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Improve JPEG output of 2-channel images #3531

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Aug 23, 2022
Merged

Improve JPEG output of 2-channel images #3531

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30 changes: 27 additions & 3 deletions src/jpeg.imageio/jpegoutput.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -163,12 +163,25 @@ JpgOutput::open(const std::string& name, const ImageSpec& newspec,
m_cinfo.image_height = m_spec.height;

// JFIF can only handle grayscale and RGB. Do the best we can with this
// limited format by truncating to 3 channels if > 3 are requested,
// truncating to 1 channel if 2 are requested.
// limited format by switching to 1 or 3 channels.
if (m_spec.nchannels >= 3) {
// For 3 or more channels, write the first 3 as RGB and drop any
// additional channels.
m_cinfo.input_components = 3;
m_cinfo.in_color_space = JCS_RGB;
} else if (m_spec.nchannels == 2) {
// Two channels are tricky. If the first channel name is "Y", assume
// it's a luminance image and write it as a single-channel grayscale.
// Otherwise, punt, write it as an RGB image with third channel black.
if (m_spec.channel_name(0) == "Y") {
m_cinfo.input_components = 1;
m_cinfo.in_color_space = JCS_GRAYSCALE;
} else {
m_cinfo.input_components = 3;
m_cinfo.in_color_space = JCS_RGB;
}
} else {
// One channel, assume it's grayscale
m_cinfo.input_components = 1;
m_cinfo.in_color_space = JCS_GRAYSCALE;
}
Expand Down Expand Up @@ -454,7 +467,18 @@ JpgOutput::write_scanline(int y, int z, TypeDesc format, const void* data,
int save_nchannels = m_spec.nchannels;
m_spec.nchannels = m_cinfo.input_components;

data = to_native_scanline(format, data, xstride, m_scratch, m_dither, y, z);
if (save_nchannels == 2 && m_spec.nchannels == 3) {
// Edge case: expanding 2 channels to 3
uint8_t* tmp = OIIO_ALLOCA(uint8_t, m_spec.width * 3);
memset(tmp, 0, m_spec.width * 3);
convert_image(2, m_spec.width, 1, 1, data, format, xstride, AutoStride,
AutoStride, tmp, TypeDesc::UINT8, 3 * sizeof(uint8_t),
AutoStride, AutoStride);
data = tmp;
} else {
data = to_native_scanline(format, data, xstride, m_scratch, m_dither, y,
z);
}
m_spec.nchannels = save_nchannels;

jpeg_write_scanlines(&m_cinfo, (JSAMPLE**)&data, 1);
Expand Down