RGBA). My intention is, to create a texture from this floating point buffer, which will be displayed by OpenGL as if it were a completely white texture, but with a varying alpha channel, where the values of the alpha channel are those from the floating point buffer. The official documentation of glTexImage2D (http://www.opengl.org/sdk/docs/man/xhtml/glTexImage2D.xml) says under "GL_ALPHA":
Quote:
GL_ALPHA Each element is a single alpha component. The GL converts it to floating point and assembles it into an RGBA element by attaching 0 for red, green, and blue. Each component is then multiplied by the signed scale factor GL_c_SCALE, added to the signed bias GL_c_BIAS, and clamped to the range [0,1] (see glPixelTransfer).
If I read that correctly, they're saying that R, G and B will be 0. But then RGB would be black instead of white, which isn't what I want. Do you know what a good method with OpenGL is to get a texture the way I described? Actually, it might be that using the GL_c_BIAS described above would do it. E.g. GL_GREEN_BIAS and such. BUT here is the very strange thing! In fact, it turns out on my computer that, even though GL_GREEN_BIAS and the others are 0, in fact, the texture already looks as if it's white! So as if it's already what I want! But since that doesn't make sense according to the official documentation, I think maybe it's only accidently correct on my computer and might be wrong on some others. What could cause this? I set both internalFormat and format to GL_ALPHA, and type to GL_FLOAT. I check with glGet and the bias values are 0. Thanks!

Share this post


Link to post
Share on other sites
Advertisement
I just implemented this, but in OpenGL 3.2 where GL_ALPHA no longer exists. Basically what I did was load it as GL_RED and then do the swizzling in my fragment shader:


in vec2 ex_TexCoord;
uniform sampler2D my_texture_id;

void main()
{
vec4 fc;
fc = texture2D(my_texture_id, ex_TexCoord);
gl_FragColor = vec4(1.0,1.0,1.0,fc.r);
}



-me

Share this post


Link to post
Share on other sites
I'm using plain old OpenGL 1.X.

My video card doesn't even support OpenGL 3 afaik (OpenGL version returns "2.1.2 NVIDIA 190.53").

Any idea how it works there and how exactly this GL_ALPHA is used?

Share this post


Link to post
Share on other sites
Well you could still use a shader. Otherwise, I don't know. OpenGL 3.x is nice because all the "how the hell does this work again" stuff was depricated [smile]. Nothing that I posted, however, is specifically OpenGL 3.2. It should work with whatever drivers you currently have for whatever video card you have.

-me

Share this post


Link to post
Share on other sites
2 things.

1.) "The GL converts it to floating point and assembles it into an RGBA element" sounds like it will still be stored as RBGA, so the obvious is if you want color with alpha, send 1,1,1, alpha....

2.) What are you trying to do then, black/white are colors, you said you only care about alpha. What type of blending are you trying to do?

Share this post


Link to post
Share on other sites
Quote:
Original post by dpadam450
2 things.

1.) "The GL converts it to floating point and assembles it into an RGBA element" sounds like it will still be stored as RBGA, so the obvious is if you want color with alpha, send 1,1,1, alpha....

2.) What are you trying to do then, black/white are colors, you said you only care about alpha. What type of blending are you trying to do?


I can't send it 1,1,1,alpha, because the buffer I send contains 1 value per color channel, not 4...

I'm trying to draw a colored mask over the screen. So the texture should be white with alpha, but I can then use glColor to draw it with any color. If it'd be black instead of white, then glColor wouldn't have effect, it'd always look black.

I think I have a theory about why it actually seems to work for me: maybe OpenGL ignores the RGB of a GL_ALPHA texture, so it doesn't matter than they're 0?

Share this post


Link to post
Share on other sites
How about rendering it to a texture? The old way, render it to the blank framebuffer, than copy the whole RGBA to the RGBA texture.

Or maybe I misunderstood something (since you have constant updating, you can add this to the beginning of the render code)

Share this post


Link to post
Share on other sites
Quote:
I can't send it 1,1,1,alpha, because the buffer I send contains 1 value per color channel, not 4...


Well to make it as easy as possible, put those values in there, make a temporary buffer and fill it 1,1,1,alpha.

You could try changing the glTexEnv to GL_REPLACE instead of modulate. That might bypass the texture color and use the glColor instead.

Share this post


Link to post
Share on other sites
I think what you want is GL_LUMINANCE. The lum value will be replicated for RGB and the alpha will be 1.0
#2, you said that your data is float, so you should not use GL_ALPHA or GL_LUMINANCE because then your float data gets converted to 8 bit integers. You need to use GL_ALPHA32F_ARB from the extension GL_ARB_texture_float.

Your call would be
glTexImage2D(GL_TEXTURE2D, 0, GL_ALPHA32F_ARB, width, height, 0, GL_ALPHA, GL_FLOAT, pixels)

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Now

  • Advertisement
  • Similar Content

    • By khawk
      We've just released all of the source code for the NeHe OpenGL lessons on our Github page at https://github.com/gamedev-net/nehe-opengl. code - 43 total platforms, configurations, and languages are included.
      Now operated by GameDev.net, NeHe is located at http://nehe.gamedev.net where it has been a valuable resource for developers wanting to learn OpenGL and graphics programming.

      View full story
    • By TheChubu
      The Khronos™ Group, an open consortium of leading hardware and software companies, announces from the SIGGRAPH 2017 Conference the immediate public availability of the OpenGL® 4.6 specification. OpenGL 4.6 integrates the functionality of numerous ARB and EXT extensions created by Khronos members AMD, Intel, and NVIDIA into core, including the capability to ingest SPIR-V™ shaders.
      SPIR-V is a Khronos-defined standard intermediate language for parallel compute and graphics, which enables content creators to simplify their shader authoring and management pipelines while providing significant source shading language flexibility. OpenGL 4.6 adds support for ingesting SPIR-V shaders to the core specification, guaranteeing that SPIR-V shaders will be widely supported by OpenGL implementations.
      OpenGL 4.6 adds the functionality of these ARB extensions to OpenGL’s core specification:
      GL_ARB_gl_spirv and GL_ARB_spirv_extensions to standardize SPIR-V support for OpenGL GL_ARB_indirect_parameters and GL_ARB_shader_draw_parameters for reducing the CPU overhead associated with rendering batches of geometry GL_ARB_pipeline_statistics_query and GL_ARB_transform_feedback_overflow_querystandardize OpenGL support for features available in Direct3D GL_ARB_texture_filter_anisotropic (based on GL_EXT_texture_filter_anisotropic) brings previously IP encumbered functionality into OpenGL to improve the visual quality of textured scenes GL_ARB_polygon_offset_clamp (based on GL_EXT_polygon_offset_clamp) suppresses a common visual artifact known as a “light leak” associated with rendering shadows GL_ARB_shader_atomic_counter_ops and GL_ARB_shader_group_vote add shader intrinsics supported by all desktop vendors to improve functionality and performance GL_KHR_no_error reduces driver overhead by allowing the application to indicate that it expects error-free operation so errors need not be generated In addition to the above features being added to OpenGL 4.6, the following are being released as extensions:
      GL_KHR_parallel_shader_compile allows applications to launch multiple shader compile threads to improve shader compile throughput WGL_ARB_create_context_no_error and GXL_ARB_create_context_no_error allow no error contexts to be created with WGL or GLX that support the GL_KHR_no_error extension “I’m proud to announce OpenGL 4.6 as the most feature-rich version of OpenGL yet. We've brought together the most popular, widely-supported extensions into a new core specification to give OpenGL developers and end users an improved baseline feature set. This includes resolving previous intellectual property roadblocks to bringing anisotropic texture filtering and polygon offset clamping into the core specification to enable widespread implementation and usage,” said Piers Daniell, chair of the OpenGL Working Group at Khronos. “The OpenGL working group will continue to respond to market needs and work with GPU vendors to ensure OpenGL remains a viable and evolving graphics API for all its customers and users across many vital industries.“
      The OpenGL 4.6 specification can be found at https://khronos.org/registry/OpenGL/index_gl.php. The GLSL to SPIR-V compiler glslang has been updated with GLSL 4.60 support, and can be found at https://github.com/KhronosGroup/glslang.
      Sophisticated graphics applications will also benefit from a set of newly released extensions for both OpenGL and OpenGL ES to enable interoperability with Vulkan and Direct3D. These extensions are named:
      GL_EXT_memory_object GL_EXT_memory_object_fd GL_EXT_memory_object_win32 GL_EXT_semaphore GL_EXT_semaphore_fd GL_EXT_semaphore_win32 GL_EXT_win32_keyed_mutex They can be found at: https://khronos.org/registry/OpenGL/index_gl.php
      Industry Support for OpenGL 4.6
      “With OpenGL 4.6 our customers have an improved set of core features available on our full range of OpenGL 4.x capable GPUs. These features provide improved rendering quality, performance and functionality. As the graphics industry’s most popular API, we fully support OpenGL and will continue to work closely with the Khronos Group on the development of new OpenGL specifications and extensions for our customers. NVIDIA has released beta OpenGL 4.6 drivers today at https://developer.nvidia.com/opengl-driver so developers can use these new features right away,” said Bob Pette, vice president, Professional Graphics at NVIDIA.
      "OpenGL 4.6 will be the first OpenGL release where conformant open source implementations based on the Mesa project will be deliverable in a reasonable timeframe after release. The open sourcing of the OpenGL conformance test suite and ongoing work between Khronos and X.org will also allow for non-vendor led open source implementations to achieve conformance in the near future," said David Airlie, senior principal engineer at Red Hat, and developer on Mesa/X.org projects.

      View full story
    • By _OskaR
      Hi,
      I have an OpenGL application but without possibility to wite own shaders.
      I need to perform small VS modification - is possible to do it in an alternative way? Do we have apps or driver modifictions which will catch the shader sent to GPU and override it?
    • By xhcao
      Does sync be needed to read texture content after access texture image in compute shader?
      My simple code is as below,
      glUseProgram(program.get());
      glBindImageTexture(0, texture[0], 0, GL_FALSE, 3, GL_READ_ONLY, GL_R32UI);
      glBindImageTexture(1, texture[1], 0, GL_FALSE, 4, GL_WRITE_ONLY, GL_R32UI);
      glDispatchCompute(1, 1, 1);
      // Does sync be needed here?
      glUseProgram(0);
      glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
      glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                                     GL_TEXTURE_CUBE_MAP_POSITIVE_X + face, texture[1], 0);
      glReadPixels(0, 0, kWidth, kHeight, GL_RED_INTEGER, GL_UNSIGNED_INT, outputValues);
       
      Compute shader is very simple, imageLoad content from texture[0], and imageStore content to texture[1]. Does need to sync after dispatchCompute?
    • By Jonathan2006
      My question: is it possible to transform multiple angular velocities so that they can be reinserted as one? My research is below:
      // This works quat quaternion1 = GEQuaternionFromAngleRadians(angleRadiansVector1); quat quaternion2 = GEMultiplyQuaternions(quaternion1, GEQuaternionFromAngleRadians(angleRadiansVector2)); quat quaternion3 = GEMultiplyQuaternions(quaternion2, GEQuaternionFromAngleRadians(angleRadiansVector3)); glMultMatrixf(GEMat4FromQuaternion(quaternion3).array); // The first two work fine but not the third. Why? quat quaternion1 = GEQuaternionFromAngleRadians(angleRadiansVector1); vec3 vector1 = GETransformQuaternionAndVector(quaternion1, angularVelocity1); quat quaternion2 = GEQuaternionFromAngleRadians(angleRadiansVector2); vec3 vector2 = GETransformQuaternionAndVector(quaternion2, angularVelocity2); // This doesn't work //quat quaternion3 = GEQuaternionFromAngleRadians(angleRadiansVector3); //vec3 vector3 = GETransformQuaternionAndVector(quaternion3, angularVelocity3); vec3 angleVelocity = GEAddVectors(vector1, vector2); // Does not work: vec3 angleVelocity = GEAddVectors(vector1, GEAddVectors(vector2, vector3)); static vec3 angleRadiansVector; vec3 angularAcceleration = GESetVector(0.0, 0.0, 0.0); // Sending it through one angular velocity later in my motion engine angleVelocity = GEAddVectors(angleVelocity, GEMultiplyVectorAndScalar(angularAcceleration, timeStep)); angleRadiansVector = GEAddVectors(angleRadiansVector, GEMultiplyVectorAndScalar(angleVelocity, timeStep)); glMultMatrixf(GEMat4FromEulerAngle(angleRadiansVector).array); Also how do I combine multiple angularAcceleration variables? Is there an easier way to transform the angular values?
  • Advertisement