Home > Articles > Programming > Graphic Programming

  • Print
  • + Share This
This chapter is from the book

Passing Data from Stage to Stage

So far, you have seen how to pass data into a vertex shader by creating a vertex attribute using the in keyword, how to communicate with fixed-function blocks by reading and writing built-in variables such as gl_VertexID and gl_Position, and how to output data from the fragment shader using the out keyword. However, it’s also possible to send your own data from shader stage to shader stage using the same in and out keywords. Just as you used the out keyword in the fragment shader to create the output variable to which it writes its color values, so you can also create an output variable in the vertex shader by using the out keyword. Anything you write to an output variable in one shader is sent to a similarly named variable declared with the in keyword in the subsequent stage. For example, if your vertex shader declares a variable called vs_color using the out keyword, it would match up with a variable named vs_color declared with the in keyword in the fragment shader stage (assuming no other stages were active in between).

If we modify our simple vertex shader as shown in Listing 3.3 to include vs_color as an output variable, and correspondingly modify our simple fragment shader to include vs_color as an input variable as shown in Listing 3.4, we can pass a value from the vertex shader to the fragment shader. Then, rather than outputting a hard-coded value, the fragment can simply output the color passed to it from the vertex shader.

Listing 3.3: Vertex shader with an output

#version 450 core

// 'offset' and 'color' are input vertex attributes
layout (location = 0) in vec4 offset;
layout (location = 1) in vec4 color;

// 'vs_color' is an output that will be sent to the next shader stage
out vec4 vs_color;

void main(void)
{
    const vec4 vertices[3] = vec4[3](vec4(0.25, -0.25, 0.5, 1.0),
                                     vec4(-0.25, -0.25, 0.5, 1.0),
                                     vec4(0.25, 0.25, 0.5, 1.0));

    // Add 'offset' to our hard-coded vertex position
    gl_Position = vertices[gl_VertexID] + offset;

    // Output a fixed value for vs_color
    vs_color = color;
}

As you can see in Listing 3.3, we declare a second input to our vertex shader, color (this time at location 1), and write its value to the vs_output output. This is picked up by the fragment shader of Listing 3.4 and written to the framebuffer. This allows us to pass a color all the way from a vertex attribute that we can set with glVertexAttrib*() through the vertex shader, into the fragment shader, and out to the framebuffer. As a consequence, we can draw different-colored triangles!

Listing 3.4: Fragment shader with an input

#version 450 core

// Input from the vertex shader
in vec4 vs_color;

// Output to the framebuffer
out vec4 color;

void main(void)
{
   // Simply assign the color we were given by the vertex shader to our output
   color = vs_color;
}

Interface Blocks

Declaring interface variables one at a time is possibly the simplest way to communicate data between shader stages. However, in most nontrivial applications, you will likely want to communicate a number of different pieces of data between stages; these may include arrays, structures, and other complex arrangements of variables. To achieve this, we can group together a number of variables into an interface block. The declaration of an interface block looks a lot like a structure declaration, except that it is declared using the in or out keyword depending on whether it is an input to or output from the shader. An example interface block definition is shown in Listing 3.5.

Listing 3.5: Vertex shader with an output interface block

#version 450 core

// 'offset' is an input vertex attribute

layout (location = 0) in vec4 offset;
layout (location = 1) in vec4 color;

// Declare VS_OUT as an output interface block
out VS_OUT
{
    vec4 color;     // Send color to the next stage
} vs_out;

void main(void)
{
     const vec4 vertices[3] = vec4[3](vec4(0.25, -0.25, 0.5, 1.0),
                                      vec4(-0.25, -0.25, 0.5, 1.0),
                                      vec4(0.25, 0.25, 0.5, 1.0));

     // Add 'offset' to our hard-coded vertex position
     gl_Position = vertices[gl_VertexID] + offset;

     // Output a fixed value for vs_color
     vs_out.color = color;
}

Note that the interface block in Listing 3.5 has both a block name (VS_OUT, uppercase) and an instance name (vs_out, lowercase). Interface blocks are matched between stages using the block name (VS_OUT in this case), but are referenced in shaders using the instance name. Thus, modifying our fragment shader to use an interface block gives the code shown in Listing 3.6.

Listing 3.6: Fragment shader with an input interface block

#version 450 core

// Declare VS_OUT as an input interface block
in VS_OUT
{
    vec4 color;     // Send color to the next stage
} fs_in;

// Output to the framebuffer
out vec4 color;

void main(void)
{
    // Simply assign the color we were given by the vertex shader to our  output
    color = fs_in.color;
}

Matching interface blocks by block name but allowing block instances to have different names in each shader stage serves two important purposes. First, it allows the name by which you refer to the block to be different in each stage, thereby avoiding confusing things such as having to use vs_out in a fragment shader. Second, it allows interfaces to go from being single items to arrays when crossing between certain shader stages, such as the vertex and tessellation or geometry shader stages, as we will see in a short while. Note that interface blocks are only for moving data from shader stage to shader stage—you can’t use them to group together inputs to the vertex shader or outputs from the fragment shader.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020