GLCONVOLUTIONFILTER2D(3G)

✨ NAME ✨

glConvolutionFilter2D — bestie, this is the one that defines a two-dimensional convolution filter 💖 (and yeah, the manual page itself lives in section 3G, so GLCONVOLUTIONFILTER2D(3G) is the whole vibe, iykyk)

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🌸 C SPECIFICATION 🌸

void glConvolutionFilter2D( GLenum target,
                            GLenum internalformat,
                            GLsizei width,
                            GLsizei height,
                            GLenum format,
                            GLenum type,
                            const GLvoid *image )

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🎀 PARAMETERS 🎀

• target — literally has to be GL_CONVOLUTION_2D. That's it, that's the whole personality. 🙅‍♀️
• internalformat — the internal format of the convolution filter kernel, okay? The allowable values are GL_ALPHA, GL_ALPHA4, GL_ALPHA8, GL_ALPHA12, GL_ALPHA16, GL_LUMINANCE, GL_LUMINANCE4, GL_LUMINANCE8, GL_LUMINANCE12, GL_LUMINANCE16, GL_LUMINANCE_ALPHA, GL_LUMINANCE4_ALPHA4, GL_LUMINANCE6_ALPHA2, GL_LUMINANCE8_ALPHA8, GL_LUMINANCE12_ALPHA4, GL_LUMINANCE12_ALPHA12, GL_LUMINANCE16_ALPHA16, GL_INTENSITY, GL_INTENSITY4, GL_INTENSITY8, GL_INTENSITY12, GL_INTENSITY16, GL_R3_G3_B2, GL_RGB, GL_RGB4, GL_RGB5, GL_RGB8, GL_RGB10, GL_RGB12, GL_RGB16, GL_RGBA, GL_RGBA2, GL_RGBA4, GL_RGB5_A1, GL_RGBA8, GL_RGB10_A2, GL_RGBA12, or GL_RGBA16. Yes that's a lot, she's got options, we love a versatile icon 💅
• width — how wide the pixel array that image points at is.
• height — how tall that pixel array is. 📐
• format — the format of the pixel data chillin' inside image. The allowable values are GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_BGR, GL_RGBA, GL_BGRA, GL_LUMINANCE, and GL_LUMINANCE_ALPHA.
• type — the type of the pixel data in image. These symbolic constants are accepted: GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP, GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT, GL_FLOAT, GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV, GL_UNSIGNED_SHORT_5_6_5, GL_UNSIGNED_SHORT_5_6_5_REV, GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV, GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2, and GL_UNSIGNED_INT_2_10_10_10_REV. Slay. ✨
• image — a pointer to a two-dimensional array of pixel data that gets processed to build the convolution filter kernel. That's the main character right there. 👑

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💖 DESCRIPTION 💖

glConvolutionFilter2D builds a two-dimensional convolution filter kernel out of an array of pixels. no cap, that's the whole job.

The pixel array specified by width, height, format, type, and image gets extracted from memory and processed exactly as if glDrawPixels were called — except processing stops right after the final expansion to RGBA is completed. 🛑

Then the R, G, B, and A components of each pixel get scaled by the four 2D GL_CONVOLUTION_FILTER_SCALE parameters and biased by the four 2D GL_CONVOLUTION_FILTER_BIAS parameters. (Those scale and bias params are set by glConvolutionParameter using the GL_CONVOLUTION_2D target and the names GL_CONVOLUTION_FILTER_SCALE and GL_CONVOLUTION_FILTER_BIAS. The parameters themselves are vectors of four values, applied to red, green, blue, and alpha, in that order.) And listen — the R, G, B, and A values are not clamped to [0,1] at any time during this process. They're free, they're unbothered. 🦋

Each pixel is then converted to the internal format specified by internalformat. This conversion is basically just mapping the component values of the pixel (R, G, B, and A) onto the values included in the internal format (red, green, blue, alpha, luminance, and intensity). The mapping goes like this:

• GL_ALPHA — Alpha = A
• GL_LUMINANCE — Luminance = R
• GL_LUMINANCE_ALPHA — Alpha = A, Luminance = R
• GL_INTENSITY — Intensity = R
• GL_RGB — Red = R, Green = G, Blue = B
• GL_RGBA — Red = R, Green = G, Blue = B, Alpha = A

The red, green, blue, alpha, luminance, and/or intensity components of the resulting pixels are stored in floating-point rather than integer values. 💾 They form a two-dimensional filter kernel image indexed with coordinates i and j, such that i starts at zero and increases from left to right, and j starts at zero and increases from bottom to top. Kernel location i,j is derived from the Nth pixel, where N is i+j*width. Yes, it's math, but it's cute math. 🧮

Heads up bestie: after a convolution is performed, the resulting color components are also scaled by their corresponding GL_POST_CONVOLUTION_c_SCALE parameters and biased by their corresponding GL_POST_CONVOLUTION_c_BIAS parameters (where c takes on the values RED, GREEN, BLUE, and ALPHA). Those parameters are set by glPixelTransfer. Double-processing energy, we love her. 💜

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🫶 NOTES 🫶

glConvolutionFilter2D is only present if GL_ARB_imaging is returned when glGetString is called with an argument of GL_EXTENSIONS. So like, vibe check first, okay? 🌈

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😭 ERRORS 😭

• GL_INVALID_ENUM is generated if target is not GL_CONVOLUTION_2D.
• GL_INVALID_ENUM is generated if internalformat is not one of the allowable values.
• GL_INVALID_VALUE is generated if width is less than zero or greater than the maximum supported value. You can query that value with glGetConvolutionParameter using target GL_CONVOLUTION_2D and name GL_MAX_CONVOLUTION_WIDTH.
• GL_INVALID_VALUE is generated if height is less than zero or greater than the maximum supported value. You can query that value with glGetConvolutionParameter using target GL_CONVOLUTION_2D and name GL_MAX_CONVOLUTION_HEIGHT.
• GL_INVALID_ENUM is generated if format is not one of the allowable values.
• GL_INVALID_ENUM is generated if type is not one of the allowable values.
• GL_INVALID_OPERATION is generated if glConvolutionFilter2D is executed between the execution of glBegin and the corresponding execution of glEnd. 🚫 (do not do this, I am begging)
• GL_INVALID_OPERATION is generated if height is one of GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV, GL_UNSIGNED_SHORT_5_6_5, or GL_UNSIGNED_SHORT_5_6_5_REV and format is not GL_RGB. (yes it literally says height there, I'm just the messenger fr 💀)
• GL_INVALID_OPERATION is generated if height is one of GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV, GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2, or GL_UNSIGNED_INT_2_10_10_10_REV and format is neither GL_RGBA nor GL_BGRA. (again: height. iconic of the manpage honestly 🙈)

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👑 ASSOCIATED GETS 👑

• glGetConvolutionParameter
• glGetConvolutionFilter

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🦋 SEE ALSO 🦋

• glConvolutionFilter1D(3G)
• glSeparableFilter2D(3G)
• glConvolutionParameter(3G)
• glPixelTransfer(3G)

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and that's the page bestie!! 🎀 the footer down at the bottom is just GLCONVOLUTIONFILTER2D(3G) all over again — no extra version, no date, just the section 3G name echoing back at you like a little mirror. periodt. 💅✨🌸